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

[Tomasz Figa <tfiga@chromium.org>]

[Fixing Niklas's address.]

On Fri, May 29, 2020 at 3:26 PM Tomasz Figa <tfiga@chromium.org> wrote:
>
> 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).
> > >