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Message-ID: <bd06f9b177d8caa7cd2748eb879f3f49c016fcce.camel@ndufresne.ca>
Subject: Re: [PATCH v3 05/15] media: mtk-vcodec: vdec: support stateless API
From:   Nicolas Dufresne <nicolas@ndufresne.ca>
To:     Alexandre Courbot <acourbot@chromium.org>,
        Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Cc:     Tiffany Lin <tiffany.lin@mediatek.com>,
        Andrew-CT Chen <andrew-ct.chen@mediatek.com>,
        Rob Herring <robh+dt@kernel.org>,
        Yunfei Dong <yunfei.dong@mediatek.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Hans Verkuil <hverkuil-cisco@xs4all.nl>,
        linux-media <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@lists.infradead.org>
Date:   Wed, 17 Mar 2021 11:09:36 -0400
In-Reply-To: <CAPBb6MUuefRgRfFFzHpJheo7mC034mXihwm5+++Dmkg=4Zgn_Q@mail.gmail.com>
References: <20210226100148.1663389-1-acourbot@chromium.org>
         <20210226100148.1663389-6-acourbot@chromium.org>
         <CAAEAJfDyHYGf+J6WDHchVATOvj5AJpO8dkAStiNGNobJ9Uz8Yw@mail.gmail.com>
         <CAPBb6MWtPuBNLpMK6U1-rfvTcPRnOEjJ06zTawxyD+pMSJ=uGQ@mail.gmail.com>
         <CAAEAJfDkS3E0YeXniBB9MuQJUnEruxGJGsLN3toVVd=7wLXGmQ@mail.gmail.com>
         <CAPBb6MUuefRgRfFFzHpJheo7mC034mXihwm5+++Dmkg=4Zgn_Q@mail.gmail.com>
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Le mercredi 17 mars 2021 à 12:13 +0900, Alexandre Courbot a écrit :
> On Tue, Mar 16, 2021 at 6:45 AM Ezequiel Garcia
> <ezequiel@vanguardiasur.com.ar> wrote:
> > 
> > Hi Alexandre,
> > 
> > On Mon, 15 Mar 2021 at 08:28, Alexandre Courbot <acourbot@chromium.org>
> > wrote:
> > > 
> > > Hi Ezequiel, thanks for the feedback!
> > > 
> > > On Thu, Mar 4, 2021 at 6:30 AM Ezequiel Garcia
> > > <ezequiel@vanguardiasur.com.ar> wrote:
> > > > 
> > > > Hello Alex,
> > > > 
> > > > Thanks for the patch.
> > > > 
> > > > On Fri, 26 Feb 2021 at 07:06, Alexandre Courbot <acourbot@chromium.org>
> > > > wrote:
> > > > > 
> > > > > From: Yunfei Dong <yunfei.dong@mediatek.com>
> > > > > 
> > > > > Support the stateless codec API that will be used by MT8183.
> > > > > 
> > > > > Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
> > > > > [acourbot: refactor, cleanup and split]
> > > > > Co-developed-by: Alexandre Courbot <acourbot@chromium.org>
> > > > > Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
> > > > > ---
> > > > >  drivers/media/platform/mtk-vcodec/Makefile    |   1 +
> > > > >  .../platform/mtk-vcodec/mtk_vcodec_dec.c      |  66 ++-
> > > > >  .../platform/mtk-vcodec/mtk_vcodec_dec.h      |   9 +-
> > > > >  .../mtk-vcodec/mtk_vcodec_dec_stateless.c     | 427
> > > > > ++++++++++++++++++
> > > > >  .../platform/mtk-vcodec/mtk_vcodec_drv.h      |   3 +
> > > > >  5 files changed, 503 insertions(+), 3 deletions(-)
> > > > >  create mode 100644 drivers/media/platform/mtk-
> > > > > vcodec/mtk_vcodec_dec_stateless.c
> > > > > 
> > > > [..]
> > > > 
> > > > > +
> > > > > +static const struct mtk_stateless_control mtk_stateless_controls[] =
> > > > > {
> > > > > +       {
> > > > > +               .cfg = {
> > > > > +                       .id = V4L2_CID_STATELESS_H264_SPS,
> > > > > +               },
> > > > > +               .codec_type = V4L2_PIX_FMT_H264_SLICE,
> > > > > +               .needed_in_request = true,
> > > > 
> > > > This "needed_in_request" is not really required, as controls
> > > > are not volatile, and their value is stored per-context (per-fd).
> > > > 
> > > > It's perfectly valid for an application to pass the SPS control
> > > > at the beginning of the sequence, and then omit it
> > > > in further requests.
> > > 
> > > If I understand how v4l2_ctrl_request_hdl_ctrl_find() works with
> > > requests, this boolean only checks that the control has been provided
> > > at least once, and not that it is provided with every request. Without
> > > it we could send a frame to the firmware without e.g. setting an SPS,
> > > which would be a problem.
> > > 
> > 
> > As Nicolas points out, in V4L2 controls have an initial value,
> > so no control can be unset.
> 
> I see. So I guess the expectation is that failure will occur later as
> the firmware reports it cannot decode properly (or returns a corrupted
> frame). Thanks for the precision.

That is identical to userspace passing bad values. We just don't want to force
userspace to pass controls that haven't changed. The control framework isn't
exactly free in CPU time.

> 
> > 
> > > > 
> > > > > +       },
> > > > > +       {
> > > > > +               .cfg = {
> > > > > +                       .id = V4L2_CID_STATELESS_H264_PPS,
> > > > > +               },
> > > > > +               .codec_type = V4L2_PIX_FMT_H264_SLICE,
> > > > > +               .needed_in_request = true,
> > > > > +       },
> > > > > +       {
> > > > > +               .cfg = {
> > > > > +                       .id = V4L2_CID_STATELESS_H264_SCALING_MATRIX,
> > > > > +               },
> > > > > +               .codec_type = V4L2_PIX_FMT_H264_SLICE,
> > > > > +               .needed_in_request = true,
> > > > > +       },
> > > > > +       {
> > > > > +               .cfg = {
> > > > > +                       .id = V4L2_CID_STATELESS_H264_DECODE_PARAMS,
> > > > > +               },
> > > > > +               .codec_type = V4L2_PIX_FMT_H264_SLICE,
> > > > > +               .needed_in_request = true,
> > > > > +       },
> > > > > +       {
> > > > > +               .cfg = {
> > > > > +                       .id = V4L2_CID_MPEG_VIDEO_H264_PROFILE,
> > > > > +                       .def = V4L2_MPEG_VIDEO_H264_PROFILE_MAIN,
> > > > > +                       .max = V4L2_MPEG_VIDEO_H264_PROFILE_HIGH,
> > > > > +                       .menu_skip_mask =
> > > > > +                              
> > > > > BIT(V4L2_MPEG_VIDEO_H264_PROFILE_BASELINE) |
> > > > > +                              
> > > > > BIT(V4L2_MPEG_VIDEO_H264_PROFILE_EXTENDED),
> > > > > +               },
> > > > > +               .codec_type = V4L2_PIX_FMT_H264_SLICE,
> > > > > +       },
> > > > > +       {
> > > > > +               .cfg = {
> > > > > +                       .id = V4L2_CID_STATELESS_H264_DECODE_MODE,
> > > > > +                       .min =
> > > > > V4L2_STATELESS_H264_DECODE_MODE_FRAME_BASED,
> > > > > +                       .def =
> > > > > V4L2_STATELESS_H264_DECODE_MODE_FRAME_BASED,
> > > > > +                       .max =
> > > > > V4L2_STATELESS_H264_DECODE_MODE_FRAME_BASED,
> > > > > +               },
> > > > > +               .codec_type = V4L2_PIX_FMT_H264_SLICE,
> > > > > +       },
> > > > > +};
> > > > 
> > > > Applications also need to know which V4L2_CID_STATELESS_H264_START_CODE
> > > > the driver supports. From a next patch, this case seems to be
> > > > V4L2_STATELESS_H264_START_CODE_ANNEX_B.
> > > 
> > > Indeed - I've added the control, thanks for catching this!
> > > 
> > > > 
> > > > > +#define NUM_CTRLS ARRAY_SIZE(mtk_stateless_controls)
> > > > > +
> > > > > +static const struct mtk_video_fmt mtk_video_formats[] = {
> > > > > +       {
> > > > > +               .fourcc = V4L2_PIX_FMT_H264_SLICE,
> > > > > +               .type = MTK_FMT_DEC,
> > > > > +               .num_planes = 1,
> > > > > +       },
> > > > > +       {
> > > > > +               .fourcc = V4L2_PIX_FMT_MM21,
> > > > > +               .type = MTK_FMT_FRAME,
> > > > > +               .num_planes = 2,
> > > > > +       },
> > > > > +};
> > > > > +#define NUM_FORMATS ARRAY_SIZE(mtk_video_formats)
> > > > > +#define DEFAULT_OUT_FMT_IDX    0
> > > > > +#define DEFAULT_CAP_FMT_IDX    1
> > > > > +
> > > > > +static const struct mtk_codec_framesizes mtk_vdec_framesizes[] = {
> > > > > +       {
> > > > > +               .fourcc = V4L2_PIX_FMT_H264_SLICE,
> > > > > +               .stepwise = {
> > > > > +                       MTK_VDEC_MIN_W, MTK_VDEC_MAX_W, 16,
> > > > > +                       MTK_VDEC_MIN_H, MTK_VDEC_MAX_H, 16,
> > > > > +               },
> > > > > +       },
> > > > > +};
> > > > > +
> > > > > +#define NUM_SUPPORTED_FRAMESIZE ARRAY_SIZE(mtk_vdec_framesizes)
> > > > > +
> > > > > +static void mtk_vdec_stateless_set_dst_payload(struct mtk_vcodec_ctx
> > > > > *ctx,
> > > > > +                                              struct vdec_fb *fb)
> > > > > +{
> > > > > +       struct mtk_video_dec_buf *vdec_frame_buf =
> > > > > +               container_of(fb, struct mtk_video_dec_buf,
> > > > > frame_buffer);
> > > > > +       struct vb2_v4l2_buffer *vb = &vdec_frame_buf->m2m_buf.vb;
> > > > > +       unsigned int cap_y_size = ctx-
> > > > > >q_data[MTK_Q_DATA_DST].sizeimage[0];
> > > > > +
> > > > > +       vb2_set_plane_payload(&vb->vb2_buf, 0, cap_y_size);
> > > > > +       if (ctx->q_data[MTK_Q_DATA_DST].fmt->num_planes == 2) {
> > > > > +               unsigned int cap_c_size =
> > > > > +                       ctx->q_data[MTK_Q_DATA_DST].sizeimage[1];
> > > > > +
> > > > > +               vb2_set_plane_payload(&vb->vb2_buf, 1, cap_c_size);
> > > > > +       }
> > > > > +}
> > > > > +
> > > > > +static struct vdec_fb *vdec_get_cap_buffer(struct mtk_vcodec_ctx
> > > > > *ctx,
> > > > > +                                          struct vb2_v4l2_buffer
> > > > > *vb2_v4l2)
> > > > > +{
> > > > > +       struct mtk_video_dec_buf *framebuf =
> > > > > +               container_of(vb2_v4l2, struct mtk_video_dec_buf,
> > > > > m2m_buf.vb);
> > > > > +       struct vdec_fb *pfb = &framebuf->frame_buffer;
> > > > > +       struct vb2_buffer *dst_buf = &vb2_v4l2->vb2_buf;
> > > > > +
> > > > > +       pfb = &framebuf->frame_buffer;
> > > > > +       pfb->base_y.va = vb2_plane_vaddr(dst_buf, 0);
> > > > 
> > > > Are you sure you need a CPU mapping? It seems strange.
> > > > I'll comment some more on the next patch(es).
> > > 
> > > I'll answer on the next patch since this is where that mapping is being
> > > used.
> > > 
> > > > 
> > > > > +       pfb->base_y.dma_addr = vb2_dma_contig_plane_dma_addr(dst_buf,
> > > > > 0);
> > > > > +       pfb->base_y.size = ctx->q_data[MTK_Q_DATA_DST].sizeimage[0];
> > > > > +
> > > > > +       if (ctx->q_data[MTK_Q_DATA_DST].fmt->num_planes == 2) {
> > > > > +               pfb->base_c.va = vb2_plane_vaddr(dst_buf, 1);
> > > > > +               pfb->base_c.dma_addr =
> > > > > +                       vb2_dma_contig_plane_dma_addr(dst_buf, 1);
> > > > > +               pfb->base_c.size = ctx-
> > > > > >q_data[MTK_Q_DATA_DST].sizeimage[1];
> > > > > +       }
> > > > > +       mtk_v4l2_debug(1,
> > > > > +               "id=%d Framebuf  pfb=%p VA=%p Y_DMA=%pad C_DMA=%pad
> > > > > Size=%zx frame_count = %d",
> > > > > +               dst_buf->index, pfb,
> > > > > +               pfb->base_y.va, &pfb->base_y.dma_addr,
> > > > > +               &pfb->base_c.dma_addr, pfb->base_y.size,
> > > > > +               ctx->decoded_frame_cnt);
> > > > > +
> > > > > +       return pfb;
> > > > > +}
> > > > > +
> > > > > +static void vb2ops_vdec_buf_request_complete(struct vb2_buffer *vb)
> > > > > +{
> > > > > +       struct mtk_vcodec_ctx *ctx = vb2_get_drv_priv(vb->vb2_queue);
> > > > > +
> > > > > +       v4l2_ctrl_request_complete(vb->req_obj.req, &ctx->ctrl_hdl);
> > > > > +}
> > > > > +
> > > > > +static int fops_media_request_validate(struct media_request *mreq)
> > > > > +{
> > > > > +       const unsigned int buffer_cnt = vb2_request_buffer_cnt(mreq);
> > > > > +       struct mtk_vcodec_ctx *ctx = NULL;
> > > > > +       struct media_request_object *req_obj;
> > > > > +       struct v4l2_ctrl_handler *parent_hdl, *hdl;
> > > > > +       struct v4l2_ctrl *ctrl;
> > > > > +       unsigned int i;
> > > > > +
> > > > > +       switch (buffer_cnt) {
> > > > > +       case 1:
> > > > > +               /* We expect exactly one buffer with the request */
> > > > > +               break;
> > > > > +       case 0:
> > > > > +               mtk_v4l2_err("No buffer provided with the request");
> > > > > +               return -ENOENT;
> > > > > +       default:
> > > > > +               mtk_v4l2_err("Too many buffers (%d) provided with the
> > > > > request",
> > > > > +                            buffer_cnt);
> > > > > +               return -EINVAL;
> > > > > +       }
> > > > > +
> > > > > +       list_for_each_entry(req_obj, &mreq->objects, list) {
> > > > > +               struct vb2_buffer *vb;
> > > > > +
> > > > > +               if (vb2_request_object_is_buffer(req_obj)) {
> > > > > +                       vb = container_of(req_obj, struct vb2_buffer,
> > > > > req_obj);
> > > > > +                       ctx = vb2_get_drv_priv(vb->vb2_queue);
> > > > > +                       break;
> > > > > +               }
> > > > > +       }
> > > > > +
> > > > > +       if (!ctx) {
> > > > > +               mtk_v4l2_err("Cannot find buffer for request");
> > > > > +               return -ENOENT;
> > > > > +       }
> > > > > +
> > > > > +       parent_hdl = &ctx->ctrl_hdl;
> > > > > +
> > > > > +       hdl = v4l2_ctrl_request_hdl_find(mreq, parent_hdl);
> > > > > +       if (!hdl) {
> > > > > +               mtk_v4l2_err("Cannot find control handler for
> > > > > request\n");
> > > > > +               return -ENOENT;
> > > > > +       }
> > > > > +
> > > > > +       for (i = 0; i < NUM_CTRLS; i++) {
> > > > > +               if (mtk_stateless_controls[i].codec_type != ctx-
> > > > > >current_codec)
> > > > > +                       continue;
> > > > > +               if (!mtk_stateless_controls[i].needed_in_request)
> > > > > +                       continue;
> > > > > +
> > > > > +               ctrl = v4l2_ctrl_request_hdl_ctrl_find(hdl,
> > > > > +                                        
> > > > > mtk_stateless_controls[i].cfg.id);
> > > > > +               if (!ctrl) {
> > > > > +                       mtk_v4l2_err("Missing required codec
> > > > > control\n");
> > > > > +                       return -ENOENT;
> > > > > +               }
> > > > > +       }
> > > > > +
> > > > > +       v4l2_ctrl_request_hdl_put(hdl);
> > > > > +
> > > > > +       return vb2_request_validate(mreq);
> > > > > +}
> > > > > +
> > > > > +static void mtk_vdec_worker(struct work_struct *work)
> > > > > +{
> > > > > +       struct mtk_vcodec_ctx *ctx =
> > > > > +               container_of(work, struct mtk_vcodec_ctx,
> > > > > decode_work);
> > > > > +       struct mtk_vcodec_dev *dev = ctx->dev;
> > > > > +       struct vb2_v4l2_buffer *vb2_v4l2_src, *vb2_v4l2_dst;
> > > > > +       struct vb2_buffer *vb2_src;
> > > > > +       struct mtk_vcodec_mem *bs_src;
> > > > > +       struct mtk_video_dec_buf *dec_buf_src;
> > > > > +       struct media_request *src_buf_req;
> > > > > +       struct vdec_fb *dst_buf;
> > > > > +       bool res_chg = false;
> > > > > +       int ret;
> > > > > +
> > > > > +       vb2_v4l2_src = v4l2_m2m_next_src_buf(ctx->m2m_ctx);
> > > > > +       if (vb2_v4l2_src == NULL) {
> > > > > +               v4l2_m2m_job_finish(dev->m2m_dev_dec, ctx->m2m_ctx);
> > > > > +               mtk_v4l2_debug(1, "[%d] no available source buffer",
> > > > > ctx->id);
> > > > > +               return;
> > > > > +       }
> > > > > +
> > > > > +       vb2_v4l2_dst = v4l2_m2m_next_dst_buf(ctx->m2m_ctx);
> > > > > +       if (vb2_v4l2_dst == NULL) {
> > > > > +               v4l2_m2m_job_finish(dev->m2m_dev_dec, ctx->m2m_ctx);
> > > > > +               mtk_v4l2_debug(1, "[%d] no available destination
> > > > > buffer", ctx->id);
> > > > > +               return;
> > > > > +       }
> > > > > +
> > > > > +       vb2_src = &vb2_v4l2_src->vb2_buf;
> > > > > +       dec_buf_src = container_of(vb2_v4l2_src, struct
> > > > > mtk_video_dec_buf,
> > > > > +                                  m2m_buf.vb);
> > > > > +       bs_src = &dec_buf_src->bs_buffer;
> > > > > +
> > > > > +       mtk_v4l2_debug(3, "[%d] (%d) id=%d, vb=%p buf_info = %p",
> > > > > +                       ctx->id, src_buf->vb2_queue->type,
> > > > > +                       src_buf->index, src_buf, src_buf_info);
> > > > > +
> > > > > +       bs_src->va = vb2_plane_vaddr(vb2_src, 0);
> > > > > +       bs_src->dma_addr = vb2_dma_contig_plane_dma_addr(vb2_src, 0);
> > > > > +       bs_src->size = (size_t)vb2_src->planes[0].bytesused;
> > > > > +       if (!bs_src->va) {
> > > > > +               v4l2_m2m_job_finish(dev->m2m_dev_dec, ctx->m2m_ctx);
> > > > > +               mtk_v4l2_err("[%d] id=%d source buffer is NULL", ctx-
> > > > > >id,
> > > > > +                            vb2_src->index);
> > > > > +               return;
> > > > > +       }
> > > > > +
> > > > > +       mtk_v4l2_debug(3, "[%d] Bitstream VA=%p DMA=%pad Size=%zx
> > > > > vb=%p",
> > > > > +                       ctx->id, buf->va, &buf->dma_addr, buf->size,
> > > > > src_buf);
> > > > > +       /* Apply request controls. */
> > > > > +       src_buf_req = vb2_src->req_obj.req;
> > > > > +       if (src_buf_req)
> > > > > +               v4l2_ctrl_request_setup(src_buf_req, &ctx->ctrl_hdl);
> > > > > +       else
> > > > > +               mtk_v4l2_err("vb2 buffer media request is NULL");
> > > > > +
> > > > > +       dst_buf = vdec_get_cap_buffer(ctx, vb2_v4l2_dst);
> > > > > +       v4l2_m2m_buf_copy_metadata(vb2_v4l2_src, vb2_v4l2_dst, true);
> > > > > +       ret = vdec_if_decode(ctx, bs_src, dst_buf, &res_chg);
> > > > > +       if (ret) {
> > > > > +               mtk_v4l2_err(
> > > > > +                       " <===[%d], src_buf[%d] sz=0x%zx pts=%llu
> > > > > vdec_if_decode() ret=%d res_chg=%d===>",
> > > > > +                       ctx->id, vb2_src->index, bs_src->size,
> > > > > +                       vb2_src->timestamp, ret, res_chg);
> > > > > +               if (ret == -EIO) {
> > > > > +                       mutex_lock(&ctx->lock);
> > > > > +                       dec_buf_src->error = true;
> > > > > +                       mutex_unlock(&ctx->lock);
> > > > > +               }
> > > > > +       }
> > > > > +
> > > > > +       mtk_vdec_stateless_set_dst_payload(ctx, dst_buf);
> > > > > +
> > > > > +       v4l2_m2m_buf_done_and_job_finish(dev->m2m_dev_dec, ctx-
> > > > > >m2m_ctx,
> > > > > +               ret ? VB2_BUF_STATE_ERROR : VB2_BUF_STATE_DONE);
> > > > > +
> > > > > +       v4l2_ctrl_request_complete(src_buf_req, &ctx->ctrl_hdl);
> > > > > +}
> > > > > +
> > > > > +static void vb2ops_vdec_stateless_buf_queue(struct vb2_buffer *vb)
> > > > > +{
> > > > > +       struct mtk_vcodec_ctx *ctx = vb2_get_drv_priv(vb->vb2_queue);
> > > > > +       struct vb2_v4l2_buffer *vb2_v4l2 = to_vb2_v4l2_buffer(vb);
> > > > > +
> > > > > +       mtk_v4l2_debug(3, "[%d] (%d) id=%d, vb=%p",
> > > > > +                       ctx->id, vb->vb2_queue->type,
> > > > > +                       vb->index, vb);
> > > > > +
> > > > > +       mutex_lock(&ctx->lock);
> > > > > +       v4l2_m2m_buf_queue(ctx->m2m_ctx, vb2_v4l2);
> > > > > +       mutex_unlock(&ctx->lock);
> > > > > +       if (vb->vb2_queue->type != V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE)
> > > > > +               return;
> > > > > +
> > > > > +       mtk_v4l2_debug(3, "(%d) id=%d, bs=%p",
> > > > > +               vb->vb2_queue->type, vb->index, src_buf);
> > > > > +
> > > > > +       /* If an OUTPUT buffer, we may need to update the state */
> > > > > +       if (ctx->state == MTK_STATE_INIT) {
> > > > > +               ctx->state = MTK_STATE_HEADER;
> > > > > +               mtk_v4l2_debug(1, "Init driver from init to header.");
> > > > 
> > > > This state thing seems just something to make the rest
> > > > of the stateful-based driver happy, right?
> > > 
> > > Correct - if anything we should either use more of the state here
> > > (i.e. set the error state when relevant) or move the state entirely in
> > > the stateful part of the driver.
> > > 
> > > > 
> > > > Makes me wonder a bit if just splitting the stateless part to its
> > > > own driver, wouldn't make your maintenance easier.
> > > > 
> > > > What's the motivation for sharing the driver?
> > > 
> > > Technically you could do it both ways. Separating the driver would
> > > result in some boilerplate code and buffer-management structs
> > > duplication (unless we keep the shared part under another module - but
> > > in this case we are basically in the same situation as now). Also
> > > despite using different userspace-facing ABIs, MT8173 and MT8183
> > > follow a similar architecture and a similar firmware interface.
> > > Considering these similarities it seems simpler from an architectural
> > > point of view to have all the Mediatek codec support under the same
> > > driver. It also probably results in less code.
> > > 
> > > That being said, the split can probably be improved as you pointed out
> > > with this state variable. But the current split is not too bad IMHO,
> > > at least not worse than how the code was originally.
> > > 
> > > > 
> > > > > +       } else {
> > > > > +               mtk_v4l2_debug(3, "[%d] already init driver %d",
> > > > > +                               ctx->id, ctx->state);
> > > > > +       }
> > > > > +}
> > > > > +
> > > > > +static int mtk_vdec_flush_decoder(struct mtk_vcodec_ctx *ctx)
> > > > > +{
> > > > > +       bool res_chg;
> > > > > +
> > > > > +       return vdec_if_decode(ctx, NULL, NULL, &res_chg);
> > > > > +}
> > > > > +
> > > > > +static const struct v4l2_ctrl_ops mtk_vcodec_dec_ctrl_ops = {
> > > > > +       .g_volatile_ctrl = mtk_vdec_g_v_ctrl,
> > > > > +};
> > > > > +
> > > > > +static int mtk_vcodec_dec_ctrls_setup(struct mtk_vcodec_ctx *ctx)
> > > > > +{
> > > > > +       struct v4l2_ctrl *ctrl;
> > > > > +       unsigned int i;
> > > > > +
> > > > > +       v4l2_ctrl_handler_init(&ctx->ctrl_hdl, NUM_CTRLS);
> > > > > +       if (ctx->ctrl_hdl.error) {
> > > > > +               mtk_v4l2_err("v4l2_ctrl_handler_init failed\n");
> > > > > +               return ctx->ctrl_hdl.error;
> > > > > +       }
> > > > > +
> > > > > +       ctrl = v4l2_ctrl_new_std(&ctx->ctrl_hdl,
> > > > > +                               &mtk_vcodec_dec_ctrl_ops,
> > > > > +                               V4L2_CID_MIN_BUFFERS_FOR_CAPTURE,
> > > > > +                               0, 32, 1, 1);
> > > > > +       ctrl->flags |= V4L2_CTRL_FLAG_VOLATILE;
> > > > 
> > > > Hm, this volatile control for MIN_BUFFERS_FOR_CAPTURE seems
> > > > to return the DPB size. However, isn't this something userspace already
> > > > knows?
> > > 
> > > True, but that's also a control the driver is supposed to provide per
> > > the spec IIUC.
> > > 
> > 
> > I don't see the specification requiring this control. TBH, I'd just drop it
> > and if needed fix the application to support this as an optional
> > control.
> > 
> > In any case, stateless devices should just need 1 output and 1 capture
> > buffer.
> 
> Mmm, you're correct indeed, and checking with our user-space it does
> not rely on this control for stateless codecs. Moving this control to
> the stateful part of the driver.
> 
> 
> > 
> > You might dislike this redundancy, note that you can also get the minimum
> > required buffers through VIDIOC_REQBUFS, where the count
> > v4l2_requestbuffers.field is returned back to userspace with the
> > number of allocated buffers.
> > 
> > If you request just 1 buffer, and your driver needed 3, you should
> > get a 3 there (vb2_ops.queue_setup takes care of that).
> > 
> > Thanks,
> > Ezequiel