Super fast Linux splashscreen

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Here’s a simple trick that I recently rediscovered when I worked on a boot time reduction project for a customer. It’s not rocket science, but you may not be aware of it.

Our customer was using fbv to display its logo right after the system booted. This is a way to show that the system is available while you’re starting the system’s main application:

fbv -d 1 /root/logo.bmp > /dev/null 2>&1

With Grabserial and using simple instrumentation with messages issued on the serial console before and after running the command, we found that this command was taking 878 ms to execute. The customer’s system had an AT91SAM9263 ARM SOC from Atmel, running at 200 MHz.

Even if fbv is a simple program (22 KB on ARM, compiled with shared libraries), decoding the logo image is still expensive. Here’s a way to get this compute cost out of your boot sequence. All you have to do is display your logo on your framebuffer, and then capture the framebuffer contents in a file:

fbv -d 1 /root/logo.bmp
cp /dev/fb0 /root/logo.fb

The new file is now a little bigger, 230400 bytes instead of 76990. However, displaying your boot logo can now be done by a simple copy:

dd if=/root/logo.fb of=/dev/fb0 bs=230400 count=1 > /dev/null 2>&1

This command now runs in only 54 ms. That’s only 6% of the initial execution time! The advantage of this approach is that it works with any kind of framebuffer pixel format, as long as you have at least one program that knows how to write to your own framebuffer.

Note that the dd command was used to read and write the logo in one shot, rather than copying in multiple chunks. We found that the equivalent cp and cat commands were slightly slower. Of course, the benchmark results will vary from one system to another. Our customer had heavily optimized their NOR flash access time. If you run this on a very slow storage device, using a much faster CPU, the time to display the logo may be several impacted by the time taken to read a bigger file from slower storage.

To get even better performance, another trick is to compress the framebuffer contents with LZO (supported by BusyBox), which is very fast at decompressing, and requires very little memory to run:

lzop -9 /root/logo.fb

The new /root/logo.fb.lzop file is now only 2987 bytes big. Of course, the compression rate will depend on your logo image. In our case, the splashscreen contains mostly white space and a simple monochrome company logo. The new command to put in your startup scripts is now:

lzopcat /root/logo.fb.lzo > /dev/fb0

The execution time is now just 52.5 ms! With a faster CPU, the time reduction would have been even bigger.

The ultimate trick for having a real and possibly animated splashscreen would be to implement your own C program, directly writing to the framebuffer memory in mmap() mode. Here’s a nice tutorial showing how easy it can be.