[alsa-devel] [PATCHv2 5/7] ASoC: TWL6030: Add support for low-power mode

[Peter Ujfalusi]

On Tuesday 13 October 2009 23:15:45 ext Lopez Cruz, Misael wrote:
> Mark,
> 
> Mark Brown wrote:
> > On Fri, Sep 25, 2009 at 09:03:30PM -0500, Lopez Cruz, Misael wrote:
> >> In high-performance mode, codec can only work at 19.2 MHz from
> >> high-performance PLL. All paths supported in the codec can be used
> >> but audio can be streamed only at 48 kHz.
> >
> > The driver should also set constraints at runtime so that applications
> > know they shouldn't use an unsupported sample rate.  See wm8988 for an
> > example of doing this - it has some similar SYSCLK based restrictions.
> 
> In OMAP4, we can have the processor connected to Audio Backend and
> then to Phoenix (OMAP4->ABE->McPDM->TWL6030), but it's also possible
> to bypass the Audio Backend (OMAP4->McPDM->TWL6030).
> 
> The McPDM link requires data to be sent at twice the desired rate.
> In the first scenario (Audio Backend enabled), OMAP4 sends the data
> at 48kHz to ABE and it upsamples it to 96kHz and then send to TWL6030
> through McPDM. In the second scenario (Audio Backend bypassed),
> OMAP4 has to send the audio data at 96kHz, since no upsampling
> blocks are available now.

Sounds interesting. Does McPDM requires that the audio is actually upsampled, or 
it requires that the for example 48KHz audio data is sent with 96KHz where for 
example the actual audio data is only in the first half?

> 
> From TWL6030 CODEC driver pespective, the DAI link supported rate
> should be 48kHz or 96kHz? If the final goal is to explicitly represent
> all links in the system (ABE, Phoenix, McBSP, etc), then ABE link
> should expose somehow that it delives upsampled data. If that assumption
> is true, then TWL6030 DAI should support 96kHz.
> 
> A second concern is that runtime constraints will prevent rates
> supported by the system, when the constraints are only valid for the
> link. For example, if the TWL6030 CODEC adds a contraint for 96kHz,
> then it prevents the whole system to play 48kHz, which is supported
> when ABE is enabled since ABE upsamples it.
> 
> -Misa
> 
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