[alsa-devel] UHCI dropped audio samples / DMA starvation fix

Monty Montgomery xiphmont at gmail.com
Wed Apr 4 23:35:30 CEST 2012

Responding to the rest inline, but unfortunately the point is moot;
after two days of running well in production and believing it fixed,
one of my boxes is once again starving UHCI DMA even with the patch.
So I did not in fact fix it.

> I've heard that the latest Intel architecture allows DMA to and from(?)
> the L3 cache.

FTR, the machines I'm testing on are Core Duo, Core2 Duo and first-gen Nehalem.

> | The Host Controller fetches the next entry from the Frame List when
> | the millisecond allotted to the current frame expires.
> ... so it is not actually possible to prefetch packets in a useful way.

That was my conclusion as well.  I could find no hooks to do so.  it also says:

|Host Controller fetches and decodes the Transfer Descriptor. Assuming
a Host-to-Function transaction, the
|Host Controller delays committing to the USB Transaction until the
FIFO fills to an appropriate “trigger point”.
|When this threshold is reached, the Host Controller can then begin
issuing the Transaction Token.

>> Unfortunately, the URB abstraction gives us no reliable way to do it
>> in the HCD, because other drivers don't necessarily pass a valid
>> CPU-side buffer address when using URB_NO_TRANSFER_DMA_MAP.
> But the USB API requires this (any HCD might use PIO or use its own
> double-buffering scheme).

Oh, it does?  I hadn't been able to come to that conclusion from the
docs I read.  If so, then that's good.

Anyway, going to keep trying.


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