Re: [PATCH 13/13] ASoC: ti: davinci-i2s: Opitonally drive DX pin during capture streams

[Bastien Curutchet <bastien.curutchet@xxxxxxxxxxx>]

Hi Péter,

On 3/21/24 19:31, Péter Ujfalusi wrote:
> Hi Bastien,
>
> On 3/21/24 17:14, Bastien Curutchet wrote:
> > > > I think the definition of the 'ti,drive-dx' is somehow odd. It allows
> > > > you to set it to 0x1234 and the DX pin will show 0x1234 when you capture
> > > > 32bit. If you capture 16bit then it will transmit 0x12 (or 0x34?), no?
> > > > If you have 4 channel capture then I won't speculate what will be on the
> > > > DX pin ;)
> > > >
> > > > Would not be better to say that the DX pin will be driven low or high
> > > > during capture _and_ disable the playback support?
> > >
> > > After some thinking, it might be still better to use the DX pin as GPIO
> > > and either have a custom machine driver which would handle it (set low
> > > when a capture trigger happens) or connect it in DAPM as a supply, bias
> > > or something and ASoC would handle it automagically.
> > >
> > > I think that would be cleaner in many ways. What do you think?
> > I agree, that would be cleaner. I ran a few tests to see if that would
> > work on my hardware. It doesn't ... So I looked back to the schematics
> > and found two reasons :
> >   * the DX pin needs to be in sync with the clock.
>
> I'm not sure what this means, sync with which clock?

Sorry, that was not very clear, I meant sync with the bit block that is
output on McBSP.CLKR pin.

> >   * the DX pin needs to be in a high-impedance state between two frames
> >     so a pull-up can drive it back up. Actually, the DX pin is also
> >     linked to the FSR pin so it provides the frame clock to the capture
> >     stream.
>
> Hrm, you are using the DX pin as FSR for the capture? Why not McBSP.FSR pin >

The McBSP.FSR pin is used for the capture but is driven by the McBSP.DX
pin. Both pins are linked together.

> Looking back to the patch, one thing stood out: you are setting the
> XDATDLY to 2.
> You have some sort of T1 framing on the bus? The pullup will make the DX
> line high in for the framing bit, right? > Or you simulate another FSR line with T1 framing DX?

Yes the goal is to simulate an FSR.

> The 'ti,drive-dx' sounds like a bad property for sure, you have T1
> framing and driving the DX to certain level.
> It is like DSP_A (1 bit delay) playing constant 0x2 ?
>
> Can you use aplay /dev/zero and a DT property to select T1 framing for
> the playback? Or that would be too coarse for timing the start of
> playback and capture?

That's a good idea, thank you. I'll try this and come back to you.


Best regards,
Bastien