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

[Emil Velikov]

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.

Or perhaps you meant something entirely different ? If so can you
please elaborate.

Thanks
Emil