Re: [RFC] drm/i915/skl: Use plane update function from mmio flips

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 04/21/2015 11:07 AM, Chris Wilson wrote:
> On Tue, Apr 21, 2015 at 11:01:03AM +0100, Tvrtko Ursulin wrote:
>>
>> Hi,
>>
>> On 04/21/2015 10:51 AM, Chris Wilson wrote:
>>> On Tue, Apr 21, 2015 at 10:29:52AM +0100, Tvrtko Ursulin wrote:
>>>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>
>>>> Avoids duplicating the code.
>>>>
>>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
>>>> Cc: Sonika Jindal <sonika.jindal@intel.com>
>>>> Cc: Damien Lespiau <damien.lespiau@intel.com>
>>>> ---
>>>> Can we do this?
>>>
>>> Sure, but I'd like to see update_primary_plane split into two in that
>>> case. One to precalcuate the parameters, then the second to apply them
>>> as we skip the first here (due to doing the setup in process context)
>>> and want the second to run inside the vblank evasion logic (and the
>>> unbounded nature of the current update_primary_plane logic scares me).
>>
>> What part is unbounded? I don't see anything blocking?
>
> The GTT view lookup may have to search through an arbitrary list, it's
> even controllable by the user. Expect synmark nastiness. This is
> "trivially" fixable, but this is only the current issue. The bigger issue

That would have to be a framebuffer object operated on from multiple 
contexts so that there are multiple vms? And a lot of them.

> is simply that we have not said that this is a timing critical function
> and now we are intending to use it from such a context.

It feels like this area is slowly going towards the "too many cooks" 
state, if not already there.

>> As a side note, watermarks seem to be not handled at all in the flip
>> path as well...
>
> The flip path should reject anything that requires a change in line size
> i.e. a change in WM.

Interesting, well, I had a look around and this means all sorts of 
"trouble" (refactoring) if proper wm param comparison is to be done.

Alternatively, in (more) cheating via embedding knowledge approach, then 
rejecing a change in tiling is simple, but rotation is only known in 
plane state and page flip is not able to compare old vs. new.

In fact, I don't even know if possible since plane properties and page 
flips look disjoint, each living in it's own timeline. If sampled when 
flip is queued it will be bad, if sampled with the flip then it is too 
late and/or properly slow.

Regards,

Tvrtko
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