Re: [PATCH v2] drm/format-helper: Only advertise supported formats for conversion

[Justin Forbes]

On Mon, Oct 31, 2022 at 11:52 AM Hector Martin <marcan@xxxxxxxxx> wrote:
>
> On 01/11/2022 01.15, Justin Forbes wrote:
> > On Thu, Oct 27, 2022 at 8:57 AM Hector Martin <marcan@xxxxxxxxx> wrote:
> >>
> >> drm_fb_build_fourcc_list() currently returns all emulated formats
> >> unconditionally as long as the native format is among them, even though
> >> not all combinations have conversion helpers. Although the list is
> >> arguably provided to userspace in precedence order, userspace can pick
> >> something out-of-order (and thus break when it shouldn't), or simply
> >> only support a format that is unsupported (and thus think it can work,
> >> which results in the appearance of a hang as FB blits fail later on,
> >> instead of the initialization error you'd expect in this case).
> >>
> >> Add checks to filter the list of emulated formats to only those
> >> supported for conversion to the native format. This presumes that there
> >> is a single native format (only the first is checked, if there are
> >> multiple). Refactoring this API to drop the native list or support it
> >> properly (by returning the appropriate emulated->native mapping table)
> >> is left for a future patch.
> >>
> >> The simpledrm driver is left as-is with a full table of emulated
> >> formats. This keeps all currently working conversions available and
> >> drops all the broken ones (i.e. this a strict bugfix patch, adding no
> >> new supported formats nor removing any actually working ones). In order
> >> to avoid proliferation of emulated formats, future drivers should
> >> advertise only XRGB8888 as the sole emulated format (since some
> >> userspace assumes its presence).
> >>
> >> This fixes a real user regression where the ?RGB2101010 support commit
> >> started advertising it unconditionally where not supported, and KWin
> >> decided to start to use it over the native format and broke, but also
> >> the fixes the spurious RGB565/RGB888 formats which have been wrongly
> >> unconditionally advertised since the dawn of simpledrm.
> >>
> >> Fixes: 6ea966fca084 ("drm/simpledrm: Add [AX]RGB210101
> >
> >
> >> Cc: stable@xxxxxxxxxxxxxxx
> >> Signed-off-by: Hector Martin <marcan@xxxxxxxxx>
> >
> > There is a CC for stable on here, but this patch does not apply in any
> > way on 6.0 or older kernels as the fourcc bits and considerable churn
> > came in with the 6.1 merge window.  You don't happen to have a
> > backport of this to 6.0 do you?
>
> v1 is probably closer to such a backport, and I offered to figure it out
> on Matrix but I heard you're already working on it ;)

i am, but I didn't want to be, so I thought I would ask.

Justin