Re: [PATCH v6,04/24] v4l: add documentation for restricted memory flag

[Tomasz Figa]

On Wed, May 22, 2024 at 02:16:22PM +0300, Laurent Pinchart wrote:
> Hi Jefrey,
> 
> Thank you for the patch.
> 
> On Thu, May 16, 2024 at 08:20:42PM +0800, Yunfei Dong wrote:
> > From: Jeffrey Kardatzke <jkardatzke@xxxxxxxxxx>
> > 
> > Adds documentation for V4L2_MEMORY_FLAG_RESTRICTED.
> > 
> > Signed-off-by: Jeffrey Kardatzke <jkardatzke@xxxxxxxxxx>
> > Signed-off-by: Yunfei Dong <yunfei.dong@xxxxxxxxxxxx>
> > ---
> >  Documentation/userspace-api/media/v4l/buffer.rst | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/Documentation/userspace-api/media/v4l/buffer.rst b/Documentation/userspace-api/media/v4l/buffer.rst
> > index 52bbee81c080..807e43bfed2b 100644
> > --- a/Documentation/userspace-api/media/v4l/buffer.rst
> > +++ b/Documentation/userspace-api/media/v4l/buffer.rst
> > @@ -696,7 +696,7 @@ enum v4l2_memory
> >  
> >  .. _memory-flags:
> >  
> > -Memory Consistency Flags
> > +Memory Flags
> >  ------------------------
> >  
> >  .. raw:: latex
> > @@ -728,6 +728,14 @@ Memory Consistency Flags
> >  	only if the buffer is used for :ref:`memory mapping <mmap>` I/O and the
> >  	queue reports the :ref:`V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS
> >  	<V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS>` capability.
> > +    * .. _`V4L2-MEMORY-FLAG-RESTRICTED`:
> > +
> > +      - ``V4L2_MEMORY_FLAG_RESTRICTED``
> > +      - 0x00000002
> > +      - The queued buffers are expected to be in restricted memory. If not, an
> > +	error will be returned. This flag can only be used with ``V4L2_MEMORY_DMABUF``.
> > +	Typically restricted buffers are allocated using a restricted dma-heap. This flag
> > +	can only be specified if the ``V4L2_BUF_CAP_SUPPORTS_RESTRICTED_MEM`` is set.
> 
> Why is this flag needed ? Given that the usage model requires the V4L2
> device to be a dma buf importer, why would userspace set the
> V4L2_BUF_CAP_SUPPORTS_RESTRICTED_MEM flag and pass a non-restricted
> buffer to the device ?

Given that the flag is specified at REQBUF / CREATE_BUFS time, it's
actually useful to tell the driver the queue is operating in restricted
(aka secure) mode.

I suppose we could handle that at the time of a first QBUF, but that
would make the driver initialization and validation quite a bit of pain.
So I'd say that the design being proposed here makes things simpler and
more clear, even if it doesn't add any extra functionality.

> 
> The V4L2_BUF_CAP_SUPPORTS_RESTRICTED_MEM flag also needs to be
> documented in the relevant section, I don't think that's done in this
> series.
> 

+1

Best regards,
Tomasz

> >  
> >  .. raw:: latex
> >  
> 
> -- 
> Regards,
> 
> Laurent Pinchart