Am Sonntag, den 06.03.2011, 12:42 +0100 schrieb Laurent Pinchart:
> Hi Alexey,
>
> On Sunday 06 March 2011 11:57:14 Alexey Fisher wrote:
> > Some webcams report wired dwMaxVideoFrameBufferSize size,
> > some times too big, some times too smole.
> >
> > I tested two webcams:
> > 046d:0809 Logitech, Inc. Webcam Pro 9000
> > 046d:0991 Logitech, Inc. QuickCam Pro for Notebooks
> > with fallowing results.
> >
> > The dwMaxVideoFrameBufferSize is not always the same. Reported
> > range is about betwene 4 and 10 bpp calculated with this formula:
> > bpp = (dwMaxVideoFrameBufferSize * 8) / (frame->wWidth * frame->wHeight)
> >
> > I tested also a real comression on this cameras. Comression
> > with noremal conditions (video chat) was about 1 bpp.
> > Worst compression was by captureing moving text on monitore
> > (lots of light, high fps) is about 3bpp.
> > So we can calculate predictable buffer size in driver, insted of
> > getting some random numers from the cam.
> >
> > Setting bpp to 4 for mjpeg should be safe.
>
> Can you but sure that it will be safe for *all* cameras ? I can't.
>
> The uvcvideo driver doesn't use the dwMaxVideoFrameBufferSize field but
> queries the device at runtime for the maximum frame size value (there's one
> exception to this though, the driver uses dwMaxVideoFrameBufferSize for
> pre-1.10 UVC devices that report a 0 frame size at runtime).
>
> The field is also totally unrelated to USB bandwidth management, the maximum
> frame size value is only used to allocate the V4L2 buffers.
Correct. Suddently i found it later. But i use
dwMaxVideoFrameBufferSize in uvc_fixup_video_ctrl to calculate
bundwidth (insted of bandwidth = frame->wWidth * frame->wHeight / 8 *
format->bpp).
The bandwidth do make a difference for usb.
>
> > Signed-off-by: Alexey Fisher <bug-tr...@fisher-privat.net>
> > ---
> > drivers/media/video/uvc/uvc_driver.c | 5 +++--
> > drivers/media/video/uvc/uvc_video.c | 3 ++-
> > 2 files changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/media/video/uvc/uvc_driver.c
> > b/drivers/media/video/uvc/uvc_driver.c index e41285a..5a74de0 100644
> > --- a/drivers/media/video/uvc/uvc_driver.c
> > +++ b/drivers/media/video/uvc/uvc_driver.c
> > @@ -347,7 +347,7 @@ static int uvc_parse_format(struct uvc_device *dev,
> > strlcpy(format->name, "MJPEG", sizeof format->name);
> > format->fcc = V4L2_PIX_FMT_MJPEG;
> > format->flags = UVC_FMT_FLAG_COMPRESSED;
> > - format->bpp = 0;
> > + format->bpp = 4;
> > ftype = UVC_VS_FRAME_MJPEG;
> > break;
> >
> > @@ -460,7 +460,8 @@ static int uvc_parse_format(struct uvc_device *dev,
> > * uncompressed formats this can be fixed by computing the
> > * value from the frame size.
> > */
> > - if (!(format->flags & UVC_FMT_FLAG_COMPRESSED))
> > + if (!(format->flags & UVC_FMT_FLAG_COMPRESSED) ||
> > + (format->fcc == V4L2_PIX_FMT_MJPEG))
> > frame->dwMaxVideoFrameBufferSize = format->bpp
> > * frame->wWidth * frame->wHeight / 8;
> >
> > diff --git a/drivers/media/video/uvc/uvc_video.c
> > b/drivers/media/video/uvc/uvc_video.c index 9a95a62..64bd1d6 100644
> > --- a/drivers/media/video/uvc/uvc_video.c
> > +++ b/drivers/media/video/uvc/uvc_video.c
> > @@ -114,7 +114,8 @@ static void uvc_fixup_video_ctrl(struct uvc_streaming
> > *stream,
> >
> > if (!(format->flags & UVC_FMT_FLAG_COMPRESSED) ||
> > (ctrl->dwMaxVideoFrameSize == 0 &&
> > - stream->dev->uvc_version < 0x0110)) {
> > + stream->dev->uvc_version < 0x0110) ||
> > + (format->fcc == V4L2_PIX_FMT_MJPEG)) {
> > pr_debug("%s: rewrite dwMaxVideoFrameSize=%u to %u.",
> > __func__, ctrl->dwMaxVideoFrameSize,
> > frame->dwMaxVideoFrameBufferSize);
>
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