Date:   Thu, 13 Sep 2018 14:58:57 +0200
From:   jacopo mondi <jacopo@...ndi.org>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     Steve Longerbeam <slongerbeam@...il.com>,
        linux-media@...r.kernel.org,
        Steve Longerbeam <steve_longerbeam@...tor.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sebastian Reichel <sre@...nel.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 06/17] media: v4l2-fwnode: Add a convenience function
 for registering subdevs with notifiers

Hi Sakari,

On Thu, Sep 13, 2018 at 03:44:25PM +0300, Sakari Ailus wrote:
> Hi Jacopo,
>
> On Thu, Sep 13, 2018 at 12:37:27PM +0200, jacopo mondi wrote:
> > Hi Steve,
> >
> > On Mon, Jul 09, 2018 at 03:39:06PM -0700, Steve Longerbeam wrote:
> > > Adds v4l2_async_register_fwnode_subdev(), which is a convenience function
> > > for parsing a sub-device's fwnode port endpoints for connected remote
> > > sub-devices, registering a sub-device notifier, and then registering
> > > the sub-device itself.
> > >
> > > Signed-off-by: Steve Longerbeam <steve_longerbeam@...tor.com>
> > > ---
> > > Changes since v5:
> > > - add call to v4l2_async_notifier_init().
> > > Changes since v4:
> > > - none
> > > Changes since v3:
> > > - remove support for port sub-devices, such sub-devices will have to
> > >   role their own.
> > > Changes since v2:
> > > - fix error-out path in v4l2_async_register_fwnode_subdev() that forgot
> > >   to put device.
> > > Changes since v1:
> > > - add #include <media/v4l2-subdev.h> to v4l2-fwnode.h for
> > >   'struct v4l2_subdev' declaration.
> > > ---
> > >  drivers/media/v4l2-core/v4l2-fwnode.c | 64 +++++++++++++++++++++++++++++++++++
> > >  include/media/v4l2-fwnode.h           | 38 +++++++++++++++++++++
> > >  2 files changed, 102 insertions(+)
> > >
> > > diff --git a/drivers/media/v4l2-core/v4l2-fwnode.c b/drivers/media/v4l2-core/v4l2-fwnode.c
> > > index 67ad333..94d867a 100644
> > > --- a/drivers/media/v4l2-core/v4l2-fwnode.c
> > > +++ b/drivers/media/v4l2-core/v4l2-fwnode.c
> > > @@ -872,6 +872,70 @@ int v4l2_async_register_subdev_sensor_common(struct v4l2_subdev *sd)
> > >  }
> > >  EXPORT_SYMBOL_GPL(v4l2_async_register_subdev_sensor_common);
> > >
> > > +int v4l2_async_register_fwnode_subdev(
> >
> > The meat of this function is to register a subdev with a notifier,
> > so I would make it clear in the function name which is otherwise
> > misleading
> >
> > > +	struct v4l2_subdev *sd, size_t asd_struct_size,
> > > +	unsigned int *ports, unsigned int num_ports,
> > > +	int (*parse_endpoint)(struct device *dev,
> > > +			      struct v4l2_fwnode_endpoint *vep,
> > > +			      struct v4l2_async_subdev *asd))
> > > +{
> > > +	struct v4l2_async_notifier *notifier;
> > > +	struct device *dev = sd->dev;
> > > +	struct fwnode_handle *fwnode;
> > > +	int ret;
> > > +
> > > +	if (WARN_ON(!dev))
> > > +		return -ENODEV;
> > > +
> > > +	fwnode = dev_fwnode(dev);
> > > +	if (!fwnode_device_is_available(fwnode))
> > > +		return -ENODEV;
> > > +
> > > +	notifier = kzalloc(sizeof(*notifier), GFP_KERNEL);
> > > +	if (!notifier)
> > > +		return -ENOMEM;
> > > +
> > > +	v4l2_async_notifier_init(notifier);
> > > +
> > > +	if (!ports) {
> > > +		ret = v4l2_async_notifier_parse_fwnode_endpoints(
> > > +			dev, notifier, asd_struct_size, parse_endpoint);
> > > +		if (ret < 0)
> > > +			goto out_cleanup;
> > > +	} else {
> > > +		unsigned int i;
> > > +
> > > +		for (i = 0; i < num_ports; i++) {
> >
> > It's not particularly exciting to iterate on pointers received from
> > callers without checking for num_ports first.
>
> The loop is not executed if num_ports is zero, so I don't see a problem
> with that.
>

I know this is internal drivers API and failures are meant to be
catched early in development, but what if the actual number of ports
identifiers is < then the num_ports parameter?

> >
> > Also the caller has to allocate an array of "ports" and keep track of it
> > just to pass it to this function and I don't see a way to set the
> > notifier's ops before the notifier gets registered here below.
>
> True; this can be seen as an omission but quite a few drivers have no need
> for this either. It could be added later on --- I think it'd make perfect
> sense.
>

In a 'notifier configuration' structure that gather these and existing
function parameters together as you suggested...

> >
> > > +			ret = v4l2_async_notifier_parse_fwnode_endpoints_by_port(
> > > +				dev, notifier, asd_struct_size,
> > > +				ports[i], parse_endpoint);
> > > +			if (ret < 0)
> > > +				goto out_cleanup;
> > > +		}
> > > +	}
> > > +
> > > +	ret = v4l2_async_subdev_notifier_register(sd, notifier);
> > > +	if (ret < 0)
> > > +		goto out_cleanup;
> > > +
> > > +	ret = v4l2_async_register_subdev(sd);
> > > +	if (ret < 0)
> > > +		goto out_unregister;
> > > +
> > > +	sd->subdev_notifier = notifier;
> >
> > This is set already by v4l2_async_subdev_notifier_register()
>
> The same pattern is actually present in
> v4l2_async_register_subdev_sensor_common(). It's used in unregistration
> that can only happen after the registration, i.e. this function, has
> completed.
>
> >
> > In general, I have doubts this function is really needed. It requires
> > the caller to reserve memory just to pass down a list of intergers,
> > and there is no way to set subdev ops.
> >
> > Could you have a look at how drivers/media/platform/rcar-vin/rcar-csi2.c
> > registers a subdevice and an associated notifier and see if in your
> > opinion it can be implemented in the same way in your imx csi/csi2 driver,
> > or you still like this one most?
>
> I was actually thinking of changing this later on a bit. I came to think of
> this after picking up the patchset to my tree... oh well.
>
> This function is meant for cases where you have multiple ports. That's not
> working very nicely at the moment, and even with my patches, you can't pass
> default configuration to e.g. v4l2_async_notifier_parse_fwnode_endpoints().
> So there's definitely work to do. I'd like to move the details of parsing
> out of drivers; every driver is doing almost the same but just in a little
> bit different way.
>

I see...

> The arguments should to be put into a struct. That way we get rid of a very
> long series of hard-to-read function arguments, as well as we don't need to
> change every caller when the function gets something new and interesting to
> do.
>
> Right now the entire patchset is so big (40 patches) that I'd prefer to get
> it in unless serious issues are found, and proceed the development on top.
>

Sure, please go ahead and thanks for the reply.

Cheers
   j

> --
> Kind regards,
>
> Sakari Ailus
> sakari.ailus@...ux.intel.com

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