Message-ID: <20200904134832.GC7518@pendragon.ideasonboard.com>
Date:   Fri, 4 Sep 2020 16:48:32 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     Jacopo Mondi <jacopo@...ndi.org>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hverkuil@...all.nl>, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        Biju Das <biju.das.jz@...renesas.com>,
        Prabhakar <prabhakar.csengg@...il.com>
Subject: Re: [PATCH v3 1/2] media: i2c: ov772x: Add support for BT656 mode

On Fri, Sep 04, 2020 at 02:00:13PM +0300, Sakari Ailus wrote:
> On Fri, Sep 04, 2020 at 12:35:50PM +0200, Jacopo Mondi wrote:
> > On Fri, Sep 04, 2020 at 12:36:26PM +0300, Sakari Ailus wrote:
> > > On Fri, Sep 04, 2020 at 11:20:49AM +0200, Jacopo Mondi wrote:
> > > > On Fri, Sep 04, 2020 at 11:21:04AM +0300, Sakari Ailus wrote:
> > > > > On Fri, Sep 04, 2020 at 09:55:53AM +0200, Jacopo Mondi wrote:
> > > > > > On Fri, Sep 04, 2020 at 04:20:00AM +0300, Laurent Pinchart wrote:
> > > > > > > Hi Prabhakar,
> > > > > > >
> > > > > > > Thank you for the patch.
> > > > > > >
> > > > > > > On Mon, Aug 24, 2020 at 08:04:05PM +0100, Lad Prabhakar wrote:
> > > > > > > > Add support to read the bus-type and enable BT656 mode if needed.
> > > > > > > >
> > > > > > > > Also fail probe if unsupported bus_type is detected.
> > > > > > > >
> > > > > > > > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> > > > > > > > Reviewed-by: Biju Das <biju.das.jz@...renesas.com>
> > > > > > > > ---
> > > > > > > >  drivers/media/i2c/ov772x.c | 32 ++++++++++++++++++++++++++++++++
> > > > > > > >  1 file changed, 32 insertions(+)
> > > > > > > >
> > > > > > > > diff --git a/drivers/media/i2c/ov772x.c b/drivers/media/i2c/ov772x.c
> > > > > > > > index 2cc6a678069a..67764d647526 100644
> > > > > > > > --- a/drivers/media/i2c/ov772x.c
> > > > > > > > +++ b/drivers/media/i2c/ov772x.c
> > > > > > > > @@ -31,6 +31,7 @@
> > > > > > > >  #include <media/v4l2-ctrls.h>
> > > > > > > >  #include <media/v4l2-device.h>
> > > > > > > >  #include <media/v4l2-event.h>
> > > > > > > > +#include <media/v4l2-fwnode.h>
> > > > > > > >  #include <media/v4l2-image-sizes.h>
> > > > > > > >  #include <media/v4l2-subdev.h>
> > > > > > > >
> > > > > > > > @@ -434,6 +435,7 @@ struct ov772x_priv {
> > > > > > > >  #ifdef CONFIG_MEDIA_CONTROLLER
> > > > > > > >  	struct media_pad pad;
> > > > > > > >  #endif
> > > > > > > > +	struct v4l2_fwnode_endpoint ep;
> > > > > > > >  };
> > > > > > > >
> > > > > > > >  /*
> > > > > > > > @@ -581,6 +583,13 @@ static int ov772x_s_stream(struct v4l2_subdev *sd, int enable)
> > > > > > > >  	if (priv->streaming == enable)
> > > > > > > >  		goto done;
> > > > > > > >
> > > > > > > > +	if (priv->ep.bus_type == V4L2_MBUS_BT656) {
> > > > > > > > +		ret = regmaup_update_bits(priv->regmap, COM7, ITU656_ON_OFF,
> > > > > > > > +					 enable ? ITU656_ON_OFF : ~ITU656_ON_OFF);
> > > > > > > > +		if (ret)
> > > > > > > > +			goto done;
> > > > > > > > +	}
> > > > > > > > +
> > > > > > > >  	ret = regmap_update_bits(priv->regmap, COM2, SOFT_SLEEP_MODE,
> > > > > > > >  				 enable ? 0 : SOFT_SLEEP_MODE);
> > > > > > > >  	if (ret)
> > > > > > > > @@ -1354,6 +1363,7 @@ static const struct v4l2_subdev_ops ov772x_subdev_ops = {
> > > > > > > >
> > > > > > > >  static int ov772x_probe(struct i2c_client *client)
> > > > > > > >  {
> > > > > > > > +	struct fwnode_handle *endpoint;
> > > > > > > >  	struct ov772x_priv	*priv;
> > > > > > > >  	int			ret;
> > > > > > > >  	static const struct regmap_config ov772x_regmap_config = {
> > > > > > > > @@ -1415,6 +1425,28 @@ static int ov772x_probe(struct i2c_client *client)
> > > > > > > >  		goto error_clk_put;
> > > > > > > >  	}
> > > > > > > >
> > > > > > > > +	endpoint = fwnode_graph_get_next_endpoint(dev_fwnode(&client->dev),
> > > > > > > > +						  NULL);
> > > > > > > > +	if (!endpoint) {
> > > > > > > > +		dev_err(&client->dev, "endpoint node not found\n");
> > > > > > > > +		ret = -EINVAL;
> > > > > > > > +		goto error_clk_put;
> > > > > > > > +	}
> > > > > > > > +
> > > > > > > > +	ret = v4l2_fwnode_endpoint_parse(endpoint, &priv->ep);
> > > > > > >
> > > > > > > v4l2_fwnode_endpoint_parse() is deprecated for new drivers,
> > > > > > > v4l2_fwnode_endpoint_alloc_parse() is recommended instead. Please note
> > > > > > > that v4l2_fwnode_endpoint_free() then needs to be called in the error
> > > > > > > path and in remove().
> > > > > >
> > > > > > Doesn't alloc_parse() differ from just _parse() as it reserve space
> > > > > > for the 'link-frequencies' array ? As this device does not support
> > > > > > CSI-2 and the 'link-frequencies' property is not allows in bindings,
> > > > > > isn't using endpoint_parse() better as it saves a call to _free() ?
> > > > >
> > > > > Yeah. I think the documentation needs to be updated.
> > > > >
> > > > > The thinking was there would be other variable size properties that drivers
> > > > > would need but that didn't happen. So feel free to continue use
> > > > > v4l2_fwnode_endpoint_parse() where it does the job.
> > > > >
> > > > > >
> > > > > > Or are we deprecating that function unconditionally ? The
> > > > > > documentation suggests "please use v4l2_fwnode_endpoint_alloc_parse()
> > > > > > in new drivers" but here it doesn't seem required..
> > > > > >
> > > > > > >
> > > > > > > On the other hand, not setting .bus_type and letting the parse()
> > > > > > > function determine the but type automatically is also deprecated, and I
> > > > > > > don't think forcing drivers to call v4l2_fwnode_endpoint_alloc_parse()
> > > > > > > once for each bus type until one succeeds is a good API. As change will
> > > > > > > be needed in that API, you can ignore v4l2_fwnode_endpoint_alloc_parse()
> > > > > > > for the time being if you want.
> > > > > >
> > > > > > But indeed relying on auto-guessing of the bus type is deprecated since
> > > > > > some time now (and the API could be improved, yes). Sorry I missed
> > > > > > that yesterday.
> > > > >
> > > > > There's one case where the bus type does not need to be set: when bindings
> > > > > require it *and* at the same time you have no default configuration that
> > > > > requires something to be set in the bus specific struct. Bindings where
> > > > > bus-type is required were added later so I think the documentation should
> > > > > be changed there, too.
> > > > >
> > > > > I can send the patches.
> > > > >
> > > > > >
> > > > > > As we support parallel and bt.656 only I must be honest I don't mind
> > > > > > it here as otherwise the code would be more complex for no real gain,
> > > > > > but I defer this to Sakari which has been fighting the battle against
> > > > > > auto-guessing since a long time now  :)
> > > > >
> > > > > I think you should require bus-type property in bindings in that case.
> > > > >
> > > > > But as it's an existing driver, bus-type will be optional. You'll need to
> > > > > default to what was supported earlier. This is actually an interesting case
> > > > > as bindings do not document it.
> > > >
> > > > For reference:
> > > > https://patchwork.linuxtv.org/project/linux-media/patch/20200903131029.18334-3-jacopo+renesas@jmondi.org/
> > > >
> > > > But yes, we might have DTBs in the wild without bus-type specified :(
> > >
> > > Shouldn't that be then that the bus-type is optional and defaults to
> > > parallel?
> > 
> > I think going forward we want to make it mandatory, don't we ? The
> > older dts will fail at dt validation time against the new yaml bindings, but
> > my understanding is that this is not a problem.
> 
> For new devices, yes. I still wouldn't make DT binding changes that render
> the old DT source invalid, at least unless it's absolutely mandatory. And
> that is not the case here.
> 
> I guess it may be a bit grey area. At least leave a comment in the driver
> on how the old bindings were so the code isn't accidentally "fixed".
> 
> > Binary compatibility, with the introduction of BT.656 support becomes
> > more complex instead :/
> > 
> > Before this series parallel was the only supported bus type and no
> > endpoint properties were required. The driver picked the default
> > settings for signal polarities and that was it.
> > 
> > With the introduction of BT.656 no signal polarity properties means
> > BT.656 when autoguess is in use. So going forward the bus-type shall
> > be explicitly set, but we might receive old DTBs with no bus-type and
> > no endpoint properties which assumes 'parallel' is in use.
> > 
> > One possible way forward could be:
> > - verify if bus-type is present in the fwnode
> > - if it is, we have a new DTB and we can rely on autoguess

It's not guessing if the bus type is specified :-)

> > - if it's not assume we have an old DTB that assumed 'parallel'. Parse
> >   the fwnode and if any relevant V4L2_MBUS_ flag is set use it,
> >   otherwise use the defaults.
> > 
> > If we make bus-type optional in new bindings, the old DTB with no
> > parallel endpoint properties would be identified as BT.656 breaking
> > capture operation, am I wrong ?
> 
> There's no technical reason why it has to be so.
> 
> You simply try endpoint parsing with parallel bus first, with the old
> defaults, and if that succeeds, then you don't attempt to parse it as
> Bt.656 anymore.

If bus-type is optional with new bindings,
v4l2_fwnode_endpoint_parse(V4L2_MBUS_PARALLEL) will always succeed if
the bus-type DT property isn't set.

> > This might require a bit more work from Prabhakar I'm sorry. The old
> > bindings were clearly falling short once BT.656 becomes supported.

-- 
Regards,

Laurent Pinchart

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