[PATCH 2/6] ASoC: samsung: i2s: configure PSR from sound card

[Padmanabhan Rajanbabu]

> -----Original Message-----
> From: Mark Brown [mailto:broonie at kernel.org]
> Sent: 21 October 2022 05:24 PM
> To: Padmanabhan Rajanbabu <p.rajanbabu at samsung.com>
> Cc: lgirdwood at gmail.com; robh+dt at kernel.org;
> krzysztof.kozlowski+dt at linaro.org; s.nawrocki at samsung.com;
> perex at perex.cz; tiwai at suse.com; pankaj.dubey at samsung.com;
> alim.akhtar at samsung.com; rcsekar at samsung.com;
> aswani.reddy at samsung.com; alsa-devel at alsa-project.org;
> devicetree at vger.kernel.org; linux-kernel at vger.kernel.org; linux-samsung-
> soc at vger.kernel.org
> Subject: Re: [PATCH 2/6] ASoC: samsung: i2s: configure PSR from sound card
> 
> On Fri, Oct 21, 2022 at 01:30:25PM +0530, Padmanabhan Rajanbabu wrote:
> 
> > We can overcome this scenario to an extent if we can get a flexibility
> > to Configure both PSR as well as RFS.
> 
> Why does it make sense for the machine driver to worry about this rather
> than having the I2S controller driver configure the clock tree?


                       ____________________________________________
_____           |                  __
| 
|         |	        |	             |   \
|
|CMU|	        |	             |     \
|
|FSD  |-  |---|-|--------->|       \        _________    _________
|
|___  |    |    | |op_clk0|         |      |               |     |
|             |
	  |    | |	             |MUX|----|  PSR       |----|  RFS
|--cdclk  |
	  |    | |              |         |      |_______|     |_______|
|
	  |    | |--------->|        /
|
	  |    |  op_clk1 |      /
|
	  |    | 	             |_ /
|
	  |    |___________________________________________|
	  |
	  |-----> To other FSD SoC Peripherals
	 
In FSD I2S, the clock source is not an independent source but a common clock
source being shared by many IPs in the same domain.

Changing the clock tree will impact other IPs in the domain as they are
dependent on the same source for functionality.

We can understand your point to bring the PSR changes under the I2S CPU DAI
driver by adding a separate compatible and data for the FSD SoC. But If we
take
the example of existing sound cards such as sound/soc/samsung/tm2_wm5110.c,
the op_clk is supplied via external audio pll to the controller and PLL
configuration
is taken care by the sound card. Since the configuration of PLL is more
specific to
the tm2 platform, it makes use of the flexibility of changing the RFS and
BFS
using the sysclk and clkdiv hooks provided by exynos7-i2s CPU DAI along with
PLL tuning for precise sampling frequency.

Similar to the above example, the choice of clock source under discussion is
not a
limitation of exynos7-i2s controller, but instead is a limitation on the FSD
SoC.
By using the proposed change, we can ensure that the exynos CPU DAI driver
is
giving additional hooks similar to existing hooks for BFS, RFS and CDCLK
direction
so that sound cards can use snd_soc_dai_set_sysclk and
snd_soc_dai_set_clkdiv
to customize the same.

An alternative approach is to use the cpu dai as bit clock and frame clock
consumer (i.e. in slave mode) so that codec can supply the MCLK to FSD for
playback and capture. But this will deviate from the actual usecase for FSD
SoC, where the CPU DAI is intended to function as master.