Re: [yavta PATCH 1/1] cache maintenance skip feature

Sakari Ailus
2016-02-02T23:22:50Z

Hi Laurent,

On Sun, Jan 24, 2016 at 10:09:24PM +0200, Laurent Pinchart wrote:
> Hi Sakari,
>
> Thank you for the patch.
>
> On Saturday 23 January 2016 23:49:54 Sakari Ailus wrote:
> > From: Samu Onkalo <samu.onkalo@intel.com>
> >
> > Add options to skip cache maintenance.
> > --dqbuf-skip-cache
> > --qbuf-skip-cache
>
> I wonder whether we should add support for this feature to yavta when it
> hasn't really been exercised much on the kernel side. I've been pondering for
> some time whether cache management shouldn't be handled in an implicit way
> instead, by letting the kernel skip cache handling when no userspace mapping
> exists instead of passing flags explicitly. I'd like to discuss and experiment
> with that approach first.

This would essentially bind two things together: the buffer and what the
user can do with it.

Supposing that most of the time you'd pass a set of buffers between two
hardware devices but would sometimes access it using the CPU, you'd be
always forced to flush the cache. For instance, capture images from one
device and process them using another. If you wanted to save the captured
raw image occasionally, you'd be forced to flush the cache always,
independently of whether you touch the buffer or not.

Avoiding the cache flush is pretty low level functionality we don't have in
a lot of kernel APIs (anywhere else?) but I think there's a reason why the
flags were added and it's still there (albeit never got properly supported;
I have a patchset for that but it requires further work).

Well, one could argue that you can mmap and munmap the DMA-BUF buffers then,
but doing that takes time as well.

This is a test program. If we end up deprecating (or removing) the flags
from the videodev2.h we can always remove the support from yavta as well.
:-)

>
> > Signed-off-by: Samu Onkalo <samu.onkalo@intel.com>
> > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > ---
> > This patch depends on my earlier patch "Fix --data-prefix option
> > documentation".
> >
> > yavta.c | 16 ++++++++++++++++
> > 1 file changed, 16 insertions(+)
> >
> > diff --git a/yavta.c b/yavta.c
> > index b21c3b3..ec4acd6 100644
> > --- a/yavta.c
> > +++ b/yavta.c
> > @@ -73,6 +73,8 @@ struct device
> > unsigned int width;
> > unsigned int height;
> > uint32_t buffer_output_flags;
> > + uint32_t buffer_qbuf_flags;
> > + uint32_t buffer_dqbuf_flags;
> > uint32_t timestamp_type;
> >
> > unsigned char num_planes;
> > @@ -1010,6 +1012,8 @@ static int video_queue_buffer(struct device *dev, int
> > index, enum buffer_fill_mo }
> > }
> >
> > + buf.flags |= dev->buffer_qbuf_flags;
> > +
> > if (video_is_mplane(dev)) {
> > buf.m.planes = planes;
> > buf.length = dev->num_planes;
> > @@ -1662,6 +1666,7 @@ static int video_do_capture(struct device *dev,
> > unsigned int nframes, buf.memory = dev->memtype;
> > buf.length = VIDEO_MAX_PLANES;
> > buf.m.planes = planes;
> > + buf.flags = dev->buffer_dqbuf_flags;
> >
> > ret = ioctl(dev->fd, VIDIOC_DQBUF, &buf);
> > if (ret < 0) {
> > @@ -1781,6 +1786,7 @@ static void usage(const char *argv0)
> > printf("-u, --userptr Use the user pointers streaming
> method\n");
> > printf("-w, --set-control 'ctrl value' Set control 'ctrl' to
> 'value'\n");
> > printf(" --data-prefix Write portions of buffer data before
> > data_offset\n"); + printf(" --[d]qbuf-skip-cache Skip cache
> > maintenance\n"); printf(" --buffer-size Buffer size in bytes\n");
> > printf(" --enum-formats Enumerate formats\n");
> > printf(" --enum-inputs Enumerate inputs\n");
> > @@ -1814,6 +1820,8 @@ static void usage(const char *argv0)
> > #define OPT_PREMULTIPLIED 269
> > #define OPT_QUEUE_LATE 270
> > #define OPT_DATA_PREFIX 271
> > +#define OPT_DQBUF_NO_CACHE 272
> > +#define OPT_QBUF_NO_CACHE 273
> >
> > static struct option opts[] = {
> > {"buffer-size", 1, 0, OPT_BUFFER_SIZE},
> > @@ -1822,6 +1830,7 @@ static struct option opts[] = {
> > {"check-overrun", 0, 0, 'C'},
> > {"data-prefix", 0, 0, OPT_DATA_PREFIX},
> > {"delay", 1, 0, 'd'},
> > + {"dqbuf-skip-cache", 0, 0, OPT_DQBUF_NO_CACHE},
> > {"enum-formats", 0, 0, OPT_ENUM_FORMATS},
> > {"enum-inputs", 0, 0, OPT_ENUM_INPUTS},
> > {"fd", 1, 0, OPT_FD},
> > @@ -1838,6 +1847,7 @@ static struct option opts[] = {
> > {"offset", 1, 0, OPT_USERPTR_OFFSET},
> > {"pause", 0, 0, 'p'},
> > {"premultiplied", 0, 0, OPT_PREMULTIPLIED},
> > + {"qbuf-skip-cache", 0, 0, OPT_QBUF_NO_CACHE},
> > {"quality", 1, 0, 'q'},
> > {"queue-late", 0, 0, OPT_QUEUE_LATE},
> > {"get-control", 1, 0, 'r'},
> > @@ -2089,6 +2099,12 @@ int main(int argc, char *argv[])
> > case OPT_DATA_PREFIX:
> > dev.write_data_prefix = true;
> > break;
> > + case OPT_DQBUF_NO_CACHE:
> > + dev.buffer_dqbuf_flags |= V4L2_BUF_FLAG_NO_CACHE_INVALIDATE;
> > + break;
> > + case OPT_QBUF_NO_CACHE:
> > + dev.buffer_qbuf_flags |= V4L2_BUF_FLAG_NO_CACHE_INVALIDATE;
> > + break;
> > default:
> > printf("Invalid option -%c\n", c);
> > printf("Run %s -h for help.\n", argv[0]);
>

--
Regards,

Sakari Ailus
e-mail: sakari.ailus@iki.fi XMPP: sailus@retiisi.org.uk