Synchronization between a crtc mode_set and page_flip?

[Archit Taneja]

On Friday 04 April 2014 02:54 AM, Daniel Vetter wrote:
> On Thu, Apr 03, 2014 at 02:28:32PM +0530, Archit Taneja wrote:
>> On Wednesday 02 April 2014 06:41 PM, Rob Clark wrote:
>>> On Wed, Apr 2, 2014 at 5:52 AM, Archit Taneja <archit at ti.com> wrote:
>>>> Hi,
>>>>
>>>> I was trying to figure out how we are supposed to manage synchronization
>>>> between a mode_set and a page_flip called on a crtc.
>>>>
>>>> Say, if a mode_set is immediately followed by a page_flip. The driver can't
>>>> process the page_flip straight away since the hardware is still completing
>>>> the mode_set.
>>>
>>> I guess setcrtc is expected to be synchronous(ish).. so a lot of
>>> userspace won't expect the first pageflip to fail with -EBUSY.
>>
>> Okay, thanks. I guess having setcrtc synchronous isn't that bad.
>
> Yeah, atm I think the rules are that pageflip can only return -EBUSY if
> there's still a pageflip ongoing. Everything else is presumed to be at
> least implicitly ordered, so if your hw can do async modesets then you
> need to synchronize. Also if there's still a pageflip outstanding you need
> to wait for that to complete in your set_config callback first (usually in
> the set_base or the crtc->disable/prepare hooks when using the crtc
> helpers).

Thanks for the info. I didn't realize that the prepare/commit hooks get 
executed when using drm_crtc_helper_set_mode() for the set_config helper.

Rob,

We disable the crtc in prepare, and enable it in commit.

If setcrtc changes the mode,  I don't see how apply worker can execute 
prepare() -> mode_set() -> commit() hooks in a row for the crtc 
drm_crtc_helper_set_mode(). We can't queue more than one 'apply' of the 
same entity. So the latter queues are bound to get rejected, I see 
omap_crtc_apply() bailing out for crtc's apply in this case.

I guess making prepare() and mode_set() wait for a vsync should fix 
this. Or, combining mode_set and commit as a single queue to apply 
should work too. Any suggestions?

Thanks,
Archit