[U-Boot] [linux-sunxi] Re: [PATCH] video: add cfb console driver for sunxi

[Michal Suchanek]

On 5 August 2014 23:03, Maxime Ripard <maxime.ripard at free-electrons.com> wrote:
> On Mon, Aug 04, 2014 at 05:05:00PM +0200, Luc Verhaegen wrote:
>> On Mon, Aug 04, 2014 at 10:39:13AM +0200, Hans de Goede wrote:
>> > Hi Luc,
>> >
>> > First of all many thanks for your work on this.
>> >
>> > ATM I don't have time to do a full review, but I don't expect there
>> > to be too many suprises when I do find the time.
>> >
>> > Really my only concern is the handover of the reserved memory, etc. to
>> > the kernel. We need to get a plan in place for that *before* this can
>> > be merged. Note I don't want to raise any artificial barriers here,
>> > I would love to see this merged ASAP. But I don't want to paint us
>> > in a corner where u-boot having hdmi console support makes it harder
>> > to get kms support in the kernel going. I think we can both agree on that.
>> >
>> > So I really want to see some plan how this will work in place before merging.
>> > Note just a plan, I don't expect kernel patches ready to be merged for this,
>> > just a good idea / sketch of how all the bits will fit together.
>>
>> Memory is not the biggest worry.
>>
>> Some kernel code needs to claim clocks if the mode is to cleanly survive
>> the start of the kernel. If not, there is no coming back until a proper
>> display driver runs. If the simplefb driver claims these clocks, then
>> the simplefb driver also must release these clocks, and this driver
>> should then never be started again, so it should set the simplefb dt
>> node status to "disabled".
>
> I'm probably missing a bit of context, but one thing I still don't get
> is why you're taking into account the simplefb <-> KMS handover. It's
> a case that shouldn't exist.
>
> By essence, simplefb has never been meant for that. It's been meant to
> have a temporary solution until a full-fledged driver is merged in the
> kernel. Which is exactly the case we're into.

It's a permanent temporary solution. Same as offb/vesafb/uefi and
other unaccelerated drivers. It will be needed for platforms on which
KMS is not (yet) fully supported or happens to break.

Also how else do you express the fact that u-boot has left the display
enabled other than generating the simplefb DT node?

Note that the KMS driver will be probably unsuitable for early console
while the simplefb driver can just write into the framebuffer set up
by u-boot.

Thanks

Michal