Re: [PATCH 1/6 v4] media: V4L2: support asynchronous subdevice registration

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

Hi Laurent

On Tue, 8 Jan 2013, Laurent Pinchart wrote:

> Hi Guennadi,
> 
> Thanks for the patch.
> 
> On Monday 07 January 2013 11:23:55 Guennadi Liakhovetski wrote:
> > >From 0e1eae338ba898dc25ec60e3dba99e5581edc199 Mon Sep 17 00:00:00 2001
> > 
> > From: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
> > Date: Fri, 19 Oct 2012 23:40:44 +0200
> > Subject: [PATCH] media: V4L2: support asynchronous subdevice registration
> > 
> > Currently bridge device drivers register devices for all subdevices
> > synchronously, tupically, during their probing. E.g. if an I2C CMOS sensor
> > is attached to a video bridge device, the bridge driver will create an I2C
> > device and wait for the respective I2C driver to probe. This makes linking
> > of devices straight forward, but this approach cannot be used with
> > intrinsically asynchronous and unordered device registration systems like
> > the Flattened Device Tree. To support such systems this patch adds an
> > asynchronous subdevice registration framework to V4L2. To use it respective
> > (e.g. I2C) subdevice drivers must request deferred probing as long as their
> > bridge driver hasn't probed. The bridge driver during its probing submits a
> > an arbitrary number of subdevice descriptor groups to the framework to
> > manage. After that it can add callbacks to each of those groups to be
> > called at various stages during subdevice probing, e.g. after completion.
> > Then the bridge driver can request single groups to be probed, finish its
> > own probing and continue its video subsystem configuration from its
> > callbacks.
> > 
> > Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
> > ---
> > 
> > v4: Fixed v4l2_async_notifier_register() for the case, when subdevices
> > probe successfully before the bridge, thanks to Prabhakar for reporting
> > 
> >  drivers/media/v4l2-core/Makefile     |    3 +-
> >  drivers/media/v4l2-core/v4l2-async.c |  284 +++++++++++++++++++++++++++++++
> >  include/media/v4l2-async.h           |  113 ++++++++++++++
> >  3 files changed, 399 insertions(+), 1 deletions(-)
> >  create mode 100644 drivers/media/v4l2-core/v4l2-async.c
> >  create mode 100644 include/media/v4l2-async.h
> 
> [snip]
> 
> > diff --git a/include/media/v4l2-async.h b/include/media/v4l2-async.h
> > new file mode 100644
> > index 0000000..91d436d
> > --- /dev/null
> > +++ b/include/media/v4l2-async.h
> > @@ -0,0 +1,113 @@
> > +/*
> > + * V4L2 asynchronous subdevice registration API
> > + *
> > + * Copyright (C) 2012, Guennadi Liakhovetski <g.liakhovetski@gmx.de>
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2 as
> > + * published by the Free Software Foundation.
> > + */
> > +
> > +#ifndef V4L2_ASYNC_H
> > +#define V4L2_ASYNC_H
> > +
> > +#include <linux/list.h>
> > +#include <linux/mutex.h>
> > +#include <linux/notifier.h>
> > +
> > +#include <media/v4l2-subdev.h>
> > +
> > +struct device;
> > +struct v4l2_device;
> > +struct v4l2_async_notifier;
> > +
> > +enum v4l2_async_bus_type {
> > +	V4L2_ASYNC_BUS_SPECIAL,
> > +	V4L2_ASYNC_BUS_PLATFORM,
> > +	V4L2_ASYNC_BUS_I2C,
> > +};
> > +
> > +struct v4l2_async_hw_device {
> > +	enum v4l2_async_bus_type bus_type;
> > +	union {
> > +		struct {
> > +			const char *name;
> > +		} platform;
> > +		struct {
> > +			int adapter_id;
> > +			unsigned short address;
> > +		} i2c;
> > +		struct {
> > +			bool (*match)(struct device *,
> > +				      struct v4l2_async_hw_device *);
> > +			void *priv;
> > +		} special;
> > +	} match;
> > +};
> > +
> > +/**
> > + * struct v4l2_async_subdev - sub-device descriptor, as known to a bridge
> > + * @hw:		this device descriptor
> > + * @list:	member in a list of subdevices
> > + */
> > +struct v4l2_async_subdev {
> > +	struct v4l2_async_hw_device hw;
> > +	struct list_head list;
> > +};
> > +
> > +/**
> > + * v4l2_async_subdev_list - provided by subdevices
> > + * @list:	member in a list of subdevices
> > + * @dev:	hardware device
> > + * @subdev:	V4L2 subdevice
> > + * @asd:	pointer to respective struct v4l2_async_subdev
> > + * @notifier:	pointer to managing notifier
> > + */
> > +struct v4l2_async_subdev_list {
> > +	struct list_head list;
> > +	struct device *dev;
> > +	struct v4l2_subdev *subdev;
> > +	struct v4l2_async_subdev *asd;
> > +	struct v4l2_async_notifier *notifier;
> > +};
> > +
> > +/**
> > + * v4l2_async_notifier - provided by bridges
> > + * @subdev_num:	number of subdevices
> > + * @subdev:	array of pointers to subdevices
> > + * @v4l2_dev:	pointer to sruct v4l2_device
> > + * @waiting:	list of subdevices, waiting for their drivers
> > + * @done:	list of subdevices, already probed
> > + * @list:	member in a global list of notifiers
> > + * @bind:	a subdevice driver is about to probe one of your subdevices
> > + * @bound:	a subdevice driver has successfully probed one of your
> > subdevices + * @complete:	all your subdevices have been probed successfully
> > + * @unbind:	a subdevice is leaving
> > + */
> > +struct v4l2_async_notifier {
> > +	int subdev_num;
> > +	struct v4l2_async_subdev **subdev;
> > +	struct v4l2_device *v4l2_dev;
> > +	struct list_head waiting;
> > +	struct list_head done;
> > +	struct list_head list;
> > +	int (*bind)(struct v4l2_async_notifier *notifier,
> > +		    struct v4l2_async_subdev_list *asdl);
> > +	int (*bound)(struct v4l2_async_notifier *notifier,
> > +		     struct v4l2_async_subdev_list *asdl);
> > +	int (*complete)(struct v4l2_async_notifier *notifier);
> > +	void (*unbind)(struct v4l2_async_notifier *notifier,
> > +		       struct v4l2_async_subdev_list *asdl);
> > +};
> > +
> > +int v4l2_async_notifier_register(struct v4l2_device *v4l2_dev,
> > +				 struct v4l2_async_notifier *notifier);
> > +void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier);
> > +/*
> > + * If subdevice probing fails any time after v4l2_async_subdev_bind(), no
> > + * clean up must be called. This function is only a message of intention.
> > + */
> > +int v4l2_async_subdev_bind(struct v4l2_async_subdev_list *asdl);
> > +int v4l2_async_subdev_bound(struct v4l2_async_subdev_list *asdl);
> 
> Could you please explain why you need both a bind notifier and a bound 
> notifier ? I was expecting a single v4l2_async_subdev_register() call in 
> subdev drivers (and, thinking about it, I would probably name it 
> v4l2_subdev_register()).

I think I can, yes. Because between .bind() and .bound() the subdevice 
driver does the actual hardware probing. So, .bind() is used to make sure 
the hardware can be accessed, most importantly to provide a clock to the 
subdevice. You can look at soc_camera_async_bind(). There I'm registering 
the clock for the subdevice, about to bind. Why I cannot do it before, is 
because I need subdevice name for clock matching. With I2C subdevices the 
subdevice name contains the name of the driver, adapter number and i2c 
address. The latter 2 I've got from host subdevice list. But not the 
driver name. I thought about also passing the driver name there, but that 
seemed too limiting to me. I also request regulators there, because before 
->bound() the sensor driver, but that could be done on the first call to 
soc_camera_power_on(), although doing this "first call" thingie is kind of 
hackish too. I could add one more soc-camera-power helper like 
soc_camera_prepare() or similar too. So, the main problem is the clock 
subdevice name. Also see the comment in soc_camera.c:

	/*
	 * It is ok to keep the clock for the whole soc_camera_device life-time,
	 * in principle it would be more logical to register the clock on icd
	 * creation, the only problem is, that at that time we don't know the
	 * driver name yet.
	 */

Thanks
Guennadi

> > +void v4l2_async_subdev_unbind(struct v4l2_async_subdev_list *asdl);
> > +#endif
> -- 
> Regards,
> 
> Laurent Pinchart
> 

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