Re: [linux-sunxi] Re: Fixing boot-time hiccups in your display

Hi Jon,

On Mon, Oct 6, 2014 at 7:34 AM, jonsmirl@gmail.com [off-list ref] wrote:

On Sun, Oct 5, 2014 at 4:01 PM, Mike Turquette [off-list ref] wrote:

quoted

Quoting jonsmirl@gmail.com (2014-10-05 10:09:52)

quoted

I edited the subject line to something more appropriate. This impacts
a lot of platforms and we should be getting more replies from people
on the ARM kernel list. This is likely something that deserves a
Kernel Summit discussion.

ELC-E and LPC are just around the corner as well. I am attending both. I
suppose some of the others interested in this topic will be present?

quoted

To summarize the problem....

The BIOS (uboot, etc) may have set various devices up into a working
state before handing off to the kernel.  The most obvious example of
this is the boot display.

So how do we transition onto the kernel provided device specific
drivers without interrupting these functioning devices?

This used to be simple, just build everything into the kernel. But
then along came multi-architecture kernels where most drivers are not
built in. Those kernels clean up everything (ie turn off unused
clocks, regulators, etc) right before user space starts. That's done
as a power saving measure.

Unfortunately that code turns off the clocks and regulators providing
the display on your screen. Which then promptly gets turned back on a
half second later when the boot scripts load the display driver. Let's
all hope the boot doesn't fail while the display is turned off.

I would say this is one half of the discussion. How do you ever really
know when it is safe to disable these things? In a world with loadable
modules the kernel cannot know that everything that is going to be
loaded has been loaded. There really is no boundary that makes it easy
to say, "OK, now it is truly safe for me to disable these things because
I know every possible piece of code that might claim these resources has
probed".

Humans know where this boundary is and can insert the clean up command
at the right point in the bootscript. It is also not fatal if the
command is inserted at the wrong point, things will just needlessly
flicker. It not even fatal if you never run this command, you'll just
leave clocks/regulators turned on that could be turned off.

What about distros? Would this "all clear" point be at the same point
in the boot process for every sub-architecture? Would it ever change?

Thanks,

-- 
Julian Calaby

Email: julian.calaby@gmail.com
Profile: http://www.google.com/profiles/julian.calaby/