Re: [PATCH v3 04/16] arm_mpam: propagate MSC read errors for mpam_msc_read_mbwu_l()

[Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>]

On Fri, 10 Jul 2026 16:45:08 +0200
Andre Przywara <andre.przywara@arm.com> wrote:

> Allow the mpam_msc_read_mbwu_l() function to return an error, and
> propagate read errors from the lower level up.
> So far we were using a special value to indicate the "unstable read"
> condition, replace that with the actual Linux error code, since it's now
> easy to do.
> 
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> ---
>  drivers/resctrl/mpam_devices.c | 54 +++++++++++++++++++++++++---------
>  1 file changed, 40 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index b1bd047da203..3b6f9e552a9f 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -1112,8 +1112,9 @@ static bool mpam_ris_has_mbwu_long_counter(struct mpam_msc_ris *ris)
>  		mpam_has_feature(mpam_feat_msmon_mbwu_44counter, &ris->props));
>  }
>  
> -static u64 mpam_msc_read_mbwu_l(struct mpam_msc *msc)
> +static int mpam_msc_read_mbwu_l(struct mpam_msc *msc, u64 *res)
>  {
> +	int ret;
>  	int retry = 3;
>  	u32 mbwu_l_low;
>  	u32 mbwu_l_high1, mbwu_l_high2;
> @@ -1123,20 +1124,30 @@ static u64 mpam_msc_read_mbwu_l(struct mpam_msc *msc)
>  	WARN_ON_ONCE((MSMON_MBWU_L + sizeof(u64)) > msc->mapped_hwpage_sz);
>  	WARN_ON_ONCE(!cpumask_test_cpu(smp_processor_id(), &msc->accessibility));
>  
> -	__mpam_read_reg(msc, MSMON_MBWU_L + 4, &mbwu_l_high2);
> +	ret = __mpam_read_reg(msc, MSMON_MBWU_L + 4, &mbwu_l_high2);
> +	if (ret)
> +		return ret;
> +
>  	do {
>  		mbwu_l_high1 = mbwu_l_high2;
> -		__mpam_read_reg(msc, MSMON_MBWU_L, &mbwu_l_low);
> -		__mpam_read_reg(msc, MSMON_MBWU_L + 4, &mbwu_l_high2);
> +		ret = __mpam_read_reg(msc, MSMON_MBWU_L, &mbwu_l_low);
> +		if (ret)
> +			return ret;
> +		ret = __mpam_read_reg(msc, MSMON_MBWU_L + 4, &mbwu_l_high2);
> +		if (ret)
> +			return ret;
>  
>  		retry--;
>  	} while (mbwu_l_high1 != mbwu_l_high2 && retry > 0);
>  
> -	if (mbwu_l_high1 == mbwu_l_high2)
> -		return ((u64)mbwu_l_high1 << 32) | mbwu_l_low;
> +	if (mbwu_l_high1 == mbwu_l_high2) {
> +		*res = ((u64)mbwu_l_high1 << 32) | mbwu_l_low;
> +	} else {
> +		pr_warn("Failed to read a stable value\n");
> +		ret = -EBUSY;

I guess this may get more complex in later patches, but for now why not just 
		return -EBUSY.

> +	}
>  
> -	pr_warn("Failed to read a stable value\n");
> -	return MSMON___L_NRDY;
> +	return ret;
>  }
>  
>  static void mpam_msc_zero_mbwu_l(struct mpam_msc *msc)
> @@ -1283,6 +1294,7 @@ static u64 mpam_msmon_overflow_val(enum mpam_device_features type,
>  static void __ris_msmon_read(void *arg)
>  {
>  	u64 now;
> +	int ret;
>  	u32 now32;
>  	bool nrdy = false;
>  	bool config_mismatch;
> @@ -1358,8 +1370,9 @@ static void __ris_msmon_read(void *arg)

Relevant bit of context isn't show, but as before looks like
ACQUIRE() and ACQUIRE_ERR() will make this simpler by just
letting you do direct returns on all error paths though
you will need to set m->err = ret in each one.  To me that
is simpler, but feel free to disagree.

>  	case mpam_feat_msmon_mbwu_44counter:
>  	case mpam_feat_msmon_mbwu_63counter:
>  		if (m->type != mpam_feat_msmon_mbwu_31counter) {
> -			now = mpam_msc_read_mbwu_l(msc);
> -			nrdy = now & MSMON___L_NRDY;
> +			ret = mpam_msc_read_mbwu_l(msc, &now);
> +			if (ret)
> +				goto out_unlock;
>  
>  			if (m->type == mpam_feat_msmon_mbwu_63counter)
>  				now = FIELD_GET(MSMON___LWD_VALUE, now);
> @@ -1397,10 +1410,15 @@ static void __ris_msmon_read(void *arg)
>  	if (nrdy)
>  		m->err = -EBUSY;
>  
> -	if (m->err)
> -		return;
> +	if (!m->err)
Above ACQUIRE() suggestion relies on it not being a problem to hold
the lock a little longer and incorporate these sets in m. I haven't
checked closely but I think that's fine.

> +		*m->val += now;
> +
> +	return;
> +
> +out_unlock:
> +	mpam_mon_sel_unlock(msc);
>  
> -	*m->val += now;
> +	m->err = ret;
>  }
>  
>  static int _msmon_read(struct mpam_component *comp, struct mon_read *arg)
> @@ -1747,6 +1765,7 @@ static int mpam_save_mbwu_state(void *arg)
>  {
>  	int i;
>  	u64 val;
> +	int ret = 0;
>  	struct mon_cfg *cfg;
>  	u32 cur_flt, cur_ctl, mon_sel;
>  	struct mpam_msc_ris *ris = arg;
> @@ -1768,7 +1787,9 @@ static int mpam_save_mbwu_state(void *arg)
>  		mpam_write_monsel_reg(msc, CFG_MBWU_CTL, 0);
>  
>  		if (mpam_ris_has_mbwu_long_counter(ris)) {
> -			val = mpam_msc_read_mbwu_l(msc);
> +			ret = mpam_msc_read_mbwu_l(msc, &val);
> +			if (ret)
> +				goto out_unlock;
>  			mpam_msc_zero_mbwu_l(msc);
>  		} else {
>  			u32 val32;
> @@ -1788,6 +1809,11 @@ static int mpam_save_mbwu_state(void *arg)
>  	}
>  
>  	return 0;
> +
> +out_unlock:
> +	mpam_mon_sel_unlock(msc);
ACQUIRE() would avoid need for this handling so I think this is another
good place to look at using it.

> +
> +	return ret;
>  }
>  
>  /*