Re: [PATCH 0/5] drm/i915/skl: Backport watermark fixes for 4.8.y

[Greg KH]

On Wed, Oct 26, 2016 at 03:36:32PM -0400, Lyude wrote:
> Now that these have finally made it into 4.9, it's time to finally backport
> these fixes. Skylake has been a mess in multi-monitor setups for a while now
> because up until recently we've been updating the watermarks on Skylake just
> like we would for previous generations of Intel GPUs. This means updating
> attributes for each plane, and then only after they've been updated writing
> their new watermark values.
> 
> The problem with this approach is Skylake has double buffered watermark
> registers that are flipped at the same time as the rest of the plane registers.
> This means that the original approach will leave planes active with new
> attributes but without the required watermark programming that would ensure the
> display pipe reads enough data from each plane. As a result, pipes start to
> underrun and the user's displays starts to flicker heavily. Usually in response
> to plugging in new monitors, or moving cursors from one screen to another
> (which triggers a plane and watermark update).
> 
> Additionally, issues were found with the original code for configuring ddb,
> display data buffer, allocations between display planes on Skylake. On Skylake
> all planes have space allocated to them in the ddb, and the hardware requires
> that these allocations never overlap at any point in time. Because ddb
> allocations were not updated alongside plane attributes despite also being
> double buffered registers armed by plane updates, planes were likely to get
> stuck momentarily with ddb allocations that overlapped one another. This would
> also lead to pipe underruns and display flickering.
> 
> The new approach fixes this problem by making sure that on Skylake, attributes
> for display planes are always updated at the same time as the watermarks, and
> pipes are updated in an order that ensures their ddb allocations don't
> overlap at any point between plane updates. This ensures the display pipes are
> always programmed correctly, and dramatically reduces the chance of display
> flickering.
> 
> (note: my e-mail has changed since these patches were upstreamed, and I updated
> the e-mails in these patches to reflect this. if this is wrong I will be happy
> to update and resend the patches).

Thanks for these, all now queued up.

greg k-h