[alsa-devel] [GIT PULL FOR 2.6.39] Media controller and OMAP3 ISP driver

[Mauro Carvalho Chehab]

Em 06-03-2011 08:38, Laurent Pinchart escreveu:
> Hi Mauro,
> 
> On Sunday 06 March 2011 11:56:04 Mauro Carvalho Chehab wrote:
>> Em 05-03-2011 20:23, Sylwester Nawrocki escreveu:
>>
>> A somewhat unrelated question that occurred to me today: what happens when
>> a format change happens while streaming?
>>
>> Considering that some formats need more bits than others, this could lead
>> into buffer overflows, either internally at the device or externally, on
>> bridges that just forward whatever it receives to the DMA buffers (there
>> are some that just does that). I didn't see anything inside the mc code
>> preventing such condition to happen, and probably implementing it won't be
>> an easy job. So, one alternative would be to require some special CAPS if
>> userspace tries to set the mbus format directly, or to recommend userspace
>> to create media controller nodes with 0600 permission.
> 
> That's not really a media controller issue. Whether formats can be changed 
> during streaming is a driver decision. The OMAP3 ISP driver won't allow 
> formats to be changed during streaming. If the hardware allows for such format 
> changes, drivers can implement support for that and make sure that no buffer 
> overflow will occur.

Such issues is caused by having two API's that allow format changes, one that
does it device-based, and another one doing it subdev-based.

Ok, drivers can implementing locks to prevent such troubles, but, without
the core providing a reliable mechanism, it is hard to implement a
correct lock. 

For example, let's suppose that some driver is using mt9m111 subdev (I just picked 
one random sensor that supports lots of MBUS formats). There's nothing
there preventing a subdev call for it to change mbus format while streaming.
Worse than that, the sensor driver has no way to block it, as it doesn't
know that the bridge driver is streaming or not.

The code at subdev_do_ioctl() is just:

case VIDIOC_SUBDEV_S_FMT: {
        struct v4l2_subdev_format *format = arg;

        if (format->which != V4L2_SUBDEV_FORMAT_TRY &&
            format->which != V4L2_SUBDEV_FORMAT_ACTIVE)
                return -EINVAL;

        if (format->pad >= sd->entity.num_pads)
                return -EINVAL;
 
        return v4l2_subdev_call(sd, pad, set_fmt, subdev_fh, format);
}

So, mc core won't be preventing it.

So, I can't see how such subdev request would be implementing a logic to
return -EBUSY on those cases.

Mauro.