[Spice-devel] [drm/qxl v2 7/7] qxl: Allow resolution which are not multiple of 8

[Christophe Fergeau]

On Thu, Nov 03, 2016 at 06:08:39PM +0100, Gerd Hoffmann wrote:
> > Or maybe other parts of the
> > kernel/userspace rely on this rounding down.
> 
> This is where I suspect we could run in trouble.  Odd resolutions simply
> don't happen on physical hardware, all usual resolutions are a multiple
> of 8, most of them are even a multiple of 16.
> 
> Various image/video formats use 16x16 blocks.
> The qemu vnc server operates on 16x16 blocks too (and we had bugs in the
> past with odd resolutions).
> 
> Also scanlines and cachelines align nicely if you don't use odd
> resolutions.
> 
> > I unfortunately don't know
> > :(
> 
> I don't have definitive answers too, just a gut feeling that this might
> cause trouble.

I think this might be fine actually, there is already one such
resolution in the kernel, which is 1366x768 (1366 is only a multiple of
2). There is already a bit of a hack to handle it anyway, see
fixup_mode_1366x768() in drm_edid.c.

> 
> Maybe we should add a module option for this?  So there is an easy
> (as-in: doesn't require a kernel rebuild) way out in case it causes
> trouble in certain setups?

This seems a bit overkill to me, but I can look into it if needed.

Christophe
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