Re: [git pull] drm for 5.8-rc1

[Daniel Stone <daniel@fooishbar.org>]

Hi,

On Fri, 14 Aug 2020 at 17:22, Thierry Reding <thierry.reding@gmail.com> wrote:
> I suspect that the reason why this works in X but not in Wayland is
> because X passes the right usage flags, whereas Weston may not. But I'll
> have to investigate more in order to be sure.

Weston allocates its own buffers for displaying the result of
composition through GBM with USE_SCANOUT, which is definitely correct.

Wayland clients (common to all compositors, in Mesa's
src/egl/drivers/dri2/platform_wayland.c) allocate with USE_SHARED but
_not_ USE_SCANOUT, which is correct in that they are guaranteed to be
shared, but not guaranteed to be scanned out. The expectation is that
non-scanout-compatible buffers would be rejected by gbm_bo_import if
not drmModeAddFB2.

One difference between Weston and all other compositors (GNOME Shell,
KWin, Sway, etc) is that Weston uses KMS planes for composition when
it can (i.e. when gbm_bo_import from dmabuf + drmModeAddFB2 from
gbm_bo handle + atomic check succeed), but the other compositors only
use the GPU. So if you have different assumptions about the layout of
imported buffers between the GPU and KMS, that would explain a fair
bit.

> Perhaps we can go and release X 1.21.0 with that modifier enablement
> patch and that'll motivate desktops to adopt it as well as the default?

Unfortunately we don't really have a good way out of this one. They
were disabled because the non-modifier path on Intel can be linear or
X-tiled (row-major), whereas the modifier path enables Y-tiled
(column-major) and compressed layouts. Y-tiled is the most efficient,
but Intel could only spare about six transistors for the global FIFO
shared between all their plane fetch engines, and Y-tiled blows
straight through it. Both X and Shell would thus fail to enable high
resolutions or many heads (2x 4K is enough even on modern platforms
IIRC), so they just turned modifiers off.

The best solution would be to do a global atomic_check across all
outputs and just blacklist modifiers until you find one which works,
but Shell doesn't yet have that code, and -modesetting ... well,
no-one's volunteered to do that yet, or probably ever.

Cheers,
Daniel
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