Re: [git pull] drm for 5.8-rc1

[Daniel Stone]

Hi,

On Wed, 1 Jul 2020 at 20:45, James Jones <jajones@xxxxxxxxxx> wrote:
> OK, I think I see what's going on.  In the Xorg modesetting driver, the
> logic is basically:
>
> if (gbm_has_modifiers && DRM_CAP_ADDFB2_MODIFIERS != 0) {
>    drmModeAddFB2WithModifiers(..., gbm_bo_get_modifier(bo->gbm));
> } else {
>    drmModeAddFB(...);
> }

I read this thread expecting to explain the correct behaviour we
implement in Weston and how modesetting needs to be fixed, but ...
that seems OK to me? As long as `gbm_has_modifiers` is a proxy for 'we
used gbm_(bo|surface)_create_with_modifiers to allocate the buffer'.

> There's no attempt to verify the DRM-KMS device supports the modifier,
> but then, why would there be?  GBM presumably chose a supported modifier
> at buffer creation time, and we don't know which plane the FB is going
> to be used with yet.  GBM doesn't actually ask the kernel which
> modifiers it supports here either though.

Right, it doesn't ask, because userspace tells it which modifiers to
use. The correct behaviour is to take the list from the KMS
`IN_FORMATS` property and then pass that to
`gbm_(bo|surface)_create_with_modifiers`; GBM must then select from
that list and only that list. If that call does not succeed and Xorg
falls back to `gbm_surface_create`, then it must not call
`gbm_bo_get_modifier` - so that would be a modesetting bug. If that
call does succeed and `gbm_bo_get_modifier` subsequently reports a
modifier which was not in the list, that's a Mesa driver bug.

> It just goes into Mesa via
> DRI and reports the modifier (unpatched) Mesa chose on its own.  Mesa
> just hard-codes the modifiers in its driver backends since its thinking
> in terms of a device's 3D engine, not display.  In theory, Mesa's DRI
> drivers could query KMS for supported modifiers if allocating from GBM
> using the non-modifiers path and the SCANOUT flag is set (perhaps some
> drivers do this or its equivalent?  Haven't checked.), but that seems
> pretty gnarly and doesn't fix the modifier-based GBM allocation path
> AFAIK.  Bit of a mess.

Two options for GBM users:
* call gbm_*_create_with_modifiers, it succeeds, call
gbm_bo_get_modifier, pass modifier into AddFB
* call gbm_*_create (without modifiers), it succeeds, do not call
gbm_bo_get_modifier, do not pass a modifier into AddFB

Anything else is a bug in the user. Note that falling back from 1 to 2
is fine: if `gbm_*_create_with_modifiers()` fails, you can fall back
to the non-modifier path, provided you don't later try to get a
modifier back out.

> For a quick userspace fix that could probably be pushed out everywhere
> (Only affects Xorg server 1.20+ AFAIK), just retrying
> drmModeAddFB2WithModifiers() without the DRM_MODE_FB_MODIFIERS flag on
> failure should be sufficient.

This would break other drivers.

> Still need to verify as I'm having
> trouble wrangling my Xorg build at the moment and I'm pressed for time.
> A more complete fix would be quite involved, as modesetting isn't really
> properly plumbed to validate GBM's modifiers against KMS planes, and it
> doesn't seem like GBM/Mesa/DRI should be responsible for this as noted
> above given the general modifier workflow/design.
>
> Most importantly, options I've considered for fixing from the kernel side:
>
> -Accept "legacy" modifiers in nouveau in addition to the new modifiers,
> though avoid reporting them to userspace as supported to avoid further
> proliferation.  This is pretty straightforward.  I'll need to modify
> both the AddFB2 handler (nouveau_validate_decode_mod) and the mode set
> plane validation logic (nv50_plane_format_mod_supported), but it should
> end up just being a few lines of code.

I do think that they should also be reported to userspace if they are
accepted. Other users can and do look at the modifier list to see if
the buffer is acceptable for a given plane, so the consistency is good
here. Of course, in Mesa you would want to prioritise the new
modifiers over the legacy ones, and not allocate or return the legacy
ones unless that was all you were asked for. This would involve
tracking the used modifier explicitly through Mesa, rather than
throwing it away at alloc time and then later divining it from the
tiling mode.

Cheers,
Daniel