[alsa-devel] [PATCH 1/1] axi-i2s: set period size register

[Lars-Peter Clausen]

On 08/27/2018 06:22 PM, Luca Ceresoli wrote:
> Hi,
> 
> thanks for your feedback.
> 
> [Adding Michal Simek (Xilinx maintainer) in Cc]
> 
> On 27/08/2018 14:27, Lars-Peter Clausen wrote:
>> On 08/24/2018 06:04 PM, Luca Ceresoli wrote:
>>> The default value of the PERIOD_LEN register is 0 and results in
>>> axi-i2s keeping TLAST always asserted in its AXI Stream output.
>>>
>>> When the AXI Stream is sent to a Xilinx AXI-DMA, this results in the
>>> DMA generating an interrupt flood and ALSA produce a corrupted
>>> recording. This is because AXI-DMA raises an interrupt whenever TLAST
>>> is active.
>>>
>>> Fix by setting the PERIOD_LEN register as soon as the period is
>>> known. This way TLAST is emitted once per period, and the DMA raises
>>> interrupts correctly.
>>
>> The patch looks OK. But I'd prefer not to merge it if possible.
>>
>> We've done some early experiments with the Xilinx AXI-DMA, but it turned out
>> to be to unreliable and we've abandoned support for it. One of the more
>> critical issues was that you can't abort a DMA transfer. That means when
>> audio capture is stopped the DMA will halt, but not complete the current
>> transfer. Then when the next audio capture start the DMA will continue with
>> the previous transfer. The observed effect of this was that the system would
>> just crash randomly (Presumably due to memory corruption).
> 
> Strange. I have done many capture experiments with arecord and didn't
> run into such bad issues. I only have a much less serious problem
> (garbage or old samples in the first few buffers), but no crashes.
> 
> Michal, are you aware of these problems?
> 
>> Have you considered using the ADI AXI-DMAC? That should work just fine.
> 
> Not until today, because AXI-DMA is working here.
> 
> I'd like to better understand what's going on before changing an IP that
> is working. Do you have additional details about your setup? How do you
> run your tests?

This was 4-5 years ago. A AXI-DMA with both TX and RX connected to the
AXI-I2S.

It might be that back then I didn't have buffer prealloc enabled, so a
new DMA buffer gets allocated for each transfer. Then you end up with
use after free and the DMA overwriting freed (and maybe reused) memory.

It was bad enough that it was a lot easier to add PL330 support to the
I2S peripheral. Not using Xilinx DMA for anything anymore has saved me
from a lot of headache.