[alsa-devel] [PATCH] ASoC: ssm2602: Fix ADC powerup sequencing

[Philipp Zabel]

Hi Rob,

On Thu, 2018-05-17 at 11:14 -0500, Rob Herring wrote:
> On Thu, May 17, 2018 at 8:30 AM, Marco Felsch <m.felsch at pengutronix.de> wrote:
> > From: Philipp Zabel <p.zabel at pengutronix.de>
> > 
> > According to the ssm2603 data sheet (control register sequencing), the
> > digital core should be activated only after all necessary bits in the
> > power register are enabled, and a delay determined by the decoupling
> > capacitor on the VMID pin has passed. If the digital core is activated
> > too early, or even before the ADC is powered up, audible artifacts
> > appear at the beginning of the recorded signal.
> > 
> > The digital core is also needed for playback, so when recording starts
> > it may already be enabled. This means we cannot get the power sequence
> > correct when we want to be able to start recording after playback.
> > 
> > As a workaround put the MIC mute switch into the DAPM routes. This
> > way we can keep the recording disabled until the MIC Bias has settled
> > and thus get rid of audible artifacts.
> > 
> > The delay we have to wait depends on a board specific capacitor
> > connected to the VMID pins, so make the delay configurable in the device
> > tree.
> > 
> > Signed-off-by: Philipp Zabel <p.zabel at pengutronix.de>
> > Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>
> > Signed-off-by: Marco Felsch <m.felsch at pengutronix.de>
> > ---
> >  .../devicetree/bindings/sound/adi,ssm2602.txt |  7 +++++
> >  sound/soc/codecs/ssm2602.c                    | 30 +++++++++++++++++--
> >  2 files changed, 35 insertions(+), 2 deletions(-)
> > 
> > diff --git a/Documentation/devicetree/bindings/sound/adi,ssm2602.txt b/Documentation/devicetree/bindings/sound/adi,ssm2602.txt
> > index 3b3302fe399b..9334d48c0462 100644
> > --- a/Documentation/devicetree/bindings/sound/adi,ssm2602.txt
> > +++ b/Documentation/devicetree/bindings/sound/adi,ssm2602.txt
> > @@ -11,9 +11,16 @@ Required properties:
> >    - reg : the I2C address of the device for I2C, the chip select
> >            number for SPI.
> > 
> > +Optional properties:
> > +
> > +  - startup-delay-us : delay between powering on and activating the digital
> > +                       core, determined by the decoupling capacitor C on the
> > +                      VMID pin: delay [µs] = C [µF] * 25000 / 3.5
> > +
> 
> We already have similarly defined property. Please reuse that. See mmc
> pwrseq binding.

Do you mean 'post-power-on-delay-ms' from 'mmc-pwseq-simple'?
It is documented as:

- post-power-on-delay-ms : Delay in ms after powering the card and
        de-asserting the reset-gpios (if any)

The startup delay here is not after powering the whole IC or deasserting
resets and before it can be used, but after powering up a specific part
of the codec (the ADC) and before unmuting the MIC input to the digital
core during start of decoding. With this in mind, do you still think the
property should be called the same as the mmc full-chip poweron delay?
If so, would it be acceptable to use post-power-on-delay-us to keep the
microsecond resolution?

thanks
Philipp