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

[James Morse <james.morse@arm.com>]

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@arm.com>
>>
>> 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