Re: [PATCH 2/2] ASoC: add core audio driver for Broadcom Cygnus SOC.

[Lori Hikichi <lhikichi-dY08KVG/lbpWk0Htik3J/w@public.gmane.org>]

On 15-04-08 12:23 PM, Mark Brown wrote:
> On Tue, Apr 07, 2015 at 07:30:57PM -0700, Lori Hikichi wrote:
>> On 15-04-06 09:19 AM, Mark Brown wrote:
> 
>>> OK, so that seems fragile - what happens if we're slightly late
>>> processing an interrupt or miss one entirely?  Most hardware has some
>>> way to read back the current position which tends to be more reliable,
>>> is that not an option here?
> 
>> The hardware updates a read pointer (rdaddr) which we feed to ALSA via the
>> ".pointer" hook. So yes, the hardware does have a register that tells us its
>> progress.  This routine (ringbuf_inc) actually updates a write pointer
>> (wraddr) which is a misnomer.  The write pointer doesn’t actually tell us
>> where we are writing to – ALSA keeps track of that. The wraddr only prevents
>> the hardware from reading past it.  We just use it, along with a low water
>> mark configuration register, to keep the periodic interrupts firing.  The
>> hardware is tracking the distance between rdaddr and wraddr and comparing
>> that to the low water mark.
> 
> The code has handling for both read and write so it's not just updating
> a write pointer.  Is there no flexibility in the hardware regarding
> interrupt generation?
> 
>> Being late processing the interrupt is okay since there are more samples to
>> read.  That is, it was only a low water mark interrupt and we have one
>> period of valid data still (we configure low water to be one period).
>> Missing an interrupt is okay since the hardware will just stop reading from
>> buffer when rdaddr has hit wraddr.
> 
> Stopping if we miss an interrupt is precisely the sort of situation we
> want to avoid if we can - if the application is sufficiently far ahead
> of the hardware everything should continue to work fine.  The minimal
> period size appears to be very small so this is a potential issue, if an
> application tries to use many very small periods it's both more
> vulnerable to some other interrupt taking longer than might be desirable
> and likely that things would be fine as the application is hopefully
> more than one period ahead of where the hardware is at.
> 
> If the hardware is always going to halt at the end of the period there's
> not a huge amount we can do about this except possibly raise the minimum
> period if systems are running into trouble but if there's a way to avoid
> doing that then that would be even better.
> 
Makes sense, thanks for clarifying the desired behaviour, I will make the 
necessary adjustments to keep the hardware supplied with valid data even if 
interrupts are held off past a whole period.

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