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

From: Lori Hikichi
Date: Tue Apr 07 2015 - 22:30:42 EST

On 15-04-06 09:19 AM, Mark Brown wrote:
On Thu, Apr 02, 2015 at 11:47:18AM -0700, Lori Hikichi wrote:
On 15-03-30 11:42 PM, Mark Brown wrote:

+ tristate "SoC platform audio for Broadcom Cygnus chips"


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+static void ringbuf_inc(void __iomem *audio_io, bool is_playback,
+ const struct ringbuf_regs *p_rbuf)

So it looks like we're getting an interrupt per period and we have to
manually advance to the next one?


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.

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 the ring buffer when rdaddr has hit wraddr.
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