[linux-sunxi] simplefb: add clock handling

Koen Kooi koen at dominion.thruhere.net
Wed Aug 13 01:21:19 PDT 2014

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. 



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