Re: [PATCH 05/10] clk: qcom: cpu-8996: move off the ACD leg while the PLL speeds up

[Konrad Dybcio]

On 10/19/26 3:41 PM, Dmitry Baryshkov wrote:
> The MSM8996 CPU clusters sometimes run at half the rate they report
> after going up between two frequencies above 600 MHz, and stay there
> until the cluster drops below 600 MHz. The PLL, the muxes and the rest
> of the cluster clock registers are all as expected: it is the ACD that
> latches at half the rate when the PLL is reprogrammed to a higher rate
> underneath it, and selecting the ACD leg again clears it. The driver
> only moves off the ACD leg when going below 600 MHz.
> 
> Like the vendor kernel, run from PLL/2 while the PLL goes up and return
> to the ACD leg afterwards. Point the SMUX at PLL/2 at probe time instead
> of XO, so that the cluster does not drop to 19.2 MHz on the first such
> switch.
> 
> Fixes: 03e342dc45c9 ("clk: qcom: Add CPU clock driver for msm8996")
> Reported-by: Christopher Obbard <christopher.obbard@xxxxxxxxxx>
> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxxxxxxxx>
> ---

LLM has the following comments:

For old rates above 1190.4 MHz the SMUX output is old/2, which
exceeds MAX_PLL_MAIN_FREQ (595.2 MHz). Downstream does not do this.
clock-cpu-8996.c acd_safe_freq() returns freq/2 only up to
1190.4 MHz and MAX_PLL_MAIN_FREQ above that, and pre_set_rate()
retunes the PLL to that value before the increase. The comment there
says the main output is what keeps voltage constraints satisfied,
which implies a 595.2 MHz limit on that output.

Up to 1190.4 MHz the two approaches are equivalent: both move the
cluster off the ACD leg and run it from the non-ACD path at half the
old rate. Above that they diverge.

Either clamp the SMUX choice to match acd_safe_freq(), or explain why
old/2 is safe above 1190.4 MHz. The second needs the hardware limit
for the non-ACD output, which is not in the patch or the cover letter.

Konrad