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(...);
}
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. 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.
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. 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.
-Don't validate modifiers in AddFB. This doesn't really gain anything
because it just pushes the failure down to mode set time, so it's not
that useful, so I don't plan on pursuing this.
As noted, need to run just now, but I should have a kernel patch to test
out either tonight or tomorrow.
If anyone's curious, the reason my testing missed this was I did most of
my verification of "old" code against the Xorg 1.19 build included with
my distro. I did hack up a Xorg 1.20-ish build to test as well that
would have included this path, but I must not have properly configured
it with GBM modifier support somehow. I was pretty focused on just
testing the forcibly-disabled atomic path in the modesetting driver in
this build, so I didn't look too closely at things beyond that.
Thanks,
-James
On 7/1/20 12:59 AM, Kirill A. Shutemov wrote:
On Wed, Jul 01, 2020 at 10:57:19AM +0300, Kirill A. Shutemov wrote:
On Tue, Jun 30, 2020 at 09:40:19PM -0700, James Jones wrote:
This implies something is trying to use one of the old
DRM_FORMAT_MOD_NVIDIA_16BX2_BLOCK format modifiers with DRM-KMS without
first checking whether it is supported by the kernel. I had tried to force
an Xorg+Mesa stack without my userspace patches to hit this error when
testing, but must have missed some permutation. If the stalled Mesa patches
go in, this would stop happening of course, but those were held up for a
long time in review, and are now waiting on me to make some modifications.
Are you using the modesetting driver in X? If so, with glamor I presume?
Yes and yes. I attached Xorg.log.
Attached now.
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