Re: [RFC PATCH 33/36] arm_mpam: Use long MBWU counters if supported

[James Morse]

Hi Ben,

On 28/07/2025 14:46, Ben Horgan wrote:
> On 7/11/25 19:36, James Morse wrote:
> > From: Rohit Mathew <rohit.mathew@xxxxxxx>
> >
> > If the 44 bit (long) or 63 bit (LWD) counters are detected on probing
> > the RIS, use long/LWD counter instead of the regular 31 bit mbwu
> > counter.
> >
> > Only 32bit accesses to the MSC are required to be supported by the
> > spec, but these registers are 64bits. The lower half may overflow
> > into the higher half between two 32bit reads. To avoid this, use
> > a helper that reads the top half twice to check for overflow.

> Slightly misleading as it may be read up to 4 times.

Meh - its referring to the high/low/high pattern. Sure if it fails you go round the whole
thing again. I'll change it 'read multiple times to check for overflow'.


> > diff --git a/drivers/platform/arm64/mpam/mpam_devices.c b/drivers/platform/arm64/mpam/
> > mpam_devices.c
> > index 774137a124f8..ace69ac2d0ee 100644
> > --- a/drivers/platform/arm64/mpam/mpam_devices.c
> > +++ b/drivers/platform/arm64/mpam/mpam_devices.c
> > @@ -1125,10 +1177,24 @@ static void __ris_msmon_read(void *arg)
> >           now = FIELD_GET(MSMON___VALUE, now);
> >           break;
> >       case mpam_feat_msmon_mbwu:
> > -        now = mpam_read_monsel_reg(msc, MBWU);
> > -        if (mpam_has_feature(mpam_feat_msmon_mbwu_hw_nrdy, rprops))
> > -            nrdy = now & MSMON___NRDY;
> > -        now = FIELD_GET(MSMON___VALUE, now);
> > +        /*
> > +         * If long or lwd counters are supported, use them, else revert
> > +         * to the 32 bit counter.
> > +         */
> 32 bit counter -> 31 bit counter

Sure,


> > +        if (mpam_ris_has_mbwu_long_counter(ris)) {
> > +            now = mpam_msc_read_mbwu_l(msc);
> > +            if (mpam_has_feature(mpam_feat_msmon_mbwu_hw_nrdy, rprops))
> > +                nrdy = now & MSMON___NRDY_L;
> > +            if (mpam_has_feature(mpam_feat_msmon_mbwu_63counter, rprops))
> > +                now = FIELD_GET(MSMON___LWD_VALUE, now);
> > +            else
> > +                now = FIELD_GET(MSMON___L_VALUE, now);
> > +        } else {
> > +            now = mpam_read_monsel_reg(msc, MBWU);
> > +            if (mpam_has_feature(mpam_feat_msmon_mbwu_hw_nrdy, rprops))
> > +                nrdy = now & MSMON___NRDY;
> > +            now = FIELD_GET(MSMON___VALUE, now);
> > +        }
> >             if (nrdy)
> >               break;

> > diff --git a/drivers/platform/arm64/mpam/mpam_internal.h b/drivers/platform/arm64/mpam/
> > mpam_internal.h
> > index fc705801c1b6..4553616f2f67 100644
> > --- a/drivers/platform/arm64/mpam/mpam_internal.h
> > +++ b/drivers/platform/arm64/mpam/mpam_internal.h
> > @@ -674,7 +675,10 @@ int mpam_get_cpumask_from_cache_id(unsigned long cache_id, u32
> > cache_level,
> >    */
> >   #define MSMON___VALUE          GENMASK(30, 0)
> >   #define MSMON___NRDY           BIT(31)
> > -#define MSMON_MBWU_L_VALUE     GENMASK(62, 0)
> > +#define MSMON___NRDY_L        BIT(63)
> > +#define MSMON___L_VALUE        GENMASK(43, 0)
> > +#define MSMON___LWD_VALUE    GENMASK(62, 0)
> > +
> As mentioned on an earlier patch. These could be added with all the other register
> definition.

Yup,


Thanks,

James