Re: [PATCH v6 3/7] media: soc-camera: switch I2C subdevice drivers to use v4l2-clk

[Guennadi Liakhovetski <g.liakhovetski@gmx.de>]

On Mon, 18 Mar 2013, Hans Verkuil wrote:

> On Fri March 15 2013 22:27:49 Guennadi Liakhovetski wrote:
> > Instead of centrally enabling and disabling subdevice master clocks in
> > soc-camera core, let subdevice drivers do that themselves, using the
> > V4L2 clock API and soc-camera convenience wrappers.
> > 
> > Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
> > ---
> > 
> > v6: clock name update
> > 
> >  drivers/media/i2c/soc_camera/imx074.c              |   18 ++-
> >  drivers/media/i2c/soc_camera/mt9m001.c             |   17 ++-
> >  drivers/media/i2c/soc_camera/mt9m111.c             |   20 ++-
> >  drivers/media/i2c/soc_camera/mt9t031.c             |   19 ++-
> >  drivers/media/i2c/soc_camera/mt9t112.c             |   19 ++-
> >  drivers/media/i2c/soc_camera/mt9v022.c             |   17 ++-
> >  drivers/media/i2c/soc_camera/ov2640.c              |   19 ++-
> >  drivers/media/i2c/soc_camera/ov5642.c              |   20 ++-
> >  drivers/media/i2c/soc_camera/ov6650.c              |   17 ++-
> >  drivers/media/i2c/soc_camera/ov772x.c              |   15 ++-
> >  drivers/media/i2c/soc_camera/ov9640.c              |   17 ++-
> >  drivers/media/i2c/soc_camera/ov9640.h              |    1 +
> >  drivers/media/i2c/soc_camera/ov9740.c              |   18 ++-
> >  drivers/media/i2c/soc_camera/rj54n1cb0c.c          |   17 ++-
> >  drivers/media/i2c/soc_camera/tw9910.c              |   18 ++-
> >  drivers/media/platform/soc_camera/soc_camera.c     |  172 +++++++++++++++-----
> >  .../platform/soc_camera/soc_camera_platform.c      |    2 +-
> >  include/media/soc_camera.h                         |   13 +-
> >  18 files changed, 355 insertions(+), 84 deletions(-)
> > 
> > diff --git a/drivers/media/i2c/soc_camera/imx074.c b/drivers/media/i2c/soc_camera/imx074.c
> > index a2a5cbb..cee5345 100644
> > --- a/drivers/media/i2c/soc_camera/imx074.c
> > +++ b/drivers/media/i2c/soc_camera/imx074.c
> > @@ -18,6 +18,7 @@
> >  #include <linux/module.h>
> >  
> >  #include <media/soc_camera.h>
> > +#include <media/v4l2-clk.h>
> >  #include <media/v4l2-subdev.h>
> >  #include <media/v4l2-chip-ident.h>
> >  
> > @@ -77,6 +78,7 @@ struct imx074_datafmt {
> >  struct imx074 {
> >  	struct v4l2_subdev		subdev;
> >  	const struct imx074_datafmt	*fmt;
> > +	struct v4l2_clk			*clk;
> >  };
> >  
> >  static const struct imx074_datafmt imx074_colour_fmts[] = {
> > @@ -272,8 +274,9 @@ static int imx074_s_power(struct v4l2_subdev *sd, int on)
> >  {
> >  	struct i2c_client *client = v4l2_get_subdevdata(sd);
> >  	struct soc_camera_subdev_desc *ssdd = soc_camera_i2c_to_desc(client);
> > +	struct imx074 *priv = to_imx074(client);
> >  
> > -	return soc_camera_set_power(&client->dev, ssdd, on);
> > +	return soc_camera_set_power(&client->dev, ssdd, priv->clk, on);
> >  }
> >  
> >  static int imx074_g_mbus_config(struct v4l2_subdev *sd,
> > @@ -431,6 +434,7 @@ static int imx074_probe(struct i2c_client *client,
> >  	struct imx074 *priv;
> >  	struct i2c_adapter *adapter = to_i2c_adapter(client->dev.parent);
> >  	struct soc_camera_subdev_desc *ssdd = soc_camera_i2c_to_desc(client);
> > +	int ret;
> >  
> >  	if (!ssdd) {
> >  		dev_err(&client->dev, "IMX074: missing platform data!\n");
> > @@ -451,13 +455,23 @@ static int imx074_probe(struct i2c_client *client,
> >  
> >  	priv->fmt	= &imx074_colour_fmts[0];
> >  
> > -	return imx074_video_probe(client);
> > +	priv->clk = v4l2_clk_get(&priv->subdev, "mclk");
> > +	if (IS_ERR(priv->clk))
> > +		return PTR_ERR(priv->clk);
> > +
> > +	ret = imx074_video_probe(client);
> > +	if (ret < 0)
> > +		v4l2_clk_put(priv->clk);
> > +
> 
> I feel uneasy about this. It's not the clock part as such but the fact that
> assumptions are made about the usage of this sensor driver. It basically
> comes down to the fact that these drivers are *still* tied to the soc-camera
> framework. I think I am going to work on this in a few weeks time to cut
> these drivers loose from soc-camera. We discussed how to do that in the past.

Sorry, not sure I understand. This is a generic (V4L2) clock, it has 
nothing specific to soc-camera.

> The whole point of the subdev API is to make drivers independent of bridge
> drivers, and these soc-camera subdev drivers are the big exception and they
> stick out like a sore thumb.

We are moving towards complete driver independency from the soc-camera 
framework, and, afaics, there's not much left. Simply noone is interested 
enough to do the work or to pay for it, noone has a really burning 
use-case. And without one it's not very easy to implement things with no 
test case. But sure, you're most welcome to work on this :)

> Anyway, w.r.t. the clock use: what happens if these drivers are used in e.g.
> a USB webcam driver? In that case there probably won't be a clock involved
> (well, there is one, but that is likely to be setup by the firmware/hardware
> itself).

Well, from the sensor driver PoV if the sensor needs a clock it seems 
logical for the driver to request it and to fail if it's not available. 
USB cameras could provide a dummy clock, or we could implement one 
centrally, however, this will lead to an undesirable result, that everyone 
will just use that dummy clock... If we make clock support optional the 
same thing will happen - noone will implement them. BTW, you're looking at 
an intermediate patch in this series, which only adds clock support. In a 
later patch the return error code for missing clock will be replaced with 
-EPROBE_DEFER which serves as a sign, that no bridge driver is available 
yes and _is_ required to support asynchronous probing.

> Wouldn't it be better if the clock name is passed on through the platform data
> (or device tree)? And if no clock name was specified, then there is no need to
> get a clock either and the driver can assume that it will always have a clock.
> That would solve this problem when this sensor driver is no longer soc-camera
> dependent.

No. Yes, this has been discussed many times - in the context of the 
generic clock API. I also proposed a patch, that did such a thing and was 
"kindly" explained, why that wasn't a good idea :-) Clock names are names 
of clock _inputs_ on the consumer. I.e. a sensor driver should request a 
clock according to its datasheet. For the clock provider it's different, 
say, a bridge driver cannot know what sensor will be connected to it and 
clock it will be expecting. That's why we have clock lookup tables, that 
connect physical clock objects (providers) with consumer clock names in 
platform data (perhaps, a similar thing is done in DT, haven't looked 
yet). I think, we could accept a compromise by using a common name for all 
clocks with the same function. I'm using "mclk" as an abbreviation for 
"master clock."

Thanks
Guennadi

> Sorry if this was discussed in earlier patches, I haven't been following this
> very closely before.
> 
> Regards,
> 
> 	Hans
> 
> > +	return ret;
> >  }
> >  
> >  static int imx074_remove(struct i2c_client *client)
> >  {
> >  	struct soc_camera_subdev_desc *ssdd = soc_camera_i2c_to_desc(client);
> > +	struct imx074 *priv = to_imx074(client);
> >  
> > +	v4l2_clk_put(priv->clk);
> >  	if (ssdd->free_bus)
> >  		ssdd->free_bus(ssdd);
> >  
> 

---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/