RE: samsung: clk: re-parent MUX to OSCCLK at run-time

["Alim Akhtar" <alim.akhtar@samsung.com>]

Hi Tudor

> -----Original Message-----
> From: Tudor Ambarus <tudor.ambarus@linaro.org>
> Sent: Wednesday, March 6, 2024 8:50 AM
> To: Sylwester Nawrocki <s.nawrocki@samsung.com>; Chanwoo Choi
> <cw00.choi@samsung.com>; Alim Akhtar <alim.akhtar@samsung.com>
> Cc: Sam Protsenko <semen.protsenko@linaro.org>; Krzysztof Kozlowski
> <krzysztof.kozlowski@linaro.org>; linux-samsung-soc@vger.kernel.org;
> linux-clk@vger.kernel.org; linux-kernel@vger.kernel.org; linux-arm-kernel
> <linux-arm-kernel@lists.infradead.org>; Peter Griffin
> <peter.griffin@linaro.org>; André Draszik <andre.draszik@linaro.org>;
> William McVicker <willmcvicker@google.com>; kernel-team@android.com
> Subject: samsung: clk: re-parent MUX to OSCCLK at run-time
> 
> Hi,
> 
> Trying to get some feedback from the samsung experts. Please consider the
> following:
> 
>                          ---------------------------------------------
>                         |                                CMU_PERIC0   |
>                         |                                             |
>                         |  MUX_USI                                    |
>                         |                                             |
>                         |  |\                                         |
>               OSCCLK ---|->| \                                        |
>                         |  |  \                                       |
>                         |  | M |                                      |
>                         |  | U |--> DIV_CLK_PERIC0_USI*_ --> GATE_USI |
>                         |  | X |        (1 ~ 16)                      |
>                         |  |  /                                       |
> DIV_CLKCMU_PERIC0_IP ---|->| /                                        |
>     (1 ~ 16)          | |  |/                                         |
>                       | |                                             |
>                       | |                                             |
>                       | |  MUX_I3C                                    |
>                       | |                                             |
>                       | |  |\                                         |
>                       --|->| \                                        |
>                         |  |  \                                       |
>                         |  | M |                                      |
>                         |  | U |--> DIV_CLK_PERIC0_I3C --> GATE_I3C   |
>                         |  | X |                                      |
>                         |  |  /                                       |
>               OSCCLK ---|->| /                                        |
>                         |  |/                                         |
>                         |                                             |
>                          ---------------------------------------------
> 
> Is it fine to re-parent the MUX_USI from above to OSCCLK at run-time,

I am not aware of the exact SOC/HW you are working on. 
It depends on the CMU design about how to achieve low power mode and clock gating for an IP/Block. 

In theory and looking at your clock diagram above, it is ok to switch to OSCCLK  for MUX_USI.

If you can just use GATE_USI clock to clock gate USI IP, you will have a low power for USI (of course there will be a leakage current still drawn).
Is that what you want to achieve (low power mode)? Or you are looking to get lowest possible operating clock for USI IP?

You need to takecare about if that clock is being shared with any other IP,
so unless all the IPs which consume this clock, goes into idle state, you can avoid MUX_USI change to OSCCLK.


> during normal operation mode? Experimentally I determined that it's fine,
> but the datasheet that I'm reading mentions OSCCLK just in the low-power
> mode context:
> i/ CMU ... "Communicates with Power Management Unit (PMU) to stop
> clocks or switch OSC clock before entering a Low-Power mode to reduce
> power consumption by minimizing clock toggling".
> ii/ "All CMUs have MUXs to change the OSCCLK during power-down mode".
> 
> Re-parenting the MUX to OSCCLK allows lower clock rates for the USI blocks
> than the DIV_CLK_PERIC0_USI can offer. For a USI clock rate below
> 6.25 MHz I have to either reparent MUX_USI to OSCCLK, or to propagate the
> clock rate to the common divider DIV_CLKCMU_PERIC0_IP. I find the
> propagation to the common DIV less desirable as a low USI clock rate affects
> I3C by lowering its clock rate too. Worse, if the common bus divider is not
> protected (using CLK_SET_RATE_GATE), USI can lower the I3C clock rate
> without I3C noticing.
> 
> Either re-parenting the MUX_USI to OSCCLK, or propagating the clock rate to
> DIV_CLKCMU_PERIC0_IP allows the same clock ranges. The first with the
> benefit of not affecting the clock rate of I3C for USI clock rates below
> 6.25 MHz. Is it fine to re-parent MUX_USI to OSCCLK at run-time?
> 
> If no feedback is received I lean towards propagating the USI clock rate to the
> common divider, but by protecting it with CLK_SET_RATE_GATE.
> 
> Feel free to add in To: or Cc: whoever might be interested. Thanks, ta