From: lhikichi@broadcom•com (Lori Hikichi)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH 2/2] ASoC: add core audio driver for Broadcom Cygnus SOC.
Date: Thu, 9 Apr 2015 19:06:22 -0700 [thread overview]
Message-ID: <5527301E.6070906@broadcom.com> (raw)
In-Reply-To: <20150408192309.GI6023@sirena.org.uk>
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.
next prev parent reply other threads:[~2015-04-10 2:06 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-31 3:16 [PATCH 0/2] Cygnus Audio Driver Scott Branden
2015-03-31 3:16 ` [PATCH 1/2] ASoC: cygnus-audio: adding device tree bindings Scott Branden
2015-03-31 5:58 ` Mark Brown
2015-04-02 18:47 ` Lori Hikichi
2015-03-31 7:00 ` Mark Brown
2015-03-31 7:26 ` [alsa-devel] " Lars-Peter Clausen
2015-04-02 22:44 ` Lori Hikichi
2015-03-31 3:16 ` [PATCH 2/2] ASoC: add core audio driver for Broadcom Cygnus SOC Scott Branden
2015-03-31 6:42 ` Mark Brown
2015-04-02 18:47 ` Lori Hikichi
2015-04-06 16:19 ` Mark Brown
2015-04-08 2:30 ` Lori Hikichi
2015-04-08 19:23 ` Mark Brown
2015-04-10 2:06 ` Lori Hikichi [this message]
2015-03-31 6:43 ` [PATCH 0/2] Cygnus Audio Driver Mark Brown
2015-04-03 19:33 ` Scott Branden
2015-04-06 9:58 ` Mark Brown
2015-04-08 2:28 ` Lori Hikichi
2015-04-08 18:54 ` Mark Brown
2015-04-11 3:06 ` Lori Hikichi
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