Re: "media: vb2: add (un)prepare_streaming queue ops" causes vb2_queue->streaming to be set earlier ?

[Hans Verkuil <hverkuil-cisco@xs4all.nl>]

Hi Hans,

On 07/12/2022 22:23, Hans de Goede wrote:
> Hi Hans,
> 
> I just fixed a crash in the out of tree driver which is caused by:
> 
> a10b21532574 ("media: vb2: add (un)prepare_streaming queue ops")
> 
> I know that breaking out of tree code is fine / not our problem,
> but I wonder if this maybe affects some in tree code too ?
> 
> The problem with the out of tree driver is that it
> used vb2_queue->streaming inside the buf_queue() callback
> to determine if the buffer was being pre-queued (from
> vb2_start_streaming() calling __enqueue_in_driver()) before
> the start_streaming queue-op is called.
> 
> Or if it was being queued after the start_streaming queue-op
> has been called.
> 
> With vb2_queue->streaming now being set before the
>  __enqueue_in_driver() calls in vb2_start_streaming() this
> broke and it went down the route to see if there was
> space available in the hw-queue which NULL pointer deref-ed
> because the hw-queue had not been setup yet (more or less).
> 
> This is fixed now as far as the ipu6 driver is concerned,
> but I wonder if we may not have in tree drivers making
> similar assumptions about vb2_queue->streaming in their
> buf_queue() callbacks ?

I checked, and there is nobody else that's doing that.

It's definitely wrong to use q->streaming like that, it was never
meant as a check for whether or not the buffer was pre-queued.

Thanks for the heads-up, though! It is a real change in vb2 behavior
that I hadn't realized could cause problems.

It's a good change, since having buf_queue called when q->streaming is
false makes no sense. However, you can use q->start_streaming_called
instead if for some reason a driver needs to know.

Regards,

	Hans