Re: [PATCH] drm/udl: Make page_flip asynchronous

Stéphane Marchesin
2017-07-13T16:26:32Z

On Mon, Jul 10, 2017 at 11:58 PM, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Fri, Jul 7, 2017 at 7:48 AM, Dawid Kurek <dawid.kurek@displaylink.com> wrote:
>> In page_flip vblank is sent with no delay. Driver does not know when the
>> actual update is present on the display and has no means for getting
>> this information from a device. It is practically impossible to say
>> exactly *when* as there is also i.e. a usb delay.
>>
>> When we are unable to determine when the vblank actually happens we may
>> assume it will behave accordingly, i.e. it will present frames with
>> proper timing. In the worst case scenario it should take up to duration
>> of one frame (we may get new frame in the device just after presenting
>> current one so we would need to wait for the whole frame).
>>
>> Because of the asynchronous nature of the delay we need to synchronize:
>> * read/write vrefresh/page_flip data when changing mode and
>> preparing/executing vblank
>> * USB requests to prevent interleaved access to URBs for two different
>> frame buffers
>>
>> All those changes are backports from ChromeOS:
>> 1. https://chromium-review.googlesource.com/250622
>> 2. https://chromium-review.googlesource.com/249450
>> partially, only change in udl_modeset.c for 'udl_flip_queue'
>> 3. https://chromium-review.googlesource.com/321378
>> 4. https://chromium-review.googlesource.com/324119
>> + fixes for checkpatch and latest drm changes
>>
>> Cc: hshi@chromium.org
>> Cc: marcheu@chromium.org
>> Cc: zachr@chromium.org
>> Cc: dbehr@google.com
>> Signed-off-by: Dawid Kurek <dawid.kurek@displaylink.com>
>
> Can't we roll this driver over to the atomic helpers instead? There
> you get nonblocking pretty much for free ... I'm not sure extending
> the old modeset code has all that much benefit really.

This code certainly has value by itself; it makes the driver more
efficient. I think the best can sometimes be the enemy of the good --
this code is here and written, but I don't think any of us is going to
tackle atomic for udl.

Stéphane

> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
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