Linux 5.7 released, Bootlin contributions

We’re late to the party as Linux 5.8 is going to be released in a few weeks, but we never published about our contribution to the current Linux stable release, Linux 5.7, so here is our usual summary! For an overview of the major changes in 5.7, KernelNewbies has a nice summary, as well as LWN, in two parts: part 1 and part 2.

Bootlin contributed 92 commits to this release, a small number of contributions compared to past releases, but nevertheless with some significant work:

  • Antoine Ténart contributed support for offloading the MACsec encryption/decryption to a PHY in the networking stack, as well as the corresponding offloading support for some specific Microchip/Vitesse Ethernet PHYs. See our blog post for more details about this feature.
  • Alexandre Belloni continued converting the Atmel/Microchip platforms to the new clock representation, with this time AT91SAM9G45, SAMA5D3, AT91SAM9N12 and AT91RM9200.
  • Alexandre Belloni, as the RTC subsystem maintainer, again did a lot of cleanup and improvements in multiple RTC drivers.
  • Kamel Bouhara contributed support for I2C recovery for the Atmel/Microchip platforms.

In terms of maintainers activity: Miquèl Raynal, as the MTD co-maintainer, merged 62 patches from other contributors, Alexandre Belloni, the RTC maintainer and Atmel/Microchip platform co-maintainer merged 49 patches from other contributors, while Grégory Clement, as the Marvell EBU platforms co-maintainer, merged 11 patches from other contributors.

Here is the detail of our contributions for 5.7:

Measured boot with a TPM 2.0 in U-Boot

A Trusted Platform Module, in short TPM, is a small piece of hardware designed to provide various security functionalities. It offers numerous features, such as storing secrets, ‘measuring’ boot, and may act as an external cryptographic engine. The Trusted Computing Group (TCG) delivers a document called TPM Interface Specifications (TIS) which describes the architecture of such devices and how they are supposed to behave as well as various details around the concepts.

These TPM chips are either compliant with the first specification (up to 1.2) or the second specification (2.0+). The TPM2.0 specification is not backward compatible and this is the one this post is about. If you need more details, there are many documents available at https://trustedcomputinggroup.org/.

Picture of a TPM wired on an EspressoBin
Trusted Platform Module connected over SPI to Marvell EspressoBin platform

Among the functions listed above, this blog post will focus on the measured boot functionality.

Measured boot principles

Measuring boot is a way to inform the last software stage if someone tampered with the platform. It is impossible to know what has been corrupted exactly, but knowing someone has is already enough to not reveal secrets. Indeed, TPMs offer a small secure locker where users can store keys, passwords, authentication tokens, etc. These secrets are not exposed anywhere (unlike with any standard storage media) and TPMs have the capability to release these secrets only under specific conditions. Here is how it works.

Starting from a root of trust (typically the SoC Boot ROM), each software stage during the boot process (BL1, BL2, BL31, BL33/U-Boot, Linux) is supposed to do some measurements and store them in a safe place. A measure is just a digest (let’s say, a SHA256) of a memory region. Usually each stage will ‘digest’ the next one. Each digest is then sent to the TPM, which will merge this measurement with the previous ones.

The hardware feature used to store and merge these measurements is called Platform Configuration Registers (PCR). At power-up, a PCR is set to a known value (either 0x00s or 0xFFs, usually). Sending a digest to the TPM is called extending a PCR because the chosen register will extend its value with the one received with the following logic:

PCR[x] := sha256(PCR[x] | digest)

This way, a PCR can only evolve in one direction and never go back unless the platform is reset.

In a typical measured boot flow, a TPM can be configured to disclose a secret only under a certain PCR state. Each software stage will be in charge of extending a set of PCRs with digests of the next software stage. Once in Linux, user software may ask the TPM to deliver its secrets but the only way to get them is having all PCRs matching a known pattern. This can only be obtained by extending the PCRs in the right order, with the right digests.

Linux support for TPM devices

A solid TPM 2.0 stack has been around for Linux for quite some time, in the form of the tpm2-tss and tpm2-tools projects. More specifically, a daemon called resourcemgr, is provided by the tpm2-tss project. For people coming from the TPM 1.2 world, this used to be called trousers. One can find some commands ready to be used in the tpm2-tools repository, useful for testing purpose.

From the Linux kernel perspective, there are device drivers for at least SPI chips (one can have a look there at files called tpm2*.c and tpm_tis*.c for implementation details).

Bootlin’s contribution: U-Boot support for TPM 2.0

Back when we worked on this topic in 2018, there was no support for TPM 2.0 in U-Boot, but one of customer needed this support. So we implemented, contributed and upstreamed to U-Boot support for TPM 2.0. Our 32 patches patch series adding TPM 2.0 support was merged, with:

  • SPI TPMs compliant with the TCG TIS v2.0
  • Commands for U-Boot shell to do minimal operations (detailed below)
  • A test framework for regression detection
  • A sandbox TPM driver emulating a fake TPM

In details, our commits related to TPM support in U-Boot:

Details of U-Boot commands

Available commands for v2.0 TPMs in U-Boot are currently:

  • STARTUP
  • SELF TEST
  • CLEAR
  • PCR EXTEND
  • PCR READ
  • GET CAPABILITY
  • DICTIONARY ATTACK LOCK RESET
  • DICTIONARY ATTACK CHANGE PARAMETERS
  • HIERARCHY CHANGE AUTH

With this set of functions, minimal handling is possible with the following sequence.

First, the TPM stack in U-Boot must be initialized with:

> tpm init

Then, the STARTUP command must be sent.

> tpm startup TPM2_SU_CLEAR

To enable full TPM capabilities, one must request to continue the self tests (or do them all again).

> tpm self_test full
> tpm self_test continue

This is enough to pursue measured boot as one just need to extend the PCR as needed, giving 1/ the PCR number and 2/ the address where the digest is stored:

> tpm pcr_extend 0 0x4000000

Reading of the extended value is of course possible with:

> tpm pcr_read 0 0x4000000

Managing passwords is about limiting some commands to be sent without previous authentication. This is also possible with the minimum set of commands recently committed, and there are two ways of implementing it. One is quite complicated and features the use of a token together with cryptographic derivations at each exchange. Another solution, less invasive, is to use a single password. Changing passwords was previously done with a single TAKE OWNERSHIP command, while today a CLEAR must precede a CHANGE AUTH. Each of them may act upon different hierarchies. Hierarchies are some kind of authority level and do not act upon the same commands. For the example, let’s use the LOCKOUT hierarchy: the locking mechanism blocking the TPM for a given amount of time after a number of failed authentications, to mitigate dictionary attacks.

> tpm clear TPM2_RH_LOCKOUT [<pw>]
> tpm change_auth TPM2_RH_LOCKOUT <new_pw> [<old_pw>]

Drawback of this implementation: as opposed to the token/hash solution, there is no protection against packet replay.

Please note that a CLEAR does much more than resetting passwords, it entirely resets the whole TPM configuration.

Finally, Dictionary Attack Mitigation (DAM) parameters can also be changed. It is possible to reset the failure counter (aka. the maximum number of attempts before lockout) as well as to disable the lockout entirely. It is possible to check the parameters have been correctly applied.

> tpm dam_reset [<pw>]
> tpm dam_parameters 0xffff 1 0 [<pw>]
> tpm get_capability 0x0006 0x020e 0x4000000 4

In the above example, the DAM parameters are reset, then the maximum number of tries before lockout is set to 0xffff, the delay before decrementing the failure counter by 1 and the lockout is entirely disabled. These parameters are for testing purpose. The third command is explained in the specification but basically retrieves 4 values starting at capability 0x6, property index 0x20e. It will display first the failure counter, followed by the three parameters previously changed.

Limitation

Although TPMs are meant to be black boxes, U-Boot current support is too light to really protect against replay attacks as one could spoof the bus and resend the exact same packets after taking ownership of the platform in order to get these secrets out. Additional developments are needed in U-Boot to protect against these attacks. Additionally, even with this extra security level, all the above logic is only safe when used in the context of a secure boot environment.

Conclusion

Thanks to this work from Bootlin, U-Boot has basic support for TPM 2.0 devices connected over SPI. Do not hesitate to contact us if you need support or help around TPM 2.0 support, either in U-Boot or Linux.

Configuring ALSA controls from an application

ALSA logoA common task when handling audio on Linux is the need to modify the configuration of the sound card, for example, adjusting the output volume or selecting the capture channels. On an embedded system, it can be enough to simply set the controls once using alsamixer or amixer and then save the configuration with alsactl store. This saves the driver state to the configuration file which, by default, is /var/lib/alsa/asound.state. Once done, this file can be included in the build system and shipped with the root filesystem. Usual distributions already include a script that will invoke alsactl at boot time to restore the settings. If it is not the case, then it is simply a matter of calling alsactl restore.

However, defining a static configuration may not be enough. For example, some codecs have advanced routing features allowing to route the audio channels to different outputs and the application may want to decide at runtime where the audio is going.

Instead of invoking amixer using system(3), even if it is not straightforward, it is possible to directly use the alsa-lib API to set controls.

Let’s start with some required includes:

#include <stdio.h>
#include <alsa/asoundlib.h>

alsa/asoundlib.h is the header that is of interest here as it is where the ALSA API lies. Then we define an id lookup function, which is actually the tricky part. Each control has a unique identifier and to be able to manipulate controls, it is necessary to find this unique identifier. In our sample application, we will be using the control name to do the lookup.

int lookup_id(snd_ctl_elem_id_t *id, snd_ctl_t *handle)
{
	int err;
	snd_ctl_elem_info_t *info;
	snd_ctl_elem_info_alloca(&info);

	snd_ctl_elem_info_set_id(info, id);
	if ((err = snd_ctl_elem_info(handle, info)) < 0) {
		fprintf(stderr, "Cannot find the given element from card\n");
		return err;
	}
	snd_ctl_elem_info_get_id(info, id);

	return 0;
}

This function allocates a snd_ctl_elem_info_t, sets its current id to the one passed as the first argument. At this point, the id only includes the control interface type and its name but not its unique id. The snd_ctl_elem_info() function looks up for the element on the sound card whose handle has been passed as the second argument. Then snd_ctl_elem_info_get_id() updates the id with the now completely filled id.

Then the controls can be modified as follows:

int main(int argc, char *argv[])
{
	int err;
	snd_ctl_t *handle;
	snd_ctl_elem_id_t *id;
	snd_ctl_elem_value_t *value;
	snd_ctl_elem_id_alloca(&id);
	snd_ctl_elem_value_alloca(&value);

This declares and allocates the necessary variables. Allocations are done using alloca so it is not necessary to free them as long as the function exits at some point.

	if ((err = snd_ctl_open(&handle, "hw:0", 0)) < 0) {
		fprintf(stderr, "Card open error: %s\n", snd_strerror(err));
		return err;
	}

Get a handle on the sound card, in this case, hw:0 which is the first sound card in the system.

	snd_ctl_elem_id_set_interface(id, SND_CTL_ELEM_IFACE_MIXER);
	snd_ctl_elem_id_set_name(id, "Headphone Playback Volume");
	if (err = lookup_id(id, handle))
		return err;

This sets the interface type and name of the control we want to modify and then call the lookup function.

	snd_ctl_elem_value_set_id(value, id);
	snd_ctl_elem_value_set_integer(value, 0, 55);
	snd_ctl_elem_value_set_integer(value, 1, 77);

	if ((err = snd_ctl_elem_write(handle, value)) < 0) {
		fprintf(stderr, "Control element write error: %s\n",
			snd_strerror(err));
		return err;
	}

Now, this changes the value of the control. snd_ctl_elem_value_set_id() sets the id of the control to be changed then snd_ctl_elem_value_set_integer() sets the actual value. There are multiple calls because this control has multiple members (in this case, left and right channels). Finally, snd_ctl_elem_write() commits the value.

Note that snd_ctl_elem_value_set_integer() is called directly because we know this control is an integer but it is actually possible to query what kind of value should be used using snd_ctl_elem_info_get_type() on the snd_ctl_elem_info_t. The scale of the integer is also device specific and can be retrieved with the snd_ctl_elem_info_get_min(), snd_ctl_elem_info_get_max() and snd_ctl_elem_info_get_step() helpers.

	snd_ctl_elem_id_clear(id);
	snd_ctl_elem_id_set_interface(id, SND_CTL_ELEM_IFACE_MIXER);
	snd_ctl_elem_id_set_name(id, "Headphone Playback Switch");
	if (err = lookup_id(id, handle))
		return err;

	snd_ctl_elem_value_clear(value);
	snd_ctl_elem_value_set_id(value, id);
	snd_ctl_elem_value_set_boolean(value, 1, 1);

	if ((err = snd_ctl_elem_write(handle, value)) < 0) {
		fprintf(stderr, "Control element write error: %s\n",
			snd_strerror(err));
		return err;
	}

This unmutes the right channel of Headphone playback, this time it is a boolean. The other common kind of element is SND_CTL_ELEM_TYPE_ENUMERATED for enumerated contents. This is used for channel muxing or selecting de-emphasis values for example. snd_ctl_elem_value_set_enumerated() has to be used to set the selected item.

	return 0;
}

This concludes this simple example and should be enough to get you started writing smarter applications that don't rely on external program to configure the sound card controls.

New feature highlights in Elixir Cross Referencer v2.0 and v2.1

The 2.1 release of the Elixir Cross Referencer is now live on https://elixir.bootlin.com/.

Development of new features has accelerated in the recent months, thanks to the contributions from Tamir Carmeli (Github), Chris White (Github) and Maxime Chrétien (Github), who was hired at Bootlin as an intern. I am going to describe the most important new features from such contributors, but the three of them actually made many smaller contributions to many aspects of Elixir.

So, here are the important new features you can now find in Elixir…

Support for symbol documentation

Thanks to Chris White, when you search for a function, you can now see where it is documented, at least when it is done in the Linux kernel way, extracting documentation from comments in the sources.

Symbol documentation in Elixir

This way, when documentation is available, you can immediately know the meaning and expected values of the parameters of a given function and its return value.

Support for Kconfig symbols

Maxime Chrétien has extended Elixir to support kernel configuration parameters. Actually, he contributed a new parser to the universal-ctags project to do so. This way, you can explore C sources and Kconfig files and find the declarations and uses of kernel parameters:

Elixir Kconfig symbols

Now, every time we mention a kernel configuration parameter in our free training materials, we can provide an Elixir link to them. Here is an example for CONFIG_SQUASHFS. Don’t hesitate to use such links in your documents and e-mails about the Linux kernel!

Note that you also have Kconfig symbol links in defconfig files, allowing to understand non-default kernel configuration settings for a given SoC family or board. See this example.

Support for Device Tree aliases

Maxime Chrétien also extended Elixir to support Device Tree labels. This way, when you explore a Device Tree source file and see a reference (phandle) to such a label, you can easily find where it’s defined and what the default properties of the corresponding node are.

Elixir Device Tree Source symbols

Following such extensions to Elixir to support new scopes for symbols, we extended the interface to allow to make searches for symbols either in specific contexts (C, Kconfig or DT), or in all contexts. In most of the cases, a single context will suffice, but we’re anyway offering a mode to perform searches in all contexts at the same time:

Elixir support for multiple symbol contexts

Support for Device Tree compatible strings

v2.1 of the Elixir Cross Referencer also adds support for Device Tree compatible strings, also contributed by Maxime Chrétien. When browsing Device Tree files, you can instantly find which drivers drivers can be bound to the corresponding devices, which properties such drivers require from such devices (as specified in the Device Tree bindings), and other Device Tree files using the same compatible string.

Elixir device tree compatible links
Elixir device tree compatible string search results

Symbol auto-completion in the search dialog

Elixir Cross Referencer v2.1 also features symbol search autocompletion, another capability implemented by Maxime Chrétien. This makes it easy to find Linux kernel function names while programming!

Elixir symbol autocompletion featur

Pygments support for Device Tree source files

In addition to this improvement for Device Tree indexing, Maxime has also contributed a new lexer to the Pygments project, which is used by Elixir for HTML syntax highlighting for all types of files.

REST API

Thanks to Tamir Carmeli, it’s now possible to access the Elixir database through a new REST API, instead of going through its web interface. This way, you can make Elixir queries from data processing scripts, for example.

Testing infrastructure

Chris White has implemented an extensive testing infrastructure to quickly detect regressions before the corresponding changes are applied to production servers. Tamir Carmeli also contributed a test system for the REST API.Thanks to this, each new commit is tested on Travis CI.

Parallel build for the Elixir database

Maxime Chrétien has managed to multithread indexing work. While Maxime is still exploring further options, this has already allowed to divide indexing time by an approximate factor of two.

Limitations

The main limitation of the Elixir Cross Referencer is that it doesn’t try to match any context. For example, the actual implementation of a symbol may depend on the value of a configuration option. When browsing a source file, Elixir also always links to all possibilities for each symbol (there can be multiple unrelated instances of the same symbol across the kernel sources) instead of narrowing the search to the definition corresponding to the currently browsed file. Elixir leave it up to the human user to find out which result matches the context of origin.

This is particularly true for Device Tree symbols that have unrelated occurrences everywhere in the source tree, such as i2c0. In a distant future, we may be able to restrict the search to the context of an originating file.

Contribute

If you have new ideas for extending the Elixir Cross Referencer to support more features and use cases, please share them on the project’s bug tracker. If they are feasible without compromising the relative simplicity and scalability of our engine, we will be happy to implement them!

Practical usage of timer counters in Linux, illustrated on Microchip platforms

Virtually all micro-controllers and micro-processors provide some form of timer counters. In the context of Linux, they are always used for kernel timers, but they can also sometimes be used for PWMs, or input capture devices able to measure external signals such as rotary encoders. In this blog post, we would like to illustrate how Linux can take advantage of such timer counters, by taking the example of the Microchip Timer Counter Block, and depict how its various features fit into existing Linux kernel subsystems.

Hardware overview

On Microchip ARM processors, the TCB (Timer Counter Block) module is a set of three independent, 16 or 32-bits, channels as illustrated in this simplified block diagram:

Microchip TCB

The exact number of TCB modules depends on which Microchip processor you’re using, this Microchip brochure gives the details. Most products have 6 or 9 timer counter channels available, which are grouped into two or three TCB modules, each having 3 channels.

Each TC channel can independently select a clock source for its counter:

  • Internal Clock: sourced from either the system bus clock (often the highest rated one with pre-defined divisors), the slow clock (crystal oscillator) and for the Microchip SAMA5D2 and SAM9X60 SOC series there is even a programmable generic clock source (GCLK) specific to each peripheral.
  • External Clock: based on the three available external input pins: TCLK0, TCLK1 or TCLK2.

The clock source choice should obviously be made depending on the accuracy required by the application.

The module has many functions declined in three different modes:

  • The input capture mode is useful to capture input signals (e.g measure a signal period) through one of the six input pins (TIOAx/TIOBx) connected to each TC module. Each pin can act as trigger source for the counter and two latch register RA/RB can be loaded and compared with a third RC register. This mode is highly configurable with lots of feature to fine tune the capture (subsambling, clock inverting, interrupt, etc.).
  • The waveform mode which provide the core function of TCs as all channels could be used as three independent free-running counters and it is also a mode used to generate PWM signals which gives an extra pool of PWMs
  • The quadrature mode is only supported on the first TC module TCB0 and two (or three) channels are required, channel 0 will decode the speed or position on TIOA0/TIOB0, channel 1 (with TIOB1 input) can be configured to store the revolution or number of rotation. Finally if speed measurement is configured the channel 2 shall define a speed time base.Something important to note is that this mode actually is only part of Microchip SAMA5 and SAM9x60 family SOCs.

Software overview

On the software side in the Linux kernel, the different functionalities offered by the Microchip TCBs will be handled by three different subsystems, which we cover in the following sections.

Clocksource susbsystem

This subsystem is the core target of any TC module as it allows the kernel to keep track of the time passing (clocksource) and program timer interrupts (clockevents). The Microchip TCB has its upstream implementation in drivers/clocksource/timer-atmel-tcb.c that uses the waveform mode to provide both clock source and clock events. The older Microchip platforms have only 16-bit timer counters, in which case two channels are needed to implement the clocksource support. Newer Microchip platforms have 32-bit timer counters, and in this case only one channel is needed to implement clocksource. In both cases, only one channel is necessary to implement clock events.

In the timer-atmel-tcb driver:

  • The clocksource is registered using a struct clocksource structure which mainly provides a ->read() callback to read the current cycle count
  • The clockevents is registered using a struct tc_clkevt_device structure, which provides callbacks to set the date of the next timer event (->set_next_event()) and to change the mode of the timer (->set_state_shutdown(), ->set_state_periodic(), ->set_state_oneshot()).

From a user-space point of view, the clocksource and clockevents subsystems are not directly visible, but they are of course used whenever one uses time or timer related functions. The available clockevents are visible in /sys/bus/clockevents and the available clocksources are visible in /sys/bus/clocksource. The file /proc/timer_list also gives a lot of information about the timers that are pending, and the available timer devices on the platform.

PWM subsystem

This subsystem is useful for many applications (fan control, leds, beepers etc.), and provides both an in-kernel APIs for other kernel drivers to use, as well as a user-space API in /sys/class/pwm, documented at https://www.kernel.org/doc/html/latest/driver-api/pwm.html.

As far as PWM functionality is concerned, the Microchip TCB module is supported by the driver at drivers/pwm/pwm-atmel-tcb.c, which also uses the waveform mode. In this mode both channels pins TIOAx/TIOBx can be used to output PWM signals which allows to provide up to 6 PWM outputs per TCB. On a high-level, this PWM driver registers a struct pwm_ops structure that provides pointers to the important callback to setup and configure PWM outputs.

The current diver implementation has the drawback of using an entire TCB module as a PWM chip: it is not possible to use 1 channel of a TCB module for PWM, and the other channels of the same TCB module for other functionality. On platforms that have only two TCB modules, this means that the first TCB module is typically used for the clockevents/clocksource functionality described previously, and therefore only the second TCB module can be used for PWM.

We are however working on lifting this limitation: Bootlin engineer Alexandre Belloni has a patch series at https://github.com/alexandrebelloni/linux/commits/at91-tcb to address this. We aim at submitting this patch series in the near future.

Thanks to the changes of this patch series, we will be able to use PWM channels as follows:

  • Configuring a 100KHz PWM signal on TIOAx:
    # echo 0 > /sys/class/pwm/pwmchip0/export
    # echo 10000 > /sys/class/pwm/pwmchip0/pwm0/period
    # echo 1000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
    # echo 1 > /sys/class/pwm/pwmchip0/pwm0/enable
    
  • Configuring a 100KHz PWM signal on TIOBx:
    # echo 1 > /sys/class/pwm/pwmchip0/export
    # echo 10000 > /sys/class/pwm/pwmchip0/pwm1/period
    # echo 1000 > /sys/class/pwm/pwmchip0/pwm1/duty_cycle
    # echo 1 > /sys/class/pwm/pwmchip0/pwm1/enable
    
  • One must note that both PWM signals of the same channel will share the same period even though we set it twice here as it is required by the PWM framework. The Microchip TCB takes the period from the RC register and RA/RB respectively for TIOAx/TIOBx duty cycles.

    Counter subsystem

    The Linux kernel counter subsystem, located in drivers/counter/ is much newer than the clocksource, clockevents and PWM subsystems described previously. Indeed, it is only in 2019 that it was added to the Linux kernel, and so far it contains only 5 drivers. This subsystem abstracts a timer counter as three entities: a Count that stores the value incremented or decremented from a measured input Signal and a Synapse that will provide edge-based trigger source.

    This subsystem was therefore very relevant to expose the input capture and quadrature decoder modes of the Microchip TCB module, and we recently submitted a patch series that implements a counter driver for the Microchip TCB module. The driver instantiates and registers a struct counter_device structure, with a variety of sub-structures and callbacks that allow the core counter subsystem to use the Microchip TCB module and expose its input capture and quadrature decoder features to user-space.

    The current user-space interface of the counter subsystem works over sysfs and is documented at https://www.kernel.org/doc/html/latest/driver-api/generic-counter.html. For example, to read the position of a rotary encoder connected to a TCB module configured as a quadradure decoder, one would do:

    # cd /sys/bus/counter/devices/counter0/count0/                    
    # echo "quadrature x4" > function                                 
    # cat count
    0
    

    However, when the device connected to the TCB is a rotary encoder, it would be much more useful to have it exposed to user-space as a standard input device so that all existing graphical libraries and frameworks can automatically make use of it. Rotary encoders connected to GPIOs can already be exposed to user-space as input devices using the rotary_encoder driver. Our goal was to achieve the same, but with a rotary encoder connected to a quadrature decoder handled by the counter subsystem. To this end, we submitted a second patch series, which:

    1. Extends the counter subsystem with an in-kernel API, so that counter devices can not only be used from user-space using sysfs, but also from other kernel subsystems. This is very much like the IIO in-kernel API, which is used in a variety of other kernel subsystems that need access to IIO devices.
    2. A new rotary-encoder-counter driver, which implements an input device based on a counter device configured in quadrature decoder mode.

    Thanks to this driver, we get an input device for our rotary encoder, which can for example be tested using evtest to decode the input events that occur when rotating the rotary encoder:

    # evtest /dev/input/event1                                        
    Input driver version is 1.0.1                                     
    Input device ID: bus 0x19 vendor 0x0 product 0x0 version 0x0      
    Input device name: "rotary@0"                                     
    Supported events:                                                 
    Event type 0 (EV_SYN)                                           
    Event type 2 (EV_REL)                                           
      Event code 0 (REL_X)                                          
    Properties:                                                       
    Testing ... (interrupt to exit)                                   
    Event: time 1325392910.906948, type 2 (EV_REL), code 0 (REL_X), value 2
    Event: time 1325392910.906948, -------------- SYN_REPORT ------------
    Event: time 1325392911.416973, type 2 (EV_REL), code 0 (REL_X), value 1
    Event: time 1325392911.416973, -------------- SYN_REPORT ------------
    Event: time 1325392913.456956, type 2 (EV_REL), code 0 (REL_X), value 2
    Event: time 1325392913.456956, -------------- SYN_REPORT ------------
    Event: time 1325392916.006937, type 2 (EV_REL), code 0 (REL_X), value 1
    Event: time 1325392916.006937, -------------- SYN_REPORT ------------
    Event: time 1325392919.066977, type 2 (EV_REL), code 0 (REL_X), value 1
    Event: time 1325392919.066977, -------------- SYN_REPORT ------------
    Event: time 1325392919.576988, type 2 (EV_REL), code 0 (REL_X), value 2
    Event: time 1325392919.576988, -------------- SYN_REPORT ------------      
    

    Device Tree

    From a Device Tree point of view, the representation is a bit more complicated than for many other hardware blocks, due to the multiple features offered by timer counters. First of all, in the .dtsi file describing the system-on-chip, we have a node that describes each TCB module. For example, for the Microchip SAMA5D2 system-on-chip, which has two TCB modules, we have in arch/arm/boot/dts/sama5d2.dtsi:

    tcb0: timer@f800c000 {
    	compatible = "atmel,at91sam9x5-tcb", "simple-mfd", "syscon";
    	#address-cells = <1>;
    	#size-cells = <0>;
    	reg = <0xf800c000 0x100>;
    	interrupts = <35 IRQ_TYPE_LEVEL_HIGH 0>;
    	clocks = <&pmc PMC_TYPE_PERIPHERAL 35>, <&clk32k>;
    	clock-names = "t0_clk", "slow_clk";
    };
    
    tcb1: timer@f8010000 {
    	compatible = "atmel,at91sam9x5-tcb", "simple-mfd", "syscon";
    	#address-cells = <1>;
    	#size-cells = <0>;
    	reg = <0xf8010000 0x100>;
    	interrupts = <36 IRQ_TYPE_LEVEL_HIGH 0>;
    	clocks = <&pmc PMC_TYPE_PERIPHERAL 36>, <&clk32k>;
    	clock-names = "t0_clk", "slow_clk";
    };
    

    This however does not define how each TCB module and each channel is going to be used. This happens at the board level, by adding sub-nodes to the appropriate TCB module node.

    First, each board needs to at least define which TCB module and channels should be used for the clocksource/clockevents. For example, arch/arm/boot/dts/at91-sama5d2_xplained.dts has:

    tcb0: timer@f800c000 {
    	timer0: timer@0 {
    		compatible = "atmel,tcb-timer";
    		reg = <0>;
    	};
    
    	timer1: timer@1 {
    		compatible = "atmel,tcb-timer";
    		reg = <1>;
    	};
    };
    

    As can be seen in this example, the timer@0 and timer@1 node are sub-nodes of the timer@f800c000 node. The SAMA5D2 has 32-bit timer counters, so only one channel is needed for the clocksource, and another channel is needed for clock events. Older platforms such as AT91SAM9260 would need:

    tcb0: timer@fffa0000 {
    	timer@0 {
    		compatible = "atmel,tcb-timer";
    		reg = <0>, <1>;
    	};
    
    	timer@2 {
    		compatible = "atmel,tcb-timer";
    		reg = <2>;
    	};
    };
    

    Where the first instance of atmel,tcb-timer uses two channels: on AT91SAM9260, each channel is only 16-bit, so we need two channels for clocksource. This is why we have reg = <0>, <1> in the first sub-node.

    Now, to use some TCB channels as PWMs, with the new patch series proposed by Alexandre, one would for example use:

    &tcb1 {
    	tcb1_pwm0: pwm@0 {
    		compatible = "atmel,tcb-pwm";
    		#pwm-cells = <3>;
    		reg = <0>;
    		pinctrl-names = "default";
    		pinctrl-0 = <&pinctrl_tcb1_tioa0 &pinctrl_tcb1_tiob0>;
    	};
    
    	tcb1_pwm1: pwm@1 {
    		compatible = "atmel,tcb-pwm";
    		#pwm-cells = <3>;
    		reg = <1>;
    		pinctrl-names = "default";
    		pinctrl-0 = <&pinctrl_tcb1_tioa1>;
    	};
    };
    

    To use the two first channels of TCB1 as PWMs. This would provide two separate PWM devices visible to user-space, and to other kernel drivers.

    Otherwise, to use a TCB as a quadrature decoder, one would use the following piece of Device Tree. Note that we must use the TCB0 module as it is the only one that supports quadrature decoding. This means that the atmel,tcb-timer nodes for clocksource/clockevents support have to use TCB1.

    &tcb0 {
    	qdec: counter@0 {
    		compatible = "atmel,tcb-capture";
    		reg = <0>, <1>;
    		pinctrl-names = "default";
    		pinctrl-0 = <&pinctrl_qdec_default>;
    	};
    };
    

    A quadrature decoder needs two channels, hence the reg = <0>, <1>.

    And if in addition you would like to setup an input device for the rotary encoder connected to the quadrature decoder, you can add:

    rotary@0 {
    	compatible = "rotary-encoder-counter";
    	counter = <&qdec>;
    	qdec-mode = <7>;
    	poll-interval = <50>;
    };
    

    Note that this is not a sub-node of the TCB node, the rotary encoder needs to be described at the top-level of the Device Tree, and has a reference to the TCB channels used as quadrature decoder by means of the counter = <&qdec>; phandle.

    Of course, these different capabilities can be combined. For example, you could use the first two channels of TCB0 to implement a quadrature decoder using the counter subsystem, and the third channel of the same TCB module for a PWM. TCB1 is used for clocksource/clockevents. In this case, the Device Tree would look like this:

    &tcb0 {
    	counter@0 {
    		compatible = "atmel,tcb-capture";
    		reg = <0>, <1>;
    		pinctrl-names = "default";
    		pinctrl-0 = <&pinctrl_qdec_default>;
    	};
    
    	pwm@2 {
    		compatible = "atmel,tcb-pwm";
    		#pwm-cells = <3>;
    		reg = <2>;
    		pinctrl-names = "default";
    		pinctrl-0 = <&pinctrl_tcb1_tioa1>;
    	};
    };
    
    &tcb1 {
    	timer@0 {
    		compatible = "atmel,tcb-timer";
    		reg = <0>, <1>;
    	};
    
    	timer@2 {
    		compatible = "atmel,tcb-timer";
    		reg = <2>;
    	};
    };
    

    Conclusion

    We hope that this blog post was useful to understand how Linux handles timer counters, and what are the Linux kernel subsystems that are involved. Even though we used the Microchip TCB to illustrate our discussion, the concepts all apply to the timer counters of other platforms that would offer similar features.