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

[Ville Syrjälä]

On Wed, Aug 22, 2018 at 09:14:08PM -0700, John Stultz 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.

I recently tried to clarify that max_width/height are simply the max
framebuffer dimensions supported by the driver. So it's perfectly legal
for a driver to declare max_height as something big that can't be
scanned out in its entirety by a single plane. For i915 I'm currently
working on bumping these limits to 32k-1 regardless of the hardware
scanout limitations.

So if you're already running with a framebuffer height >2048 and it
works then it would seem to me that you could just bump this limit in
the driver.

-- 
Ville Syrjälä
Intel