[RFC] drm/atomic+msm: add helper to implement legacy dirtyfb

Wed Apr 4 11:46:37 UTC 2018

On 04/04/2018 12:28 PM, Thomas Hellstrom wrote:
> On 04/04/2018 11:56 AM, Daniel Vetter wrote:
>> On Wed, Apr 04, 2018 at 11:10:08AM +0200, Thomas Hellstrom wrote:
>>> On 04/04/2018 10:43 AM, Daniel Vetter wrote:
>>>> On Wed, Apr 04, 2018 at 10:22:21AM +0200, Thomas Hellstrom wrote:
>>>>> Hi,
>>>>>
>>>>> On 04/04/2018 08:58 AM, Daniel Vetter wrote:
>>>>>> On Wed, Apr 4, 2018 at 12:42 AM, Rob Clark <robdclark at gmail.com> 
>>>>>> wrote:
>>>>>>> Add an atomic helper to implement dirtyfb support.  This is 
>>>>>>> needed to
>>>>>>> support DSI command-mode panels with x11 userspace (ie. when we 
>>>>>>> can't
>>>>>>> rely on pageflips to trigger a flush to the panel).
>>>>>>>
>>>>>>> To signal to the driver that the async atomic update needs to
>>>>>>> synchronize with fences, even though the fb didn't change, the
>>>>>>> drm_atomic_state::dirty flag is added.
>>>>>>>
>>>>>>> Signed-off-by: Rob Clark <robdclark at gmail.com>
>>>>>>> ---
>>>>>>> Background: there are a number of different folks working on 
>>>>>>> getting
>>>>>>> upstream kernel working on various different phones/tablets with 
>>>>>>> qcom
>>>>>>> SoC's.. many of them have command mode panels, so we kind of need a
>>>>>>> way to support the legacy dirtyfb ioctl for x11 support.
>>>>>>>
>>>>>>> I know there is work on a proprer non-legacy atomic property for
>>>>>>> userspace to communicate dirty-rect(s) to the kernel, so this can
>>>>>>> be improved from triggering a full-frame flush once that is in
>>>>>>> place.  But we kinda needa a stop-gap solution.
>>>>>>>
>>>>>>> I had considered an in-driver solution for this, but things get a
>>>>>>> bit tricky if userspace ands up combining dirtyfb ioctls with page-
>>>>>>> flips, because we need to synchronize setting various CTL.FLUSH 
>>>>>>> bits
>>>>>>> with setting the CTL.START bit.  (ie. really all we need to do for
>>>>>>> cmd mode panels is bang CTL.START, but is this ends up racing with
>>>>>>> pageflips setting FLUSH bits, then bad things.)  The easiest soln
>>>>>>> is to wrap this up as an atomic commit and rely on the worker to
>>>>>>> serialize things.  Hence adding an atomic dirtyfb helper.
>>>>>>>
>>>>>>> I guess at least the helper, with some small addition to translate
>>>>>>> and pass-thru the dirty rect(s) is useful to the final atomic 
>>>>>>> dirty-
>>>>>>> rect property solution.  Depending on how far off that is, a stop-
>>>>>>> gap solution could be useful.
>>>>>> Adding Noralf, who iirc already posted the full dirty helpers 
>>>>>> already somewhere.
>>>>>> -Daniel
>>>>> I've asked Deepak to RFC the core changes suggested for the full 
>>>>> dirty blob
>>>>> on dri-devel. It builds on DisplayLink's suggestion, with a simple 
>>>>> helper to
>>>>> get to the desired coordinates.
>>>>>
>>>>> One thing to perhaps discuss is how we would like to fit this with
>>>>> front-buffer rendering and the dirty ioctl. In the page-flip 
>>>>> context, the
>>>>> dirty rects, like egl's swapbuffer_with_damage is a hint to 
>>>>> restrict the
>>>>> damage region that can be fully ignored by the driver, new content is
>>>>> indicated by a new framebuffer.
>>>>>
>>>>> We could do the same for frontbuffer rendering: Either set a dirty 
>>>>> flag like
>>>>> you do here, or provide a content_age state member. Since we clear 
>>>>> the dirty
>>>>> flag on state copies, I guess that would be sufficient. The blob 
>>>>> rectangles
>>>>> would then become a hint to restrict the damage region.
>>>> I'm not entirely following here - I thought for frontbuffer 
>>>> rendering the
>>>> dirty rects have always just been a hint, and that the driver was 
>>>> always
>>>> free to re-upload the entire buffer to the screen.
>>>>
>>>> And through a helper like Rob's proposing here (and have floated 
>>>> around in
>>>> different versions already) we'd essentially map a frontbuffer dirtyfb
>>>> call to a fake flip with dirty rect. Manual upload drivers already 
>>>> need to
>>>> upload the entire screen if they get a flip, since some userspace uses
>>>> that to flush out frontbuffer rendering (instead of calling dirtyfb).
>>>>
>>>> So from that pov the new dirty flag is kinda not necessary imo.
>>>>
>>>>> Another approach would be to have the presence of dirty rects without
>>>>> framebuffer change to indicate frontbuffer rendering.
>>>>>
>>>>> I think I like the first approach best, although it may be 
>>>>> tempting for
>>>>> user-space apps to just set the dirty bit instead of providing the 
>>>>> full
>>>>> damage region.
>>>> Or I'm not following you here, because I don't quite see the 
>>>> difference
>>>> between dirtyfb and a flip.
>>>> -Daniel
>>> OK, let me rephrase:
>>>
>>>  From the driver's point-of-view, in the atomic world, new content 
>>> and the
>>> need for manual upload is indicated by a change in fb attached to 
>>> the plane.
>>>
>>> With Rob's patch here, (correct me if I'm wrong) in addition new 
>>> content and
>>> the need for manual upload is identified by the dirty flag, (since 
>>> the fb
>>> stays the same and the driver thus never identifies a page-flip).
>> Hm, I'm not entirely sure Rob's approach is correct. Imo a pageflip
>> (atomic or not) should result in the entire buffer getting uploaded. The
>> dirty flag is kinda redundant, a flip with the same buffer works the 
>> same
>> way as a dirtyfb with the entire buffer as the dirty rectangle.
>>
>>> In both these situations, clip rects can provide a hint to restrict the
>>> dirty region.
>>>
>>> Now I was thinking about the preferred way for user-space to 
>>> communicate
>>> front buffer rendering through the atomic ioctl:
>>>
>>> 1) Expose a dirty (or content_age property)
>>> 2) Attach a clip-rect blob property.
>>> 3) Fake a page-flip by ping-ponging two frame-buffers pointing to 
>>> the same
>>> underlying buffer object.
>>>
>>> Are you saying that people are already using 3) and we should keep 
>>> using
>>> that?
>> I'm saying they're using 3b), flip the same buffer wrapped in the same
>> drm_framebuffer, and expect it to work.
>>
>> The only advantage dirtyfb has is that it allows you to supply the
>> optimized upload rectangles, but at the cost of a funny api (it's 
>> working
>> on the fb, not the plane/crtc you want to upload) and lack of 
>> drm_event to
>> confirm when exactly you uploaded your stuff. But imo they should be the
>> same underlying operation.
>>
>> Also note that atomic helpers don't optimize out plane flips for same
>> buffers. We only optimize out some of the waiting, in a failed 
>> attempt at
>> making cursors stall less, but that's not fixed with the async plane
>> update stuff. And we can obviously optimize out the prepare/cleanup 
>> hooks,
>> because the buffer should be pinned already.
>>
>
> I'm a bit confused.
>
> Apparently we have different opinions in when an uploading driver 
> should assume altered plane content and the need for re-upload.
> vmwgfx is from what I know currently assuming that this happens only 
> on changed fb attachment (what we call a page-flip) whereas if I 
> understand you correctly it should happen on each atomic state commit?
>
> If we should assume the latter, then it has odd implications, let's 
> say you have 8 screens up, and you pan one of them on the large fb, 
> why would you upload the contents of the other 7?
>
> Likewise for cursors,  why would you want to upload the cursor image 
> on each cursor move?
>
> So in my POW, option 1) is the option that aligns with the current 
> vmwgfx implementation and from what I can tell, what Rob has 
> implemented in his patch.
>

Hmm.

After doing some apparently well needed reading up on the code it looks 
like vmwgfx is actually doing a full upload on each plane state change,
only those planes that actually got changed are referenced in the 
update. So that takes care of the panning example, assuming user-space 
is smart enough to leave the unchanged planes / crtcs out of the update.

However, the cursor example still holds, and IMHO we should have a 
better way to define content change than plane state update...

/Thomas

> /Thomas
>
>