Re: [PATCH 1/3] media: videobuf2: Add a transfer error event

[Michael Rodin]

Hi Hans,

On Tue, Aug 02, 2022 at 11:32:03AM +0200, Hans Verkuil wrote:
> Hi Michael,
> 
> Apologies for the late reply...

Thank you very much for your feedback, very appreciated!

> On 7/15/22 18:15, Michael Rodin wrote:
> > Hi Nicolas,
> > 
> > On Mon, Jul 04, 2022 at 11:59:58AM -0400, Nicolas Dufresne wrote:
> >> Hi Micheal,
> >>
> >> thanks for your work, I have some questions below ...
> > 
> > Thank you for your feedback!
> > 
> >> Le mardi 28 juin 2022 à 20:00 +0200, Michael Rodin a écrit :
> >>> From: Niklas Söderlund <https://urldefense.proofpoint.com/v2/url?u=http-3A__niklas.soderlund-2Brenesas-40ragnatech.se&d=DwIFaQ&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=sWsgk3pKkv5GeIDM2RZlPY8TjNFU2D0oBeOj6QNBadE&m=7ktiIpDjee6bMSPLXXR7KVvJ_y234VytWEydKF2TWEo&s=-GUWUbGDkkrTAXiF_75xnL13cn3HYL2r2ZN0XwlG41U&e=>
> >>>
> >>> Add a new V4L2_EVENT_XFER_ERROR event to signal if an error happens during
> >>> video transfer.
> >>>
> >>> The use-case that sparked this new event is to signal to the video
> >>> device driver that an error has happen on the CSI-2 bus from the CSI-2
> >>> receiver subdevice.
> >>>
> >>> Signed-off-by: Niklas Söderlund <https://urldefense.proofpoint.com/v2/url?u=http-3A__niklas.soderlund-2Brenesas-40ragnatech.se&d=DwIFaQ&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=sWsgk3pKkv5GeIDM2RZlPY8TjNFU2D0oBeOj6QNBadE&m=7ktiIpDjee6bMSPLXXR7KVvJ_y234VytWEydKF2TWEo&s=-GUWUbGDkkrTAXiF_75xnL13cn3HYL2r2ZN0XwlG41U&e=>
> >>> [mrodin@xxxxxxxxxxxxxx: adapted information what to do if this new event is received]
> >>> Signed-off-by: Michael Rodin <mrodin@xxxxxxxxxxxxxx>
> >>> ---
> >>>  .../userspace-api/media/v4l/vidioc-dqevent.rst         | 10 ++++++++++
> >>>  .../userspace-api/media/videodev2.h.rst.exceptions     |  1 +
> >>>  include/uapi/linux/videodev2.h                         |  1 +
> >>>  3 files changed, 12 insertions(+)
> >>>
> >>> diff --git a/Documentation/userspace-api/media/v4l/vidioc-dqevent.rst b/Documentation/userspace-api/media/v4l/vidioc-dqevent.rst
> >>> index 6eb40073c906..3cf0b4859784 100644
> >>> --- a/Documentation/userspace-api/media/v4l/vidioc-dqevent.rst
> >>> +++ b/Documentation/userspace-api/media/v4l/vidioc-dqevent.rst
> >>> @@ -182,6 +182,16 @@ call.
> >>>  	the regions changes. This event has a struct
> >>>  	:c:type:`v4l2_event_motion_det`
> >>>  	associated with it.
> >>> +    * - ``V4L2_EVENT_XFER_ERROR``
> >>
> >> I'm not sure why this event is specific to XFER. Is there uses cases were a
> >> future implementation would have both XFER and RECEIVER error ?
> > 
> > I am not sure whether I understand you correctly, do you mean that there is
> > already a method to signal a receiver error? Or that we should name it
> > V4L2_EVENT_RECEIVER_ERROR? I think that "transfer error" is a good name for
> > this event, because it could be sent by receiver or by transmitter drivers,
> > depending on their hardware error detection capabilities. We could have
> > e.g. a video transmitter which can detect an error coupled with a video
> > receiver which can not detect any errors.
> > 
> >>> +      - 7
> >>> +      - This event is triggered when an transfer error is detected while
> >>> +	streaming. For example if an error is detected on a video bus in
> >>> +	the pipeline. If a driver receives this event from an upstream
> >>> +	subdevice, it has to forward the event to userspace. The streaming
> >>> +	application has to check if the transfer error is unrecoverable,
> >>> +	i.e. no new buffers can be dequeued from the kernel after the
> >>> +	expected time. If the error is unrecoverable, the streaming
> >>> +	application should restart streaming if it wants to continue.
> >>
> >> The process to determine if an error is recoverable or not isn't clear to me. As
> >> an application developer, I would not know what to do here. Recoverable error
> >> already have a designed mechanism, it consist of marking done a buffer with the
> >> flag V4L2_BUF_FLAG_ERROR. I would like to understand what the existing mechanism
> >> needed to be replaced, and the placement should be documented.
> > 
> > "Recoverable" means in this context that kernel space continues to capture
> > video buffers (which do not necessarily have the flag V4L2_BUF_FLAG_ERROR).
> > So probably we should not say "recoverable" or "unrecoverable" in the
> > context of this event to avoid confusion. V4L2_EVENT_XFER_ERROR just tells
> > userspace that it should restart streaming if the buffer flow stops after
> > this event. So would it be sufficient for an application developer if we
> > drop all statements about "recoverability" from the event description?
> 
> Here you touch on the core problem of this patch: you are basically saying
> that userspace has to 1) subscribe to this event, 2) poll for it, 3) if it
> arrives start a timer, 4) if the timer triggers and no new buffers have been
> received in the meantime, then 5) restart streaming.
> 
> So in other words, you are just too lazy to do this in the driver and want
> to hand it off to userspace.
> 
> That's not how it works. Usually the driver will know if the error is
> recoverable or not (i.e. if an HDMI receiver loses signal, that's definitely
> unrecoverable, and it's something the driver can know and call vb2_queue_error).
> 
> If it is really unknown, then you indeed need some monitoring thread. And
> that's fine. Even better if you can make some helper things in the V4L2 core.
> 
> But you can't just kick that to userspace IMHO. I can guarantee that almost
> no userspace application will do this and it is really not the job of userspace
> to deal with such issues.

>From my understanding this means that my previous patch [1] already went in
the right direction by implementing a monitoring thread in rcar-vin. But on
the other hand Niklas has pointed out that it's not good to have this in a
driver [2]. Therefore it sounds like the only acceptable solution would be to
move this monitoring thread to the V4L2/VB2 core, which would then monitor
capture drivers for frame timeouts and maybe also notify userspace based
on this. What do you think? If you already have a solution in mind, I would
very appreciate if you could give me a few hints for an implementation!

[1] https://lore.kernel.org/lkml/1652983210-1194-4-git-send-email-mrodin@xxxxxxxxxxxxxx/
[2] https://lore.kernel.org/lkml/YqEO3%2FKekkZhVjW+@xxxxxxxxxxxxxxxxx/

> Regards,
> 
> 	Hans
> 
> > 
> >> Nicolas
> >>
> >>>      * - ``V4L2_EVENT_PRIVATE_START``
> >>>        - 0x08000000
> >>>        - Base event number for driver-private events.
> >>> diff --git a/Documentation/userspace-api/media/videodev2.h.rst.exceptions b/Documentation/userspace-api/media/videodev2.h.rst.exceptions
> >>> index 9cbb7a0c354a..25bde61a1519 100644
> >>> --- a/Documentation/userspace-api/media/videodev2.h.rst.exceptions
> >>> +++ b/Documentation/userspace-api/media/videodev2.h.rst.exceptions
> >>> @@ -500,6 +500,7 @@ replace define V4L2_EVENT_CTRL event-type
> >>>  replace define V4L2_EVENT_FRAME_SYNC event-type
> >>>  replace define V4L2_EVENT_SOURCE_CHANGE event-type
> >>>  replace define V4L2_EVENT_MOTION_DET event-type
> >>> +replace define V4L2_EVENT_XFER_ERROR event-type
> >>>  replace define V4L2_EVENT_PRIVATE_START event-type
> >>>  
> >>>  replace define V4L2_EVENT_CTRL_CH_VALUE ctrl-changes-flags
> >>> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> >>> index 5311ac4fde35..44db724d4541 100644
> >>> --- a/include/uapi/linux/videodev2.h
> >>> +++ b/include/uapi/linux/videodev2.h
> >>> @@ -2385,6 +2385,7 @@ struct v4l2_streamparm {
> >>>  #define V4L2_EVENT_FRAME_SYNC			4
> >>>  #define V4L2_EVENT_SOURCE_CHANGE		5
> >>>  #define V4L2_EVENT_MOTION_DET			6
> >>> +#define V4L2_EVENT_XFER_ERROR			7
> >>>  #define V4L2_EVENT_PRIVATE_START		0x08000000
> >>>  
> >>>  /* Payload for V4L2_EVENT_VSYNC */
> >>
> > 
> 

-- 
Best Regards,
Michael