Re: [PATCH] ASoC: fsl_sai: Clean code for synchronize mode

On Tue, Aug 04, 2020 at 10:35:12AM +0800, Shengjiu Wang wrote:

On Tue, Aug 4, 2020 at 10:11 AM Nicolin Chen [off-list ref] wrote:

quoted

On Tue, Aug 04, 2020 at 09:39:44AM +0800, Shengjiu Wang wrote:

quoted

On Tue, Aug 4, 2020 at 5:57 AM Nicolin Chen [off-list ref] wrote:

quoted

On Mon, Aug 03, 2020 at 04:04:23PM +0800, Shengjiu Wang wrote:

quoted

quoted

quoted

clock generation. The TCSR.TE is no need to enabled when only RX
is enabled.

You are correct if there's only RX running without TX joining.
However, that's something we can't guarantee. Then we'd enable
TE after RE is enabled, which is against what RM recommends:

# From 54.3.3.1 Synchronous mode in IMX6SXRM
# If the receiver bit clock and frame sync are to be used by
# both the transmitter and receiver, it is recommended that
# the receiver is the last enabled and the first disabled.

I remember I did this "ugly" design by strictly following what
RM says. If hardware team has updated the RM or removed this
limitation, please quote in the commit logs.

There is no change in RM and same recommandation.

My change does not violate the RM. The direction which generates
the clock is still last enabled.

Using Tx syncing with Rx clock for example,
T1: arecord (non-stop) => set RE
T2: aplay => set TE then RE (but RE is already set at T1)

Anything that I am missing?

This is a good example.
We have used this change locally for a long time, so I think it is
safe to do this change, a little different with the recommandation.

Any reason for we have to go against the recommendation?

Previous code will enable TE and RE  together even for asynchronous
mode.
And for recommendation, previous code just consider the RX sync with
TX, but still violates the recommendation for TX sync with RX case.
So at least this new change is some kind of improvement.

Okay. Let's change it then. Please make sure to update/remove
those old comments in the trigger(). And it's probably better
to mention that what we do now is a bit different from RM:
	/*
	 * Enable the opposite direction for synchronous mode
	 * 1. Tx sync with Rx: only set RE for Rx; set TE & RE for Tx
	 * 2. Rx sync with Tx: only set TE for Tx; set RE & TE for Rx
	 *
	 * RM recommends to enable RE after TE for case 1 and to enable
	 * TE after RE for case 2, but we here may not always guarantee
	 * that happens: "arecord 1.wav; aplay 2.wav" in case 1 enables
	 * TE after RE, which is against what RM recommends but should
	 * be safe to do, judging by years of testing results.
	 */

Btw, do we need similar change for TRIGGER_STOP?