linux-kernel - Re: [PATCH] drm/mediatek: Set sensible cursor width/height values to fix crash

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Message-ID: <CAPj87rPZRjmMPjaOY-UH4auTuMS6mh9N7=maRBzxut2OgtALbw@mail.gmail.com>
Date: Thu, 18 Jul 2024 12:10:32 +0100
From: Daniel Stone <daniel@...ishbar.org>
To: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
Cc: Fei Shao <fshao@...omium.org>, Chen-Yu Tsai <wenst@...omium.org>, chunkuang.hu@...nel.org, 
	p.zabel@...gutronix.de, airlied@...il.com, daniel@...ll.ch, 
	matthias.bgg@...il.com, shawn.sung@...iatek.com, ck.hu@...iatek.com, 
	dri-devel@...ts.freedesktop.org, linux-mediatek@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	kernel@...labora.com
Subject: Re: [PATCH] drm/mediatek: Set sensible cursor width/height values to
 fix crash

Hi all,

On Thu, 18 Jul 2024 at 11:24, AngeloGioacchino Del Regno
<angelogioacchino.delregno@...labora.com> wrote:
> Il 18/07/24 11:27, Fei Shao ha scritto:
> > This matches my preference in [1], so of course I'd like to see it
> > merged... if maintainers are okay with it.
> > Given I've tested the exact same change before:
> > Reviewed-by: Fei Shao <fshao@...omium.org>
> > Tested-by: Fei Shao <fshao@...omium.org>
>
> Thanks!

And:
Reviewed-by: Daniel Stone <daniels@...labora.com>

> >> OOTH, Intel recently added a feature for enumerating "suggested"
> >> cursor sizes. See https://patchwork.freedesktop.org/patch/583299/
> >>
> >> Not sure if other compositors will end up using it or not.
>
> Yeah, that's good, and we might do that as well in MediaTek DRM... in a slightly
> different way, as it looks like they are simply hinting the same values as the
> mode_config is declaring... while we'd be adding a hint with a sensible size that
> is less than the maximum supported one from the overlay.
>
> In reality, here, the issue is that the most popular compositors do not support
> overlay planes (as in, they don't use them at all)... my first idea was to remove
> the CURSOR plane entirely and declare it as per what it is for real (an OVERLAY),
> but that would only give a performance penalty as that'd become yet another unused
> plane and nothing else.
>
> If at least the most popular compositors did support overlay planes, I'd have done
> that instead... but oh, well!
>
> And anyway I hope that the maintainers are okay with this because, well, otherwise
> MediaTek SoCs won't be usable with any popular WM.

Every compositor is going to use it, yeah. But until it does, people
are just going to use cursor_width and cursor_size. A lot of older
desktop hardware supports only a single fixed dimension for the cursor
plane (hence the single values), so rather than guess if it needs to
be 32x32 or 64x64 or whatever, people just allocate to the size. Not
to mention that the old pre-atomic cursor ioctls actually require that
you allocate for cursor_width x cursor_height.

So yeah, this is the right fix - though you could even be more
aggressive and reduce it to 256x256 - and supporting the CURSOR_SIZE
property would be even more useful again.

Cheers,
Daniel