Re: [PATCH 1/2] media: V4L2: add temporary clock helpers

Sylwester Nawrocki
2012-10-21T18:52:25Z

Hi Guennadi,

On 10/20/2012 12:20 AM, Guennadi Liakhovetski wrote:
> Typical video devices like camera sensors require an external clock source.
> Many such devices cannot even access their hardware registers without a
> running clock. These clock sources should be controlled by their consumers.
> This should be performed, using the generic clock framework. Unfortunately
> so far only very few systems have been ported to that framework. This patch
> adds a set of temporary helpers, mimicking the generic clock API, to V4L2.
> Platforms, adopting the clock API, should switch to using it. Eventually
> this temporary API should be removed.

So I gave this patch a try this weekend. I would have a few comments/
questions. Thank you for sharing this!

> Signed-off-by: Guennadi Liakhovetski<g.liakhovetski@gmx.de>
> ---
> drivers/media/v4l2-core/Makefile | 2 +-
> drivers/media/v4l2-core/v4l2-clk.c | 126 ++++++++++++++++++++++++++++++++++++
> include/media/v4l2-clk.h | 48 ++++++++++++++
> 3 files changed, 175 insertions(+), 1 deletions(-)
> create mode 100644 drivers/media/v4l2-core/v4l2-clk.c
> create mode 100644 include/media/v4l2-clk.h
>
> diff --git a/drivers/media/v4l2-core/Makefile b/drivers/media/v4l2-core/Makefile
> index 00f64d6..cb5fede 100644
> --- a/drivers/media/v4l2-core/Makefile
> +++ b/drivers/media/v4l2-core/Makefile
> @@ -5,7 +5,7 @@
> tuner-objs := tuner-core.o
>
> videodev-objs := v4l2-dev.o v4l2-ioctl.o v4l2-device.o v4l2-fh.o \
> - v4l2-event.o v4l2-ctrls.o v4l2-subdev.o
> + v4l2-event.o v4l2-ctrls.o v4l2-subdev.o v4l2-clk.o
> ifeq ($(CONFIG_COMPAT),y)
> videodev-objs += v4l2-compat-ioctl32.o
> endif
> diff --git a/drivers/media/v4l2-core/v4l2-clk.c b/drivers/media/v4l2-core/v4l2-clk.c
> new file mode 100644
> index 0000000..7d457e4
> --- /dev/null
> +++ b/drivers/media/v4l2-core/v4l2-clk.c
> @@ -0,0 +1,126 @@
> +/*
> + * V4L2 clock service

A like the name :-D

> + *
> + * 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.
> + */
> +
> +#include<linux/errno.h>
> +#include<linux/list.h>
> +#include<linux/module.h>
> +#include<linux/mutex.h>
> +#include<linux/string.h>
> +
> +#include<media/v4l2-clk.h>
> +#include<media/v4l2-subdev.h>
> +
> +static DEFINE_MUTEX(clk_lock);
> +static LIST_HEAD(v4l2_clk);

nit: how about naming this lists v4l2_clks ?

> +
> +struct v4l2_clk *v4l2_clk_get(struct v4l2_subdev *sd, const char *id)
> +{
> + struct v4l2_clk *clk = NULL;
> +
> + mutex_lock(&clk_lock);
> + if (!id) {
> + if (list_is_singular(&v4l2_clk)) {

Hmm, the clock list is global, why should we assume there will be only one
entry with NULL v4l2_clk::id ? It would be useful to not provide the per
subdev clock id when there is only one clock used per a sub-device, which
is a majority of cases AFAICT.

> + clk = list_entry(&v4l2_clk, struct v4l2_clk, list);
> + if (!strstr(sd->name, clk->dev_id))

Ok, then clk->dev_id is supposed to be a sub-string of sd->name,
looks good...

> + clk = ERR_PTR(-ENODEV);
> + } else {
> + clk = ERR_PTR(-EINVAL);
> + }
> + } else {
> + list_for_each_entry(clk,&v4l2_clk, list) {
> + if (!strcmp(id, clk->id)&&
> + !strcmp(sd->name, clk->dev_id))

but why we are doing a "strong" check here ? Couldn't the second strcmp()
be just strstr() ?

> + break;
> + }
> + if (&clk->list ==&v4l2_clk)
> + clk = ERR_PTR(-ENODEV);
> + }
> + mutex_unlock(&clk_lock);
> +
> + if (!IS_ERR(clk)&&
> + !try_module_get(clk->ops->owner))
> + clk = ERR_PTR(-ENODEV);
> +
> + return clk;
> +}
> +EXPORT_SYMBOL(v4l2_clk_get);
>
> +void v4l2_clk_put(struct v4l2_clk *clk)
> +{
> + if (!IS_ERR(clk))
> + module_put(clk->ops->owner);
> +}
> +EXPORT_SYMBOL(v4l2_clk_put);
> +
> +int v4l2_clk_enable(struct v4l2_clk *clk)
> +{
> + if (!clk->ops->enable)
> + return -ENOSYS;
> + return clk->ops->enable(clk);
> +}
> +EXPORT_SYMBOL(v4l2_clk_enable);
> +
> +void v4l2_clk_disable(struct v4l2_clk *clk)
> +{
> + if (clk->ops->disable)
> + clk->ops->disable(clk);
> +}
> +EXPORT_SYMBOL(v4l2_clk_disable);
> +
> +unsigned long v4l2_clk_get_rate(struct v4l2_clk *clk)
> +{
> + if (!clk->ops->get_rate)
> + return -ENOSYS;
> + return clk->ops->get_rate(clk);
> +}
> +EXPORT_SYMBOL(v4l2_clk_get_rate);
> +
> +int v4l2_clk_set_rate(struct v4l2_clk *clk, unsigned long rate)
> +{
> + if (!clk->ops->set_rate)
> + return -ENOSYS;
> + return clk->ops->set_rate(clk, rate);
> +}
> +EXPORT_SYMBOL(v4l2_clk_set_rate);
> +
> +struct v4l2_clk *v4l2_clk_register(const struct v4l2_clk_ops *ops,
> + const char *dev_name,
> + const char *name)
> +{
> + struct v4l2_clk *clk;
> +
> + if (!ops || !ops->owner || (!list_empty(&v4l2_clk)&& !name))

ops->owner can be null when the clock provider module is built-in, not
a loadable module. I actually hit this problem. ops->owner needs to be
removed and I think the clocks list check could be removed as well,
please see my comment above.

Also it might be useful to check if a particular clocks is already
registered, to make this more foolproof and easier to debug.

> + return ERR_PTR(-EINVAL);
> +
> + clk = kzalloc(sizeof(struct v4l2_clk), GFP_KERNEL);
> + if (!clk)
> + return ERR_PTR(-ENOMEM);
> +
> + clk->ops = ops;
> + clk->id = name;
> + clk->dev_id = dev_name;
> +
> + mutex_lock(&clk_lock);
> + list_add_tail(&clk->list,&v4l2_clk);
> + mutex_unlock(&clk_lock);
> +
> + return clk;
> +}
> +EXPORT_SYMBOL(v4l2_clk_register);
> +
> +void v4l2_clk_unregister(struct v4l2_clk *clk)
> +{
> + mutex_lock(&clk_lock);
> + list_del(&clk->list);
> + mutex_unlock(&clk_lock);
> +
> + kfree(clk);
> +}
> +EXPORT_SYMBOL(v4l2_clk_unregister);

I have reworked some of the functions found here while trying to use your
work with s3c-camif and ov9650 sensor drivers [1]. Please feel free to
take (part of) these changes, if there are any you agree with.

I planned to also rework s3c-camif and add asynchronous subdev registration
to it, but didn't quite managed to do it yet, it's going to be next step.

> diff --git a/include/media/v4l2-clk.h b/include/media/v4l2-clk.h
> new file mode 100644
> index 0000000..0c05ab3
> --- /dev/null
> +++ b/include/media/v4l2-clk.h
> @@ -0,0 +1,48 @@
> +/*
> + * V4L2 clock service
> + *
> + * 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.
> + *
> + * ATTENTION: This is a temporary API and it shall be replaced by the generic
> + * clock API, when the latter becomes widely available.
> + */
> +
> +#ifndef MEDIA_V4L2_CLK_H
> +#define MEDIA_V4L2_CLK_H
> +
> +#include<linux/list.h>
> +
> +struct module;
> +struct v4l2_subdev;
> +
> +struct v4l2_clk {
> + struct list_head list;
> + const struct v4l2_clk_ops *ops;
> + const char *dev_id;
> + const char *id;

I've found it helpful to add a

void *priv;

field here, so the clock provider module can use it as a cookie,
which can be passed in a call to v4l2_clk_register() and then
retrieved in the clock ops. I'm not sure if this could be replaced
with some container_of() magic.

> +};
> +
> +struct v4l2_clk_ops {
> + struct module *owner;
> + int (*enable)(struct v4l2_clk *clk);
> + void (*disable)(struct v4l2_clk *clk);
> + unsigned long (*get_rate)(struct v4l2_clk *clk);
> + int (*set_rate)(struct v4l2_clk *clk, unsigned long);
> +};
> +
> +struct v4l2_clk *v4l2_clk_register(const struct v4l2_clk_ops *ops,
> + const char *dev_name,
> + const char *name);
> +void v4l2_clk_unregister(struct v4l2_clk *clk);
> +struct v4l2_clk *v4l2_clk_get(struct v4l2_subdev *sd, const char *id);
> +void v4l2_clk_put(struct v4l2_clk *clk);
> +int v4l2_clk_enable(struct v4l2_clk *clk);
> +void v4l2_clk_disable(struct v4l2_clk *clk);
> +unsigned long v4l2_clk_get_rate(struct v4l2_clk *clk);
> +int v4l2_clk_set_rate(struct v4l2_clk *clk, unsigned long rate);
> +
> +#endif

[1] http://git.linuxtv.org/snawrocki/media.git/shortlog/refs/heads/s3c-camif-devel

Regards,
Sylwester