Re: [PATCH v8 03/36] media: subdev: add 'which' to subdev state

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

On 27/09/2021 03:48, Laurent Pinchart wrote:
> Hi Tomi,
> 
> On Thu, Sep 16, 2021 at 04:24:19PM +0300, Tomi Valkeinen wrote:
>> On 16/09/2021 16:07, Jacopo Mondi wrote:
>>
>>> Also note that operations like s_stream do not take a state as
>>> parameter. The driver has to fetch it from the subdev anyway
>>> (this in reply to the idea of having the active state as parameter vs
>>> retrieving it from the subdev if ACTIVE)
>>>
>>> While porting the R-Car drivers on top of this series I found myself
>>> in the need to (in the s_stream call chain)
>>>
>>> static int rcsi2_start_receiver(struct rcar_csi2 *priv)
>>> {
>>> 	const struct v4l2_subdev_state *state = priv->subdev.state;
>>> 	const struct v4l2_subdev_stream_configs *configs = &state->stream_configs;
>>>
>>>           ...
>>>
>>> 	/*
>>> 	 * Configure field handling inspecting the formats of the
>>> 	 * single sink pad streams.
>>> 	 */
>>> 	for (i = 0; i < configs->num_configs; ++i) {
>>> 		const struct v4l2_subdev_stream_config *config = configs->configs;
>>> 		if (config->pad != RCAR_CSI2_SINK)
>>> 			continue;
>>>
>>> 		if (config->fmt.field != V4L2_FIELD_ALTERNATE)
>>> 			continue;
>>>
>>> 		fld |= FLD_DET_SEL(1);
>>> 		fld |= FLD_FLD_EN(config->stream);
>>>
>>> 		/* PAL vs NTSC. */
>>> 		if (config->fmt.height == 240)
>>> 			fld |= FLD_FLD_NUM(0);
>>> 		else
>>> 			fld |= FLD_FLD_NUM(1);
>>> 	}
>>>
>>>           ...
>>>
>>> }
>>>
>>> Am I doing it wrong, or is this a case for the subdev to have to
>>> directly access sd->state ?
>>
>> In s_stream path you should:
>>
>> 	state = v4l2_subdev_lock_active_state(sd);
>>
>> 	<do the work with the state>
>>
>> 	v4l2_subdev_unlock_state(state);
>>
>> If you already have the state, e.g. in set_fmt:
>>
>> 	state = v4l2_subdev_validate_and_lock_state(sd, state);
>>
>> 	<do the work with the state>
>>
>> 	v4l2_subdev_unlock_state(state);
>>
>> Accessing the stream_configs directly is fine but not that nice. I did
>> think about some helpers, perhaps for_each_stream_config(), but I didn't
>> add that as I didn't have the need.
>>
>> There's v4l2_state_get_stream_format() which can be used in many cases,
>> but we probably need something else if you need to iterate over all the
>> configs.
> 
> I really like forcing drivers to call functions that will lock the
> state, at least until we can move the locks to the core (if ever). We
> should move the fields of v4l2_subdev that drivers are not supposed to
> access directly under a big PRIVATE comment.

Can you clarify what you mean here? Did you mean you like functions that 
will _check_ the lock? Or did you just mean that you like 
v4l2_subdev_lock_state() better than mutex_lock(state->lock)?

Well, in any case, I think my series does both =). I can add the private 
comment to subdev. In fact, at one time I did not have sd->state, but 
sd->_state, just to make sure no one accesses it.

  Tomi