[PATCH v4 2/2] ASoC: fsl_ssi: add 20-bit sample format for AC'97 and use it for capture

[Maciej S. Szmigiero]

On 01.12.2017 00:53, Nicolin Chen wrote:
> On Thu, Nov 30, 2017 at 08:20:08PM +0100, Maciej S. Szmigiero wrote:
> 
>> In the AC'97 mode we have to differentiate two things:
>> 1) Bit width of the physical AC'97 interface ("AC-link"),
>> 2) Bit width of samples that are accepted during a playback and output
>> during a recording by the SSI (and, so by the sound card that it driven
>> by this CPU).
>>
>> Bit width of the physical AC'97 interface is fixed at 20 bits per sample
>> (both in playback and capture direction).
>>
>> Bit width of samples that are accepted (or produced) by the SSI could
>> be set in its STCCR and SRCCR registers (although in the AC'97 mode
>> only settings of either 16 or 20 bits are possible).
>> Each direction could be set independently from the other one.
> 
> I checked the reference manual regarding the AC97 configurations.
> It seems that AC97 sets SYNC bit in SCR register. However, unlike
> I2S which only uses STCCR for both TX and RX, AC97 does use STCCR
> and SRCCR separately. So bypassing SRCCR if SYNC bit is set isn't
> correct for AC97 cases.

I assume by 'SYNC' bit you mean the 'SYN' bit, not the 'SYNC_TX_FS' one.

You are right, we can't ignore SRCCR only because 'SYN' is set (at least
not in the AC'97 mode).
That's why the current driver doesn't do it this way, the
'symmetric_rates' flag is used instead.

> 
> I will clean up the driver a bit and I think the change would be
> highly related to AC97 code. So I'll later need you review/test.

OK.

>From my perspective it would be great if the whole cleanup was in one
series, so the whole testing doesn't need to be repeated per patch
(it involves a lot of manual work).

>> Regarding a sample rate in AC'97 mode its effective value isn't really
>> controlled by the CPU (that is, SSI), but by a CODEC since it is
>> the CODEC which tells the CPU when it should send a next sample for
>> playback and when a next capture sample is ready.
>> There are no problems if they are different (as long as the CODEC
>> supports this, naturally, but it's up to its driver to restrict the
>> sample rate space accordingly).
> 
> It's because CODEC drives the bit clock and framesync clock, isn't
> it? 

Strictly speaking, the frame sync is driven by the controller (SSI),
but it is simply the CODEC-provided bit clock divided by 256.
And the CODEC-provided bit clock is fixed at 12.288MHz by the AC'97
specs.

But every frame from CODEC also has 'TAG' bits which tell the
controller whether this frame contains valid capture samples or not.
If the capture sample rate currently programmed in CODEC is less
than 48kHz (the frame rate) it simply means that some of incoming
frames will contain 'TAG' bits indicating that these frames do not
contain valid capture samples (for example, if the capture rate is
24kHz then only half of the frames, on average, will be marked by CODEC
as containing valid capture samples).

The situation with playback is similar: the frame from CODEC also has
'SLOTREQ' bits which tell the controller if it should send playback
samples (and which) in the next frame - for example, if the playback
rate is 24kHz then in half of the frames, on average, the CODEC will
request playback samples.

Hope it is clear now.

> Could SSI act as the clock master side? It doesn't seem to be
> configurable to do so according to the reference manual though.

In the AC'97 mode?
I couldn't find in the AC'97 specs any reference to a mode of
operation in which the controller generates the bit clock.

Also, the SSI manual (from imx6q) states that in the AC'97 mode:
"TXDIR bit (SSI.STCR[5]) is forced to 0 internally to select external
generated bit clock".
This suggests it isn't possible to make the SSI generate the bit clock
in this mode of operation.

> 
>> A comment above "fsl,ssi-asynchronous" property says that it is about
>> whether "the RX and the TX clocks [are] locked".
>> They are on the physical AC'97 interface, but they aren't on the logical
>> playback / capture interface.
>> IMHO, this configurable property simply makes no sense in the AC'97
>> mode.
> 
> The property is more on the hardware level. And we should refer to
> the DT binding doc:
>     fsl,ssi-asynchronous:
>          If specified, the SSI is to be programmed in asynchronous
>          mode.  In this mode, pins SRCK, STCK, SRFS, and STFS must
>          all be connected to valid signals.  In synchronous mode,
>          SRCK and SRFS are ignored.  Asynchronous mode allows
>          playback and capture to use different sample sizes and
>          sample rates.  Some drivers may require that SRCK and STCK
>          be connected together, and SRFS and STFS be connected
>          together.  This would still allow different sample sizes,
>          but not different sample rates.
> 
> If AC97 doesn't need SRFS and SRCK at all, it's fine to ignore it.

Yes, the AC'97 mode doesn't use SRFS and SRCK at all.

> Thanks
> Nic

Best regards,
Maciej