Re: In-tree version of new FireWire drivers available

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Robert Crocombe wrote:
On 1/25/07, Pieter Palmers <[email protected]> wrote:
I'd like to make one note here:
We should have a way to use smaller DMA buffers than one page size. If I
remember correctly, the page size on my system is 4096 bytes, being 1024
quadlets. If we assume a 4 channel audio stream, this corresponds to 256
audio samples. This means that the controller generates an interrupt
every 256 samples, making that we can achieve a latency of 512 samples
at best. This is unacceptable in a pro-audio environment.

The current stack exhibits this problem, and I solve it by recalculating
the max packet size, based upon the stream composition (i.e. expected
packet size) and the requested audio buffer size, such that the
interrupts are generated at a high enough frequency.

I'm not a kernel hacker, but when looking through the code I had the
impression that smaller DMA buffers were possible (aren't smaller
buffers used in packet-per-buffer mode?).

I am using isochronous receive in RAW1394_DMA_PACKET_PER_BUFFER mode
because I am closing a simulation loop around the data that is
received/transmitted.  Just for giggles I cranked up a test
isochronous stream from a bus analyzer at 1kB per packet at 8kHz at
the S400 rate (i.e., one packet on each cycle start: 8MBps ), set the
machine up to listen, and was able to maintain 8kHz interrupts at ~12%
CPU utilization on a 2.8GHz Opteron.

   1744719 interrupts int 218.112 seconds is 7999.193 ints/sec

I wasn't doing anything with the data for this test, but I have had
the aforementioned sim running steady at a somewhat lower rate.  This
test ran under 2.6.20-rc5-rt10, but the more "productiony" system is
on 2.6.16-rt29.

So hopefully you can get markedly lower latencies.  Myself, I'm
tickled pink by the performance that can be achieved.

I don't really understand what you are trying to say here. The overhead
of running in RAW1394_DMA_PACKET_PER_BUFFER mode is only acceptable for
very small buffer sizes. Usually one packet consists of 8 to 32 frames
(depending on the framerate of the stream), a frame being one sample of
all audio channels.

Currently I prefer about 4 interrupts per period, as we need some slack
to cope with the variable amount of no-data packets. So the
RAW1394_DMA_PACKET_PER_BUFFER mode is needed only for buffer sizes of 32
frames (assuming 8 frames per packet). Higher buffer sizes should use
another mode, because otherwise we're burning CPU cycles for no good
reason (12% cpu load is a little too high for me). The most frequently
used buffer sizes are around 128 frames, so that would mean 16
interrupts per period (4 times too much).

The way I currently solve this is by using the BUFFERFILL mode, but I inform the kernel that I expect packets that are larger than what I will effectively receive. If you specify a max_packet_size of 4096/4 bytes, every 4 packets the DMA buffer will be full and an interrupt will be generated. Internally it's called buff_stride if I'm not mistaking.

But again, what exactly is your point in this message?

Pieter



-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux