linux-kernel - Re: [RFC][PATCH 05/15] videobuf2: handle V4L2_FLAG_MEMORY_NON

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Message-ID: <CAAFQd5C=Zj7h5Xe1w=0czX5ge1Kh=2cj6yEkN6binPgmmpj6Wg@mail.gmail.com>
Date:   Tue, 28 Jan 2020 13:45:29 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Sakari Ailus <sakari.ailus@....fi>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Pawel Osciak <posciak@...omium.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 05/15] videobuf2: handle V4L2_FLAG_MEMORY_NON_CONSISTENT
 in REQBUFS

On Thu, Jan 23, 2020 at 8:08 PM Hans Verkuil <hverkuil@...all.nl> wrote:
>
> On 1/22/20 4:48 AM, Sergey Senozhatsky wrote:
> > On (20/01/22 11:18), Sergey Senozhatsky wrote:
> > [..]
> >>>> +    * -
> >>>> +      - __u32
> >>>>        - ``reserved``\ [1]
> >>>>        - A place holder for future extensions. Drivers and applications
> >>>> -  must set the array to zero.
> >>>> +  must set the array to zero, unless application wants to specify
> >>>> +        buffer management ``flags``.
> >>>
> >>> I think support for this flag should be signaled as a V4L2_BUF_CAP capability.
> >>> If the capability is not set, then vb2 should set 'flags' to 0 to preserve the
> >>> old 'Drivers and applications must set the array to zero' behavior.
> >>
> >> The patch set adds V4L2_BUF_CAP_SUPPORTS_CACHE_HINTS towards the end of the
> >> series, I guess I can shuffle the patches and change the wording here.
> >
> > Or I can add separate queue flag and V4L2_BUF_CAP:
> >
> > struct vb2_queue {
> > ...
> >       allow_cache_hints:1
> > +     allow_consistency_hints:1
> > ...
> > }
> >
> > and then have CAP_SUPPORTS_CACHE_HINTS/CAP_SUPPORTS_CONSISTENCY_HINTS.
>
> Don't these two go hand-in-hand? I.e. either neither are supported, or
> both are supported? If so, then one queue flag is sufficient.

Cache sync hints are already part of the standard UAPI, so I think
there isn't any capability bit needed for them. That said, they aren't
really tied to non-consistent MMAP buffers. Userspace using USERPTR
can also use them.

MMAP buffer consistency hint deserves a capability bit indeed.

Best regards,
Tomasz