[PATCH v5 4/8] drm/cma-helper: Use the generic fbdev emulation

[Daniel Vetter]

On Thu, Aug 23, 2018 at 6:14 AM, John Stultz <john.stultz at linaro.org> wrote:
> On Mon, Aug 20, 2018 at 11:44 PM, John Stultz <john.stultz at linaro.org> wrote:
>> Hey Noralf, all,
>>   I've been digging for a bit on the regression that this patch has
>> tripped on the HiKey board as reported here:
>> https://lkml.org/lkml/2018/8/16/81
>>
>> The first issue was that the kirin driver was setting
>> mode_config.max_width/height = 2048, which was causing errors as the
>> the requested resolution was 1920x2160 (due to surfaceflinger
>> requesting y*2 for page flipping).
>
> Hey Noralf,
>   Sorry, I know your probably sick of me. But I just wanted to circle
> around on this little bit. So part of the issue I found earlier, was
> that I'm running w/ CONFIG_DRM_FBDEV_OVERALLOC=200, to support
> Surfaceflinger's request for page flipping. This is what makes the Y
> resolution 2160, which runs afoul of the new max_height check of 2048
> in the generic code.
>
> I was checking with Xinliang, who know the kirin display hardware,
> about the max_height being set to 2048 to ensure bumping it up wasn't
> a problem, but he said 2048x2048  was unfortunately not arbitrary, and
> that was the hard limit of the display hardware. However, with
> overalloc, the 1920x2160 res fbdev should still be ok, as only
> 1920x1080 is actually displayed at one time.
>
> So it seems like we might need to multiply the max_height by the
> overalloc factor when we are checking it in
> drm_internal_framebuffer_create?
>
> Does that approach sound sane, or would folks prefer something different?

I guess we could simply not check against the height limit when
allocating framebuffers. But we've done that for userspace buffers
since forever (they just allocate 2 buffers for page-flipping), so I
have no idea what would all break if we'd suddenly lift this
restriction. And whether we'd lift it for fbdev only or for everyone
doesn't really make much of a difference, since either this works, or
it doesn't (across all chips).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch