Hi Hans,
On Wed, Nov 15, 2017 at 7:12 PM, Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
> Hi Alexandre,
>
> On 15/11/17 10:38, Alexandre Courbot wrote:
>> Hi Hans!
>>
>> Thanks for the patchset! It looks quite good at first sight, a few comments and
>> questions follow though.
>>
>> On Monday, November 13, 2017 11:34:02 PM JST, Hans Verkuil wrote:
>>> From: Hans Verkuil <hans.verkuil@xxxxxxxxx>
>>>
>>> Hi Alexandre,
>>>
>>> This is a first implementation of the request API in the
>>> control framework. It is fairly simplistic at the moment in that
>>> it just clones all the control values (so no refcounting yet for
>>> values as Laurent proposed, I will work on that later). But this
>>> should not be a problem for codecs since there aren't all that many
>>> controls involved.
>>
>> Regarding value refcounting, I think we can probably do without it if we parse
>> the requests queue when looking values up. It may be more practical (having a
>> kref for each v4l2_ctrl_ref in a request sounds overkill to me), and maybe also
>> more predictible since we would have less chance of having dangling old values.
>>
>>> The API is as follows:
>>>
>>> struct v4l2_ctrl_handler *v4l2_ctrl_request_alloc(void);
>>>
>>> This allocates a struct v4l2_ctrl_handler that is empty (i.e. has
>>> no controls) but is refcounted and is marked as representing a
>>> request.
>>>
>>> int v4l2_ctrl_request_clone(struct v4l2_ctrl_handler *hdl,
>>> const struct v4l2_ctrl_handler *from,
>>> bool (*filter)(const struct v4l2_ctrl *ctrl));
>>>
>>> Delete any existing controls in handler 'hdl', then clone the values
>>> from an existing handler 'from' into 'hdl'. If 'from' == NULL, then
>>> this just clears the handler. 'from' can either be another request
>>> control handler or a regular control handler in which case the
>>> current values are cloned. If 'filter' != NULL then you can
>>> filter which controls you want to clone.
>>
>> One thing that seems to be missing is, what happens if you try to set a control
>> on an empty request? IIUC this would currently fail because find_ref() would
>> not be able to find the control. The value ref should probably be created in
>> that case so we can create requests with a handful of controls.
>
> Wasn't the intention that we never have an empty request but always clone?
> I.e. in your code the _alloc call is always followed by a _clone call.
>
> The reason I have a separate _alloc function is that you use that when you
> want to create a new control handler ('new request'). If the user wants to reuse an
> existing request, then _clone can be called directly on the existing handler.
>
>> Also, if you clone a handler that is not a request, I understand that all
>> controls will be deduplicated, creating a full-state copy? That could be useful,
>> but since this is the only way to make the current code work, I hope that the
>> current impossibility to set a control on an empty request is a bug (or misunderstanding from my part).
>
> I think it is a misunderstanding. Seen from userspace you'll never have an empty
> request.
It depends on what we want requests to be:
(1) A representation of what the complete state of the device should
be when processing buffers, or
(2) A set of controls to be applied before processing buffers.
IIUC your patchset currently implements (1): as we clone a request (or
the current state of the device), we create a new ref for every
control that is not inherited. And when applying the request, we
consider all these controls again.
There is the fact that on more complex pipelines, the number of
controls may be big enough that we may want to limit the number we
need to manage per request, but my main concern is that this will
probably disallow some use-cases that we discussed in Prague.
For instance, let's say I create a request for a camera sensor by
cloning the device's control_handler, and set a few controls like
exposure that I want to be used for a give frame. But let's also say
that I have another thread taking care of focus on the same device -
this thread uses some other input to constantly adjust the focus by
calling S_EXT_CTRLS directly (i.e. without a request). When my request
gets processed, the focus will be reset to whatever it was when I
cloned the request, which we want to avoid (if you remember, I
interrupted Laurent to ask whether this is the behavior we wanted, and
we all agreed it was).
Or maybe I am missing something in your implementation?
That does not change the fact that I keep working on codecs using the
current model, but I think we will want to address this. The simplest
way to do this would be to only create refs for controls that we
explicitly change (or when cloning a request and not the
state_handler). It would also have the added benefit of reducing the
number of refs. :)
Cheers,
Alex.
