Message-ID: <20200904110013.GG4392@valkosipuli.retiisi.org.uk>
Date:   Fri, 4 Sep 2020 14:00:13 +0300
From:   Sakari Ailus <sakari.ailus@....fi>
To:     Jacopo Mondi <jacopo@...ndi.org>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        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

Hi Jacopo,

On Fri, Sep 04, 2020 at 12:35:50PM +0200, Jacopo Mondi wrote:
> Hi Sakari,
> 
> 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:
> > > Hi Sakari,
> > >
> > > On Fri, Sep 04, 2020 at 11:21:04AM +0300, Sakari Ailus wrote:
> > > > Hi Laurent, Jacopo,
> > > >
> > > > On Fri, Sep 04, 2020 at 09:55:53AM +0200, Jacopo Mondi wrote:
> > > > > Hi Laurent,
> > > > >
> > > > > 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 = regmap_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
> - 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.

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

-- 
Kind regards,

Sakari Ailus

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