From mboxrd@z Thu Jan  1 00:00:00 1970
From: Clemens Ladisch <clemens@ladisch.de>
Subject: Re: Questions about virtual ALSA driver (dummy),
 PortAudio and full-duplex drops
Date: Tue, 06 Aug 2013 15:04:13 +0200
Message-ID: <5200F44D.4070104@ladisch.de>
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To: David Henningsson <david.henningsson@canonical.com>
Cc: alsa-devel@alsa-project.org, Smilen Dimitrov <sd@imi.aau.dk>
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David Henningsson wrote:
> On 08/06/2013 12:59 PM, Clemens Ladisch wrote:
>>> On 2013-07-25 10:37, Clemens Ladisch wrote:
>>>> Your driver's .pointer callback must report the *actual* position at
>>>> which the hardware has finished reading from the buffer
>>
>> ... for a playback stream, or finished reading, for a capture stream.
>
> What if the pointer granularity is very coarse? E g, some hardware might
> only be able what period you're in (IIRC, I've seen this on the Tegra
> platform), rather than the actual sample. Would you recommend to report
> the latest period boundary in that case, or interpolating it with timers?

By reporting position x, the driver guarantees that the device has
finished reading (for a playback stream) before x, and that the
application is allowed to overwrite the buffer before x with new sample
data.

When the driver does not know the current position of the DMA
controller, it must report the last known 'safe' position (and set
SNDRV_PCM_INFO_BLOCK_TRANSFER).


Regards,
Clemens