Re: [PATCH v2 24/45] arm_mpam: resctrl: Convert to/from MPAMs fixed-point formats

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Fri, 19 Dec 2025 18:11:26 +0000
Ben Horgan <ben.horgan@arm.com> wrote:

> From: Dave Martin <Dave.Martin@arm.com>
> 
> MPAM uses a fixed-point formats for some hardware controls.  Resctrl
> provides the bandwidth controls as a percentage. Add helpers to convert
> between these.
> 
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>

Perhaps it's just me, but I found this hard to check mostly because
of the spec being very light when describing this (that I could
find, maybe there is more somewhere else).

So whilst I'll give a tag, I'd like ideally a little more documentation,
particularly around the oddity of the left aligned field and max value
of bwa_wd being 16.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

> ---
>  drivers/resctrl/mpam_resctrl.c | 41 ++++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
> index bdbc5504964b..a20656e49edc 100644
> --- a/drivers/resctrl/mpam_resctrl.c
> +++ b/drivers/resctrl/mpam_resctrl.c
> @@ -10,6 +10,7 @@
>  #include <linux/errno.h>
>  #include <linux/limits.h>
>  #include <linux/list.h>
> +#include <linux/math.h>
>  #include <linux/printk.h>
>  #include <linux/rculist.h>
>  #include <linux/resctrl.h>
> @@ -246,6 +247,46 @@ static bool cache_has_usable_cpor(struct mpam_class *class)
>  	return class->props.cpbm_wd <= 32;
>  }
>  
> +/*
> + * Each fixed-point hardware value architecturally represents a range
> + * of values: the full range 0% - 100% is split contiguously into
> + * (1 << cprops->bwa_wd) equal bands.

This bit of the spec is really confusing.  I particularly like
BWA_WD being 6 bits but only allowed to take values up to 16. That took
me a while to spot as I couldn't work out what this was supposed to do if that
was 32.

Anyhow this is a request for a bit more documentation for this function such
as explaining the field is left aligned to bit 16.


> + * Find the nearest percentage value to the upper bound of the selected band:
> + */
> +static u32 mbw_max_to_percent(u16 mbw_max, struct mpam_props *cprops)
> +{
> +	u32 val = mbw_max;
> +
> +	val >>= 16 - cprops->bwa_wd;
> +	val += 1;
> +	val *= MAX_MBA_BW;
> +	val = DIV_ROUND_CLOSEST(val, 1 << cprops->bwa_wd);
> +
> +	return val;
> +}
> +
> +/*
> + * Find the band whose upper bound is closest to the specified percentage.
> + *
> + * A round-to-nearest policy is followed here as a balanced compromise
> + * between unexpected under-commit of the resource (where the total of
> + * a set of resource allocations after conversion is less than the
> + * expected total, due to rounding of the individual converted
> + * percentages) and over-commit (where the total of the converted
> + * allocations is greater than expected).
> + */
> +static u16 percent_to_mbw_max(u8 pc, struct mpam_props *cprops)
> +{
> +	u32 val = pc;
> +
> +	val <<= cprops->bwa_wd;
> +	val = DIV_ROUND_CLOSEST(val, MAX_MBA_BW);
> +	val = max(val, 1) - 1;
> +	val <<= 16 - cprops->bwa_wd;
> +
> +	return val;
> +}
> +
>  /* Test whether we can export MPAM_CLASS_CACHE:{2,3}? */
>  static void mpam_resctrl_pick_caches(void)
>  {