Re: [alsa-devel] [PATCH] ASoC: fsl_ssi: call _fsl_ssi_set_dai_fmt() just once in AC'97 mode

[Nicolin Chen <nicoleotsuka@gmail.com>]

On Tue, Nov 21, 2017 at 01:32:09AM +0100, Maciej S. Szmigiero wrote:
> On 21.11.2017 01:00, Nicolin Chen wrote:
> > On Mon, Nov 20, 2017 at 11:13:45PM +0100, Maciej S. Szmigiero wrote:
> (..)
> >> We need to make sure, however, that only proper channel slots are enabled
> >> at playback start time since some AC'97 CODECs (like VT1613) were observed
> >> requesting via SLOTREQ (and so enabling at SSI) spurious ones just after
> >> an AC'97 link is started but before the CODEC is configured by its driver.
> > 
> > I don't really understand this part. Why do we need to *make sure*
> > and set SACCDIS and SACCEN again since they're initialized already> 
> > Could you please elaborate a bit more?
> 
> SACCDIS and SACCEN aren't just normal registers.
> They are a way to disable or enable bits in SACCST register - writing
> a '1' bit at some position in SACCDIS unsets the relevant bit in SACCST,
> writing a '1' bit at some position in SACCEN sets the relevant bit in
> SACCST.
> 
> But a bit in SACCST can also be set (and so an AC'97 channel slot
> enabled) if a CODEC sets the relevant bit in its SLOTREQ request (it is
> enough if this bit was set in SLOTREQ just once, bits in SACCST are
> 'sticky').

This makes sense now. It could be mentioned a bit in the commit log
as well.

> If an extra slot gets enabled that's a disaster for playback because some
> of normal left or right channel samples are instead redirected to this
> extra slot.
> 
> Unfortunately, the VT1613 codec on UDOO board (which is the only user of
> fsl_ssi driver in the AC'97 mode) is a bit broken and likes to (sometimes)
> set extra bits in its SLOTREQ request - I've confirmed this on two boards
> bought a few months apart directly from UDOO shop.
> 
> A workaround implemented in fsl-asoc-card of setting an appropriate
> CODEC register so that slots 3/4 (the normal stereo playback slots) are
> used for S/PDIF seems to mostly fix this issue but since this codec is so
> untrustworthy then:

If this move is also a workaround, probably it'd be better to have
an fsl_ssi_ac97_xxx_war() function with some comments/descriptions,
and include it in the config(). Since it doesn't hurt to set these
registers again, I am personally fine with that. But it needs to be
clear -- otherwise, people may try to remove it later with a change
like: Removing redundant SACCEN/SACCDIS settings since they're done
during probe().