Re: [CI] drm/i915/display: change pipe allocation order for discrete platforms

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Apr 02, 2026 at 12:43:57PM +0300, Jani Nikula wrote:
> On Mon, 30 Mar 2026, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> > On Mon, Mar 30, 2026 at 02:37:47PM +0300, Jani Nikula wrote:
> >> On Mon, 16 Mar 2026, Jani Nikula <jani.nikula@intel.com> wrote:
> >> > When big joiner is enabled, it reserves the adjacent pipe as the
> >> > secondary pipe. This happens without the user space knowing, and
> >> > subsequent attempts at using the CRTC with that pipe will fail. If the
> >> > user space does not have a coping mechanism, i.e. trying another CRTC,
> >> > this leads to a black screen.
> >> >
> >> > Try to reduce the impact of the problem on discrete platforms by mapping
> >> > the CRTCs to pipes in order A, C, B, and D. If the user space reserves
> >> > CRTCs in order, this should trick it to using pipes that are more likely
> >> > to be available for and after joining.
> >> >
> >> > Limit this to discrete platforms, which have four pipes, and no eDP, a
> >> > combination that should benefit the most with least drawbacks.
> >> 
> >> Ville, I think it's time to review and, pretty soon, merge this.
> >> 
> >> Our IGT changes to deconflate CRTCs and pipes have been merged, and
> >> there's the removal of invalid igt_crtc_t at [1] left. The trybot CI
> >> results on i915 for swapping pipes B and C on all platforms, not just
> >> discrete like here, didn't break anything either anymore [2].
> >> 
> >> I'm contemplating slapping Cc: stable on this too.
> >> 
> >> There's the FIXME on the CRTC index warning.
> >
> > IIRC we already concluded that the WARN is unnecessary. I'd have to
> > look through the previous mails to see what I actually said there.
> >
> >> With the A+C and B+D
> >> pairing there's no issue, the CRTC indexes remain in that order. But can
> >> we ever really end up with B+C pairing?
> >
> > It might be rare if userspace picks crtcs in order. But IIRC we had
> > bugs where it was clear userspace was just picking random crtcs willy
> > nilly. IIRC it was sway doing it, and I think I even proposed a
> > uapi documentation update to suggest using crtcs in order. Can't 
> > remember that happened to that one.
> >
> > But I think we still want the "walk the crtcs in pipe order" change,
> > mainly to keep the more optimal commit sequence. Also I'm not quite
> > 100% convinced we don't have some subtle assumption somewhere about
> > the order.
> 
> You mean convert all for_each_intel_crtc*() iterators to pipe order?

Yes.

-- 
Ville Syrjälä
Intel