Monday, July 20, 2009

GPU accelerated H.264 decoding

A while ago I bought a new camera and I had to learn that my dual core machine isn't able to play footage encoded in highest quality level in realtime (second highest quality works). Fortunately, ffplay can't do that either so it's not gmerlins fault.

H.264 decoding on the graphics card can be done with vdpau or vaapi. The former is Nvidia specific and libavcodec can use it for H.264. The latter is vendor independent (it can use vdpau as backend on nvidia cards) but H.264 decoding with vaapi is not supported by ffmpeg yet.

In principle I prefer vendor independent solutions, but since I need H.264 support and ATI cards suck anyway on Linux, I tried VDPAU first.

The implementation in my libavcodec video frontend was straightforward after studying the MPlayer source. The VDPAU codecs are completely separated from the other codecs. They can simply be selected e.g. with avcodec_find_decoder_by_name("h264_vdpau"). Then, one must supply callback functions for get_buffer, release_buffer and draw_horiz_band. That's because the rendering targets are no longer frames in memory but rather handles of data-structures on the GPU. See here and here to see the details.

After the decoding, the image data is copied to memory by calling VdpVideoSurfaceGetBitsYCbCr. This brings of course a severe slowdown. A much better way would be to keep the frames in graphics memory as long as possible. But this needs to be done in a much more generic way: Images can be VDPAU or VAAPI video surfaces, OpenGL textures or whatever. Implementing generic support for video frames, which are not in regular RAM, will be another project.

Wednesday, July 1, 2009

Linux serial port driver madness

Serial ports are almost extinct on desktop PCs because everyone uses USB or ethernet. But for many automation tasks, RS-232 connections are still state of the art. That's mostly because they are well standardized since the 60s and UART chips are cheap.

I wanted to write a program, which uses a serial port for controlling 2 stepper motors. I wanted to write it such that users have the least possible trouble. Especially during the startup phase, a dialog box should show all available serial ports and let the user select the right one.

And while trying to get all available serial ports, I stumbled across 3 nasty linux bugs:

Bug 1: Always 4 serial port devices
One of the intentions of udev was that only the devices, which are physically present on the system, appear as nodes in the /dev directory. A big step forward compared to the situation before. Unfortunately, the serial driver always creates /dev/ttyS[0-3]. The reason (forgot the link) is that the ports of some exotic multi I/O board aren't detected properly. So the fix was to get 1000s of users into trouble instead of just making one special module parameter for that board.

Bug 2: open() succeeds for nonexistant ports
The manual page of open(2) says, that if one tries to open a device node with no physical device is behind it, errno is set to ENODEV or ENXIO. In the case of a non existing serial port, a valid filedescriptor is returned

Bug 3: tcgetattr returns EIO
According to the glibc manual EIO is usually used for physical read/write errors. For regular files this error means, that you should backup your data because the disk is about to die. A physical read/write error can never happen on a device which is physically nonexistant.

The second best thing would be to return ENXIO (no such device or address). The best thing would be to return EBADF (bad file descriptor) because the open() call before would have returned -1 already.

At least tcgetattr() doesn't succeed like open() so it can be used for the detection routine below.

The solution
The good thing is that once there is a workaround, the problem is quickly forgotten. Here's mine (returns 1 if the port is a physically existant serial device, 0 else):
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>

int is_serial_port(const char * device)
int fd, ret = 1;
struct termios t;
fd = open(device, O_RDWR);

if(fd < 0)
return 0;

if(tcgetattr(fd, &t))
ret = 0;
return ret;

Of course this won't find ports, which are currently opened by another application.