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

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

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.

BTW I just realized that DG2 may also get a slight extra benefit from
the reordering because A+C has twice the dbuf space compared to A+B.

-- 
Ville Syrjälä
Intel