Re: [PATCH v4 0/5] media: add v4l2_pipeline_stream_{enable,disable}

[Tomasz Figa <tfiga@chromium.org>]

On Thu, May 28, 2020 at 6:21 PM Dafna Hirschfeld
<dafna.hirschfeld@collabora.com> wrote:
>
> Hi Tomasz, Helen, Laurent
>
> On 26.05.20 20:57, Laurent Pinchart wrote:
> > Hi Tomasz,
> >
> > On Tue, May 26, 2020 at 06:11:00PM +0200, Tomasz Figa wrote:
> >> On Fri, May 22, 2020 at 11:06 AM Helen Koike <helen.koike@collabora.com> wrote:
> >>> On 5/22/20 4:55 AM, Dafna Hirschfeld wrote:
> >>>> Hi,
> >>>> This is v4 of the patchset that was sent by Helen Koike.
> >>>>
> >>>> Media drivers need to iterate through the pipeline and call .s_stream()
> >>>> callbacks in the subdevices.
> >>>>
> >>>> Instead of repeating code, add helpers for this.
> >>>>
> >>>> These helpers will go walk through the pipeline only visiting entities
> >>>> that participates in the stream, i.e. it follows links from sink to source
> >>>> (and not the opposite).
> >>>> For example, in a topology like this https://bit.ly/3b2MxjI
> >>>> calling v4l2_pipeline_stream_enable() from rkisp1_mainpath won't call
> >>>> .s_stream(true) for rkisp1_resizer_selfpath.
> >>>>
> >>>> stream_count variable was added in v4l2_subdevice to handle nested calls
> >>>> to the helpers.
> >>>> This is useful when the driver allows streaming from more then one
> >>>> capture device sharing subdevices.
> >>>
> >>> If I understand correctly, this isn't  true anymore right? Nested calls aren't
> >>> possible anymore since this version doesn't contain stream_count in struct v4l2_subdevice.
> >>>
> >>> Documentation of v4l2_pipeline_stream_*() should also be updated.
> >>>
> >>> Just to be clear, without the nested call, vimc will require to add its own
> >>> counters, patch https://patchwork.kernel.org/patch/10948833/ will be
> >>> required again to allow multi streaming.
> >>>
> >>> Also, patch "media: staging: rkisp1: cap: use v4l2_pipeline_stream_{enable,disable}"
> >>> is cleaner in the previous version (with stream_count in struct v4l2_subdevice).
> >>>
> >>> All drivers that allows multi streaming will need to implement some special handling.
> >>>
> >>> Adding stream_count in struct v4l2_subdevice still seems cleaner to me. I'd like to hear
> >>> what others think.
> >>
> >> I certainly would see this reference counting done in generic code,
> >> because requiring every driver to do it simply adds to the endless
>
> It is required only for drivers that support multistreaming. I don't know much
> about other driver except of the ones I am working on, is it a common case?
>

I'm not very familiar with the older camera I/F drivers, but multiple
streams isn't anything unusual for camera hardware. I recall the old
Samsung FIMC already having support for separate preview and capture
outputs.

Also adding the reference counting on framework level probably
wouldn't really hurt drivers which only have 1 stream anyway.

> >> amount of boiler plate that V4L2 currently requires from the drivers.
> >> :(
> >>
> >> I wonder if it wouldn't be possible to redesign the framework so that
> >> .s_stream() at the subdev level is only called when it's expected to
> >> either start or stop this particular subdev and driver's
> >> implementation can simply execute the requested action.
>
> You mean that a generic code similar to the helper functions in this patchset
> will be used for all drivers, so that drivers don't call s_stream for subdevices
> anymore?
> Anyway, this patchset just adds helper functions, it does not redesign the code.
> Maybe the stream_count can be updated in the v4l2_subdev_call macro ?
> This why it can be used not just for the helper functions.

Sorry, just thinking out loud. Generally if we look at other kAPIs,
such as the drm_crtc_helper_funcs [1] or regulator_ops [2], the driver
provided implementation doesn't have to care about duplicate
enable/disable requests.

[1] https://elixir.bootlin.com/linux/v5.7-rc7/source/include/drm/drm_modeset_helper_vtables.h#L61
[2] https://elixir.bootlin.com/linux/v5.7-rc7/source/include/linux/regulator/driver.h#L144

If we could prohibit calling v4l2_subdev_ops::s_stream() by the
drivers directly and instead add something like
v4l2_subdev_s_stream(), the latter could do reference counting on its
own and then only call v4l2_subdev_ops::s_stream() when the reference
count changes between 0 and 1.

One problem I see with this series is that I'm not sure if it's always
guaranteed that all the drivers in the pipeline would actually use the
generic helpers. If there is a driver in the pipeline which just calls
v4l2_subdev_ops::s_stream() on some other subdev on its own, it
wouldn't be aware of the reference count and bad things could happen
(e.g. the subdev stopped despite the count being > 0).

However, I'm afraid this is more of the kAPI design issue and could be
require quite a significant effort to be straightened out.

Best regards,
Tomasz

>
> Thanks,
> Dafna
>
> >
> > I'd very much like that. Note that I think a few drivers abuse the on
> > parameter to the function to pass other values than 0 or 1. We'd have to
> > fix those first (or maybe it has been done already, it's been a long
> > time since I last checked).
> >