linux-kernel - Re: [PATCH 1/4] interconnect: qcom: icc-rpm: Add AB/IB calculations coefficients

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Message-ID: <ZMFU-hFSOHLr3hFP@gerhold.net>
Date:   Wed, 26 Jul 2023 19:16:42 +0200
From:   Stephan Gerhold <stephan@...hold.net>
To:     Konrad Dybcio <konrad.dybcio@...aro.org>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Georgi Djakov <djakov@...nel.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] interconnect: qcom: icc-rpm: Add AB/IB calculations
 coefficients

On Wed, Jul 26, 2023 at 06:25:43PM +0200, Konrad Dybcio wrote:
> Presumably due to the hardware being so complex, some nodes (or busses)
> have different (usually higher) requirements for bandwidth than what
> the usual calculations would suggest.
> 

Weird. I just hope this was never abused to workaround other broken
configuration. A nice round ib_percent = 200 has mostly the same effect as

  - Doubling the requested peek bandwidth in the consumer driver (perhaps
    they were too lazy to fix the driver in downstream at some point)
  - Halving the node buswidth

It's probably hard to say for sure...

> Looking at the available downstream files, it seems like AB values are
> adjusted per-bus and IB values are adjusted per-node.
> With that in mind, introduce percentage-based coefficient struct members
> and use them in the calculations.
> 
> One thing to note is that downstream does (X%)*AB and IB/(Y%) which
> feels a bit backwards, especially given that the divisors for IB turn
> out to always be 25, 50, 200 making this a convenient conversion to 4x,
> 2x, 0.5x.. This commit uses the more sane, non-inverse approach.
> 
> Signed-off-by: Konrad Dybcio <konrad.dybcio@...aro.org>
> ---
>  drivers/interconnect/qcom/icc-rpm.c | 10 +++++++++-
>  drivers/interconnect/qcom/icc-rpm.h |  5 +++++
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/interconnect/qcom/icc-rpm.c b/drivers/interconnect/qcom/icc-rpm.c
> index 2c16917ba1fd..2de0e1dfe225 100644
> --- a/drivers/interconnect/qcom/icc-rpm.c
> +++ b/drivers/interconnect/qcom/icc-rpm.c
> @@ -298,9 +298,11 @@ static int qcom_icc_bw_aggregate(struct icc_node *node, u32 tag, u32 avg_bw,
>   */
>  static void qcom_icc_bus_aggregate(struct icc_provider *provider, u64 *agg_clk_rate)
>  {
> -	u64 agg_avg_rate, agg_rate;
> +	struct qcom_icc_provider *qp = to_qcom_provider(provider);
> +	u64 agg_avg_rate, agg_peak_rate, agg_rate;
>  	struct qcom_icc_node *qn;
>  	struct icc_node *node;
> +	u16 percent;
>  	int i;
>  
>  	/*
> @@ -315,6 +317,12 @@ static void qcom_icc_bus_aggregate(struct icc_provider *provider, u64 *agg_clk_r
>  			else
>  				agg_avg_rate = qn->sum_avg[i];
>  
> +			percent = qp->ab_percent ? qp->ab_percent : 100;
> +			agg_avg_rate = mult_frac(percent, agg_avg_rate, 100);

			if (qp->ab_percent)
				agg_avg_rate = mult_frac(qp->ab_percent, agg_avg_rate, 100);

Would be likely more efficient (no calculation if unspecified) and not
much harder to read.

> +
> +			percent = qn->ib_percent ? qn->ib_percent : 100;
> +			agg_peak_rate = mult_frac(percent, qn->max_peak[i], 100);
> +

agg_peak_rate doesn't seem to be used anywhere else? 🤔

Thanks,
Stephan