linux-kernel - Re: [PATCH v8 4/8] media: platform: Add Cedrus VPU decoder driver

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Date:   Thu, 6 Sep 2018 16:25:34 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     contact@...lk.fr,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        devicetree@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <linux-arm-kernel@...ts.infradead.org>,
        devel@...verdev.osuosl.org,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        thomas.petazzoni@...tlin.com, ayaka <ayaka@...lik.info>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Alexandre Courbot <acourbot@...omium.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        linux-sunxi@...glegroups.com
Subject: Re: [PATCH v8 4/8] media: platform: Add Cedrus VPU decoder driver

On Thu, Sep 6, 2018 at 4:01 PM Hans Verkuil <hverkuil@...all.nl> wrote:
>
> On 09/05/2018 06:29 PM, Paul Kocialkowski wrote:
> > Hi and thanks for the review!
> >
> > Le lundi 03 septembre 2018 à 11:11 +0200, Hans Verkuil a écrit :
> >> On 08/28/2018 09:34 AM, Paul Kocialkowski wrote:
> >>> +static int cedrus_request_validate(struct media_request *req)
> >>> +{
> >>> +   struct media_request_object *obj, *obj_safe;
> >>> +   struct v4l2_ctrl_handler *parent_hdl, *hdl;
> >>> +   struct cedrus_ctx *ctx = NULL;
> >>> +   struct v4l2_ctrl *ctrl_test;
> >>> +   unsigned int i;
> >>> +
> >>> +   list_for_each_entry_safe(obj, obj_safe, &req->objects, list) {
> >>
> >> You don't need to use the _safe variant during validation.
> >
> > Okay, I'll use the regular one then.
> >
> >>> +           struct vb2_buffer *vb;
> >>> +
> >>> +           if (vb2_request_object_is_buffer(obj)) {
> >>> +                   vb = container_of(obj, struct vb2_buffer, req_obj);
> >>> +                   ctx = vb2_get_drv_priv(vb->vb2_queue);
> >>> +
> >>> +                   break;
> >>> +           }
> >>> +   }
> >>
> >> Interesting question: what happens if more than one buffer is queued in the
> >> request? This is allowed by the request API and in that case the associated
> >> controls in the request apply to all queued buffers.
> >>
> >> Would this make sense at all for this driver? If not, then you need to
> >> check here if there is more than one buffer in the request and document in
> >> the spec that this is not allowed.
> >
> > Well, our driver was written with the (unformal) assumption that we
> > only deal with a pair of one output and one capture buffer. So I will
> > add a check for this at request validation time and document it in the
> > spec. Should that be part of the MPEG-2 PIXFMT documentation (and
> > duplicated for future formats we add support for)?
>
> Can you make a patch for vb2_request_has_buffers() in videobuf2-core.c
> renaming it to vb2_request_buffer_cnt() and returning the number of buffers
> in the request?
>
> Then you can call it here to check that you have only one buffer.
>
> And this has to be documented with the PIXFMT.
>
> Multiple buffers are certainly possible in non-codec scenarios (vim2m and
> vivid happily accept that), so this is an exception that should be
> documented and checked in the codec driver.

Hmm, isn't it still 1 buffer per 1 queue and just multiple queues
included in the request?

If we indeed allow multiple buffers for the same queue in a request,
we shouldn't restrict this on a per-driver basis. It's definitely not
a hardware limitation, since the driver could just do the same as if 2
requests with the same controls were given.

Best regards,
Tomasz