On Tue, 13 Apr 2021 16:11:29 +0200 Daniel Vetter daniel@ffwll.ch wrote:
On Tue, Apr 13, 2021 at 02:56:02PM +0300, Pekka Paalanen wrote:
On Tue, 13 Apr 2021 11:49:03 +0200 Daniel Vetter daniel.vetter@ffwll.ch wrote:
It's very confusing for userspace to have to deal with inconsistencies here, and some drivers screwed this up a bit. Most just ommitted the format list when they meant to say that only linear modifier is allowed, but some also meant that only implied modifiers are acceptable (because actually none of the planes registered supported modifiers).
Now that this is all done consistently across all drivers, document the rules and enforce it in the drm core.
Cc: Pekka Paalanen pekka.paalanen@collabora.com Signed-off-by: Daniel Vetter daniel.vetter@intel.com Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com Cc: Maxime Ripard mripard@kernel.org Cc: Thomas Zimmermann tzimmermann@suse.de Cc: David Airlie airlied@linux.ie Cc: Daniel Vetter daniel@ffwll.ch
drivers/gpu/drm/drm_plane.c | 16 +++++++++++++++- include/drm/drm_mode_config.h | 2 ++ 2 files changed, 17 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c index 0dd43882fe7c..16a7e3e57f7f 100644 --- a/drivers/gpu/drm/drm_plane.c +++ b/drivers/gpu/drm/drm_plane.c @@ -128,6 +128,11 @@
pairs supported by this plane. The blob is a structdrm_format_modifier_blob. Without this property the plane doesn'tsupport buffers with modifiers. Userspace cannot change this property.
Note that userspace can check the DRM_CAP_ADDFB2_MODIFIERS driver
capability for general modifier support. If this flag is set then every
plane will have the IN_FORMATS property, even when it only supports
DRM_FORMAT_MOD_LINEAR.Ooh, that's even better. But isn't that changing the meaning of the cap? Isn't the cap older than IN_FORMATS?
Hm indeed. But also how exactly are you going to user modifiers without IN_FORMATS ... it's a bit hard.
Easy for at least one specific case, as Daniel Stone said in IRC. Use GBM to allocate using the no-modifiers API but specify USE_LINEAR. That basically gives you MOD_LINEAR buffer. Then you can try to make a DRM FB for it using AddFB2-with-modifiers.
Does anyone do this, I have no idea.
Actually, I think this semantic change is fine. Old userspace did not know that the cap means all planes have IN_FORMATS, so they can deal with IN_FORMATS missing, but if it is never missing, no problem.
It could be a problem with new userspace and old kernel, but that's by definition not a kernel bug, right? Just... inconvenient for userspace as they can't make full use of the flag and need to keep the fallback path for missing IN_FORMATS.
As long as there are KMS drivers that don't support modifiers, generic userspace probably needs the fallback path anyway.
I think this is all because we've enabled modifiers piece-by-piece and never across the entire thing (e.g. with compositor and protocols), so the missing pieces only became apparent later on.
I'm not sure whether compositors really want to support this, I guess worst case we could disable the cap on these old kernels.
What about the opposite? Is it allowed to have even a single IN_FORMATS if you don't have the cap?
That direction is enforced since 5.1, because some drivers screwed it up and confusion in userspace ensued.
Should I add a bug that on kernels older than 5.1 the situation is more murky and there's lots of bugs?
Yes, that would help to set expectations.
I'm currently on Debian stable FWIW, so 4.19 based kernel with I don't know what patches.
On Tue, 13 Apr 2021 16:19:10 +0200 Daniel Vetter daniel@ffwll.ch wrote:
On Tue, Apr 13, 2021 at 04:11:29PM +0200, Daniel Vetter wrote:
Should I add a bug that on kernels older than 5.1 the situation is more murky and there's lots of bugs?
I guess we should recommend to userspace that if they spot an inconsistency between IN_FORMATS across planes and the cap then maybe they want to disable modifier support because it might be all kinds of broken?
Yes please!
------
*/
static unsigned int drm_num_planes(struct drm_device *dev) @@ -277,8 +282,14 @@ static int __drm_universal_plane_init(struct drm_device *dev, format_modifier_count++; }
- if (format_modifier_count)
- /* autoset the cap and check for consistency across all planes */
- if (format_modifier_count) {
WARN_ON(!config->allow_fb_modifiers &&!list_empty(&config->plane_list));What does this mean?
If allow_fb_modifiers isn't set yet (we do that in the line below) and we are _not_ the first plane that gets added to the driver (that's done towards the end of the function) then that means there's already a plane registered without modifiers and hence IN_FORMAT. Which we then warn about.
Ah, ok! Would have taken a while for me to decipher that, and impossible with just this patch context.
config->allow_fb_modifiers = true;
- } else {
WARN_ON(config->allow_fb_modifiers);This warning here checks the other case of an earlier plane with modifiers, but the one we're adding now doesn't have them.
Cool.
Thanks, pq