[PATCH v4 2/5] media: ov2640: add async probe function

Josh Wu josh.wu at atmel.com
Thu Dec 25 22:37:14 PST 2014

Hi, Guennadi

Thanks for the reply.
And Merry Christmas and happy new year.

On 12/25/2014 6:39 AM, Guennadi Liakhovetski wrote:
> Hi Josh,
> On Mon, 22 Dec 2014, Josh Wu wrote:
>> Hi, Guennadi
>> On 12/20/2014 6:16 AM, Guennadi Liakhovetski wrote:
>>> On Fri, 19 Dec 2014, Josh Wu wrote:
>>>> Hi, Guennadi
>>>> Thanks for the review.
>>>> On 12/19/2014 5:59 AM, Guennadi Liakhovetski wrote:
>>>>> Hi Josh,
>>>>> Thanks for your patches!
>>>>> On Thu, 18 Dec 2014, Josh Wu wrote:
>>>>>> To support async probe for ov2640, we need remove the code to get
>>>>>> 'mclk'
>>>>>> in ov2640_probe() function. oterwise, if soc_camera host is not probed
>>>>>> in the moment, then we will fail to get 'mclk' and quit the
>>>>>> ov2640_probe()
>>>>>> function.
>>>>>> So in this patch, we move such 'mclk' getting code to ov2640_s_power()
>>>>>> function. That make ov2640 survive, as we can pass a NULL (priv-clk)
>>>>>> to
>>>>>> soc_camera_set_power() function.
>>>>>> And if soc_camera host is probed, the when ov2640_s_power() is called,
>>>>>> then we can get the 'mclk' and that make us enable/disable soc_camera
>>>>>> host's clock as well.
>>>>>> Signed-off-by: Josh Wu <josh.wu at atmel.com>
>>>>>> ---
>>>>>> v3 -> v4:
>>>>>> v2 -> v3:
>>>>>> v1 -> v2:
>>>>>>      no changes.
>>>>>>     drivers/media/i2c/soc_camera/ov2640.c | 31
>>>>>> +++++++++++++++++++++----------
>>>>>>     1 file changed, 21 insertions(+), 10 deletions(-)
>>>>>> diff --git a/drivers/media/i2c/soc_camera/ov2640.c
>>>>>> b/drivers/media/i2c/soc_camera/ov2640.c
>>>>>> index 1fdce2f..9ee910d 100644
>>>>>> --- a/drivers/media/i2c/soc_camera/ov2640.c
>>>>>> +++ b/drivers/media/i2c/soc_camera/ov2640.c
>>>>>> @@ -739,6 +739,15 @@ static int ov2640_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 ov2640_priv *priv = to_ov2640(client);
>>>>>> +	struct v4l2_clk *clk;
>>>>>> +
>>>>>> +	if (!priv->clk) {
>>>>>> +		clk = v4l2_clk_get(&client->dev, "mclk");
>>>>>> +		if (IS_ERR(clk))
>>>>>> +			dev_warn(&client->dev, "Cannot get the mclk.
>>>>>> maybe
>>>>>> soc-camera host is not probed yet.\n");
>>>>>> +		else
>>>>>> +			priv->clk = clk;
>>>>>> +	}
>>>>>>       	return soc_camera_set_power(&client->dev, ssdd, priv->clk,
>>>>>> on);
>>>>>>     }
>> Just let me explained a little more details at first:
>> As my understanding, current the priv->clk is a v4l2_clk: mclk, which is a
>> wrapper clock in soc_camera.c.
>> it can make soc_camera to call camera host's clock_start() clock_stop().
>> As in ov2640, the real mck (pck1) is in ov2640 dt node (xvclk). So the camera
>> host's clock_start()/stop() only need to enable/disable his peripheral clock.
> I'm looking at the ov2640 datasheet. In the block diagram I only see one
> input clock - the xvclk. Yes, it can be supplied by the camera host
> controller, in which case it is natural for the camera host driver to own
> and control it, or it can be a separate clock device - either static or
> configurable. This is just a note to myself to clarify, that it's one and
> the same clock pin we're talking about.
> Now, from the hardware / DT PoV, I think, the DT should look like:
> a) in the ov2640 I2C DT node we should have a clock consumer entry,
> linking to a board-specific source.

That's what this patch series do right now.
In my patch 5/5 DT document said, ov2640 need a clock consumer which 
refer to the xvclk input clock.
And it is a required property.

> b) if the ov2640 clock is supplied by a camera host, its DT entry should
> have a clock source subnode, to which ov2640 clock consumer entry should
> link. The respective camera host driver should then parse that clock
> subnode and register the respective clock with the V4L2 framework, by
> calling v4l2_clk_register().
Ok, So in this case, I need to wait for the "mclk" in probe of ov2640 
driver. So that I can be compatible for the camera host which provide 
the clock source.

> c) if the ov2640 clock is supplied by a different clock source, the
> respective driver should parse it and also eventually call
> v4l2_clk_register().
> Implementing case (b) above is so far up to each individual (soc-camera)
> camera host driver. In soc-camera host drivers don't register V4L2 clocks
> themselves, as you correctly noticed, they just provide a .clock_start()
> and a .clock_stop() callbacks. The registration is done by the soc-camera
> core.
> If I understand correctly you have case (c). Unfortunately, this case
> isn't supported atm. I think, a suitable way to do this would be:
> (1) modify soc-camera to not register a V4L2 clock if the host doesn't
> provide the required callbacks.
> (2) hosts should recognise configurations, in which they don't supply the
> master clock to clients and not provide the callbacks then.
> (3) a separate driver should register a suitable V4L2 clock.
> Whereas I don't think we need to modify camera drivers. Their requesting
> of a V4L2 clock is correct as is.
> Some more fine-print: if the clock is supplied by a generic device, it
> would be wrong for it to register a V4L2 clock. It should register a
> normal CCF clock, and a separate V4L2 driver should create a V4L2 clock
> from it. This isn't implemented either and we've been talking about it for
> a while now...

I think I understand your point now.

>> That is the motivation I want ov2640 be probed even without "mclk".
> ov2640 can be used with other boards and camera hosts, not only your
> specific board. In other configurations your change will break the driver.
>>> Ok, think about this: you check whether priv->clk is set on each
>>> .s_power() call, which is already a bit awkward.
>> yes, it is.
>>> Such approach can be used
>>> when there's no other way to perform a one-time action, but here we have
>>> one. But never mind, that's not the main problem. If priv->clk isn't set,
>>> you try to acquire it. But during probing, when this function is called
>>> for the first time clock isn't available yet, but you still want to
>>> succeed probing. So, you just issue a warning and continue. But then later
>>> an application opens the camera, .s_power() is called again, but for some
>>> reason the clock might still be not available, and this time you should
>>> fail.
>>> But you don't, you succeed and then you'll fail somewhere later,
>>> presumably, with a timeout waiting for frames. Am I right?
>> if the clock (v4l2 clock: mclk) is not available, then, there is no camera
>> host available.
> This isn't true - from the hardware perspective. The clock can be supplied
> by a different source.
>> So the system should have no v4l2 device found.
>> I think in this case the application cannot call the camera sensor .s_power()
>> via v4l2 ioctl.
>> So the timeout case should not happened.
> No, sorry, I meant a physical clock, not aclock object. You can register
> the complete framework and try to use it, but if the physical clock isn't
> enabled, the camera sensor won't function correctly.
>>>>>> @@ -1078,21 +1087,21 @@ static int ov2640_probe(struct i2c_client
>>>>>> *client,
>>>>>>     	if (priv->hdl.error)
>>>>>>     		return priv->hdl.error;
>>>>>>     -	priv->clk = v4l2_clk_get(&client->dev, "mclk");
>>>>>> -	if (IS_ERR(priv->clk)) {
>>>>>> -		ret = PTR_ERR(priv->clk);
>>>>>> -		goto eclkget;
>>>>>> -	}
>>>>>> -
>>>>>>     	ret = ov2640_video_probe(client);
>>>>> The first thing the above ov2640_video_probe() function will do is call
>>>>> ov2640_s_power(), which will request the clock. So, by moving requesting
>>>>> the clock from ov2640_probe() to ov2640_s_power() doesn't change how
>>>>> probing will be performed, am I right?
>>>> yes, you are right. In this patch, the "mclk" will requested by
>>>> ov2640_s_power().
>>>> The reason why I put the getting "mclk" code from ov2640_probe() to
>>>> ov2640_s_power() is : as the "mclk" here is camera host's peripheral
>>>> clock.
>>>> That means ov2640 still can be probed properly (read ov2640 id) even no
>>>> "mclk". So when I move this code to ov2640_s_power(), otherwise the
>>>> ov2640_probe() will be failed or DEFER_PROBE.
>>>> Is this true for all camera host? If it's not true, then I think use
>>>> -EPROBE_DEFER would be a proper way.
>>> Sorry, not sure what your question is.
>> Sorry, I don't make me clear here.
>> My question should be: Are all the camera host's clock_start()/stop() only
>> operate their peripheral clock?
> Yes, that's all camera hosts have. They cannot operate other unrelated
> clock sources.
>>> And I'm not sure ov2640's registers
>>> can be accessed with no running clock.
>> No, it seems there is a misunderstanding here.
>> I didn't mean ov2640 can be probed without xvclk.
>> What I try to say is the ov2640 can be probed without camera host's peripheral
>> clock.
> Ok, then your patch will break the driver even earlier - when trying to
> access ov2640 registers without enabling the clock.
>>>    I think some camera sensors can do
>>> this, but I have no idea about this one. How did you verify? Is it
>>> mentioned in a datasheet? Or did you really disconnected (grounded) the
>>> sensor clock input and tried to access its reqisters?
>>> If you just
>>> verified, that it's working without requesting the clock, are you sure
>>> your clock output isn't running permanently all the time anyway?
>> I didn't verify the those method as I only probed the ov2640 without ISI
>> enabled. ISI peripheral clock is disabled and etc.
> Right, this means a separate (probably always-on) clock source is used on
> your specific board, but this doesn't have to be the case on all other
> boards, where ov2640 is used.
>>>>> Or are there any other patched,
>>>>> that change that, that I'm overseeing?
>>>>> If I'm right, then I would propose an approach, already used in other
>>>>> drivers instead of this one: return -EPROBE_DEFER if the clock isn't
>>>>> available during probing. See ef6672ea35b5bb64ab42e18c1a1ffc717c31588a
>>>>> for
>>>>> an example. Or did I misunderstand anything?
>> I can implement with your method. like in probe() function, request the
>> v4l2_clk "mclk", if failed then return -EPROBE_DEFER.
> Yes, please, I think this would be a correct solution.
According to my understanding, if I implement this way I can be 
compatible with the camera host which provide the xvclk for ov2640.

So I will prepare the next version with this way.
BTW: do you have any comment for the 1/5 patches?

Best Regards,
Josh Wu

>> But I remember you mentioned that you will remove the v4l2 clock in future.
>> See ff5430de commit message.
>> So I just want to not so depends on the v4l2_clk "mclk".
> This might or might not happen. We so far depend on it. And we might also
> keep it, just adding a CCF V4L2 clock driver to handle generic clock
> sources like in case (c) above.
> Thanks
> Guennadi
>> Best Regards,
>> Josh Wu
>>>> Actually months ago I already done a version of ov2640 patch which use
>>>> -EPROBE_DEFER way.
>>>> But now I think the ov2640 can be probed correctly without "mclk", so it
>>>> is no
>>>> need to return -EPROBE_DEFER.
>>>> And the v4l2 asyn API can handle the synchronization of host. So I prefer
>>>> to
>>>> use this way.
>>>> What do you think about this?
>>>> Best Regards,
>>>> Josh Wu
>>>>> Thanks
>>>>> Guennadi
>>>>>>     	if (ret) {
>>>>>> -		v4l2_clk_put(priv->clk);
>>>>>> -eclkget:
>>>>>> -		v4l2_ctrl_handler_free(&priv->hdl);
>>>>>> +		goto evideoprobe;
>>>>>>     	} else {
>>>>>>     		dev_info(&adapter->dev, "OV2640 Probed\n");
>>>>>>     	}
>>>>>>     +	ret = v4l2_async_register_subdev(&priv->subdev);
>>>>>> +	if (ret < 0)
>>>>>> +		goto evideoprobe;
>>>>>> +
>>>>>> +	return 0;
>>>>>> +
>>>>>> +evideoprobe:
>>>>>> +	v4l2_ctrl_handler_free(&priv->hdl);
>>>>>>     	return ret;
>>>>>>     }
>>>>>>     @@ -1100,7 +1109,9 @@ static int ov2640_remove(struct i2c_client
>>>>>> *client)
>>>>>>     {
>>>>>>     	struct ov2640_priv       *priv = to_ov2640(client);
>>>>>>     -	v4l2_clk_put(priv->clk);
>>>>>> +	v4l2_async_unregister_subdev(&priv->subdev);
>>>>>> +	if (priv->clk)
>>>>>> +		v4l2_clk_put(priv->clk);
>>>>>>     	v4l2_device_unregister_subdev(&priv->subdev);
>>>>>>     	v4l2_ctrl_handler_free(&priv->hdl);
>>>>>>     	return 0;
>>>>>> -- 
>>>>>> 1.9.1
>>>>> --
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