Re: [PATCH v7 08/13] xen/cpufreq: implement amd-cppc driver for CPPC in passive mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.08.2025 12:52, Penny Zheng wrote:
> @@ -50,10 +141,335 @@ int __init amd_cppc_cmdline_parse(const char *s, const 
> char *e)
>      return 0;
>  }
>  
> +/*
> + * If CPPC lowest_freq and nominal_freq registers are exposed then we can
> + * use them to convert perf to freq and vice versa. The conversion is
> + * extrapolated as an linear function passing by the 2 points:
> + *  - (Low perf, Low freq)
> + *  - (Nominal perf, Nominal freq)
> + * Parameter freq is always in kHz.
> + */
> +static int amd_cppc_khz_to_perf(const struct amd_cppc_drv_data *data,
> +                                unsigned int freq, uint8_t *perf)
> +{
> +    const struct xen_processor_cppc *cppc_data = data->cppc_data;
> +    unsigned int mul, div;
> +    int offset = 0, res;
> +
> +    if ( cppc_data->cpc.lowest_mhz &&
> +         data->caps.nominal_perf > data->caps.lowest_perf &&
> +         cppc_data->cpc.nominal_mhz > cppc_data->cpc.lowest_mhz )
> +    {
> +        mul = data->caps.nominal_perf - data->caps.lowest_perf;
> +        div = cppc_data->cpc.nominal_mhz - cppc_data->cpc.lowest_mhz;
> +
> +        /*
> +         * We don't need to convert to kHz for computing offset and can
> +         * directly use nominal_mhz and lowest_mhz as the division
> +         * will remove the frequency unit.
> +         */
> +        offset = data->caps.nominal_perf -
> +                 (mul * cppc_data->cpc.nominal_mhz) / div;
> +    }
> +    else
> +    {
> +        /* Read Processor Max Speed(MHz) as anchor point */
> +        mul = data->caps.highest_perf;
> +        div = this_cpu(pxfreq_mhz);
> +        if ( !div )
> +            return -EOPNOTSUPP;
> +    }
> +
> +    res = offset + (mul * freq) / (div * 1000);
> +    if ( res > UINT8_MAX )
> +    {
> +        printk_once(XENLOG_WARNING
> +                    "Perf value exceeds maximum value 255: %d\n", res);
> +        *perf = UINT8_MAX;
> +        return 0;
> +    }
> +    if ( res <= 0 )
> +    {
> +        printk_once(XENLOG_WARNING
> +                    "Perf value smaller than minimum value 0: %d\n", res);

The message text doesn't fit the if() condition anymore. Perhaps simply
omit the "0" from the text? Then:
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan