[PATCH v6 10/11] modesetting: Disable Reverse PRIME for i915

[Chris Wilson]

On Sun, Jun 12, 2016 at 06:23:25PM +0100, Emil Velikov wrote:
> On 12 June 2016 at 17:09, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > On Sun, Jun 12, 2016 at 02:35:43PM +0100, Emil Velikov wrote:
> >> Hi all,
> >>
> >> On 11 June 2016 at 01:21, Alex Goins <agoins at nvidia.com> wrote:
> >> > Reverse PRIME seems to be designed with discrete graphics as a sink in
> >> > mind, designed to do an extra copy from sysmem to vidmem to prevent a
> >> > discrete chip from needing to scan out from sysmem.
> >> >
> >> > The criteria it used to detect this case is if we are a GPU screen and
> >> > Glamor accelerated. It's possible for i915 to fulfill these conditions,
> >> > despite the fact that the additional copy doesn't make sense for i915.
> >> >
> >> > Normally, you could just set AccelMethod = none as an option for the device
> >> > and call it a day. However, when running with modesetting as both the sink
> >> > and the source, Glamor must be enabled.
> >> >
> >> > Ideally, you would be able to set AccelMethod individually for devices
> >> > using the same driver, but there seems to be a bug in X option parsing that
> >> > makes all devices on a driver inherit the options from the first detected
> >> > device. Thus, glamor needs to be enabled for all or for none until that bug
> >> > (if it's even a bug) is fixed.
> >> >
> >> > Nonetheless, it probably doesn't make sense to do the extra copy on i915
> >> > even if Glamor is enabled for the device, so this is more user friendly by
> >> > not requiring users to disable acceleration for i915.
> >> >
> >> IMHO the proposed patch does not sound like a reasonable long-term
> >> solution. Ideally the driver will expose a flag, based on which Xorg
> >> will enable/disable reverse prime. That said, as a short-term fix this
> >> is fine, barring the issues mentioned below.
> >
> > The decision as to whether or not the slave can use the passed pixmap as
> > its own scanout (or whether it requires a copy) is not part of the
> > master's policy.
> Hi Chris, it's this precisely what I've said/meant :-)
> 
> To put in other words:
> IMHO the master/slave device should expose all their capabilities.
> Based on these, Xorg should {en,dis}able reverse prime/etc.

Yes. But I would prefer it if the user made the choice, it may require
to jump through 20 different hoops, but if it is the only way to achieve
the user's configuration, that is what is need to be done.

This patch seems to be second guessing what the slave might be doing
instead, as you said, exposing a method by which the master/slave can
negotiate on what is being passed between them.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre