[linux-sunxi] simplefb: add clock handling

[Hans de Goede]

Hi,

On 08/13/2014 10:21 AM, Koen Kooi wrote:
> 
> Op 13 aug. 2014, om 09:54 heeft David Herrmann <dh.herrmann at gmail.com> het volgende geschreven:
> 
>> Hi
>>
>> On Wed, Aug 13, 2014 at 9:17 AM, Luc Verhaegen <libv at skynet.be> wrote:
>>> This is needed for the sunxi platform, where the u-boot initialized display
>>> engine gets disabled by the clocks framework if certain clocks are not
>>> claimed. Once these clocks are disabled, register content is lost, and there
>>> is no turning back unless a full display driver is loaded, which kind of
>>> beats the purpose of having simplefb running.
>>>
>>> The lack of clock handling should plague more hardware, but so far rpi is the
>>> best known user of simplefb, and its stepmotherly handling of the arm core
>>> has kept these sort of issues from the kernel.
>>>
>>> The sunxi u-boot side code can be found here:
>>> https://groups.google.com/forum/#!topic/linux-sunxi/dPs958sIXvY
>>>
>>> Patch 3 might be controversial, as this does not achieve anything real today,
>>> since the status property in dt is only really evaluated when dealing with a
>>> nodes memory. It still seems like a good idea to at least flag this memory or
>>> node as disabled, as we really have no way back when the clocks get disabled.
>>
>> Hans de Goede shortly told me about this and, tbh, I am not very
>> pleased. Stephen tried to keep simplefd "as simple as possible", your
>> patch now adds hardware-specific features. To be fair, the patch is
>> simple and clocks are easy to handle, but I somehow fear we have to
>> add more and more hardware-support that is required to keep the
>> framebuffer active. This really defeats the purpose of simplefb.
>>
>> The biggest question I have, is why do you add the clocks to your DT
>> at all? The framebuffer is set up by your boot-loader (or maybe
>> platform code) and should prepare the clocks. I don't see why we add
>> the clocks to DT now. If they're not added, then no-one will disable
>> them and simplefb works just fine, right?
> 
> All clocks known to linux without a consumer will get disabled on most (all?) ARM systems to save power. Years ago OMAP had a Kconfig option to change that behaviour and add printk warnings for the clocks it would touch. 
> To be honest, I don't get why sunxi needs a simplefb to begin with, only a proper kms/drm driver is needed which would register the clocks it needs properly. These patches and discussion seem like a lot of effort wasted on the wrong thing. But I can't complain about that since I'm not the one doing the work. 

I believe that having some simple generic fb driver will be useful
on non x86, since we don't have vga-console there, and most distros
will build kms drivers as modules. Having the kernel / initrd code being
able to show output (like e.g. missing symbols in the kms drivers) seems
a very useful feature to me.

The way I envision this to work is:

u-boot lights up display, if it fails to load the kernel / ftd / ramdisk,
it can show this on the display

kernel takes over using something like simplefb (built into the kernel)
for its initial output / any error messages.

initrd loads kms, kms takes over.

This way we've a way to show error messages during boot at all times.

As we start supporting more ARM htpc boxes out of the box, telling the
user to hook up a serial console (which often involves soldering wires
to some test points) when things don't work really is not a viable
answer.

Regards,

Hans