[libcamera-devel] Video recording on PinePhone Re: libcamera on pinephone

[Pavel Machek]

Hi!

> > Wow. Ok, uncached explains things. Is it easy to switch to do read()
> > from /dev/videoX? That should be easy to do, not require ABI changes,
> > and should improve performance in this case...
> 
> read() is deprecated and won't help much as there will still be a copy
> reading from uncached memory. You can get the same result with a
> memcpy() of the whole frame in the application.

First, let me begin by saying that passing uncached memory to userland
is rude and maybe dangerous. There are some CPU errata, for
example. Placing futex in uncached memory might be fun. Userspace is
not really equipped to deal with uncached memory.

Second, no. If we do read(), it is simple interface, kernel will be
doing the copying, and it is free to do the usual magic to speed
things up. (I suppose simply working with memory as cached and using
flush_dcache_range). 

With current interface, I get a ~18fps with 1920x1080 images.

With read/write, I get 82fps when doing file-to-file copy (with
additional open/close syscalls that should not really be needed) and
145 fps when copying from /dev/null.

You mentioned that new interface for passing cacheable buffers would
be needed. That sounds like a lot of work but I'll gladly help with
testing.

If we start using read(), we will get speedup over current situation
-- I should be able to get 30fps on the pinephone -- and advantage is
that it is existing interface. All we need to do is to remove the
"deprecated" tag and implement it for our hardware.

Do you see any other short-term solutions?

Best regards,
								Pavel

PS: Benchmark attached. It is 190MB/sec on the pinephone; I kind of
expected more, but.. it crosses kernel-user boundary and we really
only need 60MB/sec for fully using all camera capabilities.

#include <stdio.h>
#include <fcntl.h>

/* cat /dev/zero | head -c 2073600 > /tmp/delme.0 
 
mobian at mobian:~/g/tui/pp$ time ./syscalltest 
0.00user 1.19system 1.55 (0m1.556s) elapsed 76.82%CPU 
echo $? 
 
file-to-file: 
100 frames in 1.219 seconds -> 82fps. 
zero-to-file: 
in 0m0.690s -> 145fps. 
 */

#define	ZERO
#define SIZE 2073600

int main(void)
{
  int f1, f2;
  int i;
  static char buf[SIZE];

#ifdef ZERO
  f1 = open("/dev/zero", O_RDWR);
#endif
  f2 = open("/tmp/delme.1", O_CREAT | O_RDWR, 0777);

  for (i=0; i<100; i++) {
#ifndef ZERO
    f1 = open("/tmp/delme.0", O_RDWR);
#endif
    if (SIZE != read(f1, buf, SIZE))
      return 1;
#ifndef ZERO
    close(f1);
#endif
    if (SIZE != write(f2, buf, SIZE))
      return 2;
  }
  return 0;
}

-- 
People of Russia, stop Putin before his war on Ukraine escalates.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 195 bytes
Desc: not available
URL: <https://lists.libcamera.org/pipermail/libcamera-devel/attachments/20220715/0d41f052/attachment.sig>