Hi
On 6/17/2024 9:54 AM, Brian Starkey wrote:
Hi,
On Mon, Jun 17, 2024 at 05:16:36PM +0200, Daniel Vetter wrote:
On Mon, Jun 17, 2024 at 01:41:59PM +0000, Hoosier, Matt wrote:
Hi,
There is a discussion ongoing over in the compositor world about the implication of this cautionary wording found in the documentation for the DRM_MODE_CONNECTOR_WRITEBACK connectors:
* "WRITEBACK_OUT_FENCE_PTR":
* Userspace can use this property to provide a pointer for the kernel to
* fill with a sync_file file descriptor, which will signal once the
* writeback is finished. The value should be the address of a 32-bit
* signed integer, cast to a u64.
* Userspace should wait for this fence to signal before making another
* commit affecting any of the same CRTCs, Planes or Connectors.
* **Failure to do so will result in undefined behaviour.**
* For this reason it is strongly recommended that all userspace
* applications making use of writeback connectors *always* retrieve an
* out-fence for the commit and use it appropriately.
* From userspace, this property will always read as zero.
The question is whether it's realistic to hope that a DRM writeback
connector can produce results on every frame, and do so without dragging
down the frame-rate for the connector.
The wording in the documentation above suggests that it is very likely
the fence fd won't signal userspace until after the vblank following the
scanout during which the writeback was applied (call that frame N). This
would mean that the compositor driving the connector would typically be
unable to legally queue a page flip for frame N+1.
Is this the right interpretation? Is the writeback hardware typically
even designed with a streaming use-case in mind? Maybe it's just
intended for occasional static screenshots.
So typically writeback hardware needs its separate crtc (at least the
examples I know of) and doesn't make a lot of guarantees that it's fast
enough for real time use. Since it's a separate crtc it shouldn't hold up
the main composition loop, and so this should be all fine.
On Mali-DP and Komeda at least, you can use writeback on the same CRTC
that is driving a "real" display, and it should generally work. If the
writeback doesn't keep up then the HW will signal an error, but it was
designed to work in-sync with real scanout, on the same pipe.
Same with MSM hardware. You can use writeback with same CRTC that is
driving a "real" display and yes we call it concurrent writeback. So I
think it is correct in the documentation to expect to wait till this is
signaled if the same CRTC is being used.
If/when we have hardware and driver support where you can use the
writeback connector as a real-time streamout kind of thing, then we need
to change all this, because with the current implementation, there's
indeed the possibility that funny things can happen if you ignore the
notice (funny as in data corruption, not funny as the kernel crashes of
course).
Indeed, the wording was added (from what I remember from so long
ago...) because it sounded like different HW made very different
guarantees/non-guarantees about what data would be written when, so
perhaps you'd end up with some pixels from the next frame in your
buffer or something.
Taking Mali-DP/Komeda again, the writeback configuration is latched
along with everything else, and writeback throughput permitting, it
should "just work" if you submit a new writeback every frame. It
drains out the last of the data during vblank, before starting on the
next frame. That doesn't help the "general case" though.
Would it be fair to summarize it like below:
1) If the same CRTC is shared with the real time display, then the
hardware is expected to fire this every frame so userspace should wait
till this is signaled.
2) If a different CRTC is used for the writeback, then the composition
loop for the real time display should not block on this unless its a
mirroring use-case, then we will be throttled by the lowest refresh rate
anyway.
If we already have devices where you can use writeback together with real
outputs, then I guess that counts as an oopsie :-/
Well "works fine" fits into the "undefined behaviour" bucket, just as
well as "corrupts your fb" does :-)
-Brian
Cheers, Sima
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
