The MSMON_MBWU register accumulates counts monotonically forward and
would not automatically cleared to zero on overflow. The overflow portion
is exactly what mpam_msmon_overflow_val() computes, there is no need to
additionally subtract mbwu_state->prev_val.

Before invoking write_msmon_ctl_flt_vals(), the overflow bit of the
MSMON_MBWU register must first be read to prevent it from being
inadvertently cleared by the write operation.

Finally, use the overflow bit instead of relying on counter wrap-around
to determine whether an overflow has occurred, that avoids the case where
a wrap-around (now > prev_val) is overlooked. So with this, prev_val no
longer has any use and remove it.

CC: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: Zeng Heng <redacted>
---
 drivers/resctrl/mpam_devices.c  | 22 +++++++++++++++++-----
 drivers/resctrl/mpam_internal.h |  3 ---
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
index 0dd048279e02..db4cec710091 100644
--- a/drivers/resctrl/mpam_devices.c
+++ b/drivers/resctrl/mpam_devices.c

@@ -1039,7 +1039,6 @@ static void write_msmon_ctl_flt_vals(struct mon_read *m, u32 ctl_val,
 		mpam_write_monsel_reg(msc, CFG_MBWU_CTL, ctl_val | MSMON_CFG_x_CTL_EN);

 		mbwu_state = &m->ris->mbwu_state[m->ctx->mon];
-		mbwu_state->prev_val = 0;

 		break;
 	default:

@@ -1062,6 +1061,16 @@ static u64 mpam_msmon_overflow_val(enum mpam_device_features type)
 	}
 }

+static bool read_msmon_mbwu_is_overflow(struct mpam_msc *msc)
+{
+	u32 ctl;
+
+	ctl = mpam_read_monsel_reg(msc, CFG_MBWU_CTL);
+	return ctl & (MSMON_CFG_x_CTL_OFLOW_STATUS |
+		      MSMON_CFG_MBWU_CTL_OFLOW_STATUS_L) ?
+		      true : false;
+}
+
 /* Call with MSC lock held */
 static void __ris_msmon_read(void *arg)
 {

@@ -1069,6 +1078,7 @@ static void __ris_msmon_read(void *arg)
 	bool config_mismatch;
 	struct mon_read *m = arg;
 	u64 now, overflow_val = 0;
+	bool mbwu_overflow = false;
 	struct mon_cfg *ctx = m->ctx;
 	bool reset_on_next_read = false;
 	struct mpam_msc_ris *ris = m->ris;

@@ -1091,6 +1101,7 @@ static void __ris_msmon_read(void *arg)
 			reset_on_next_read = mbwu_state->reset_on_next_read;
 			mbwu_state->reset_on_next_read = false;
 		}
+		mbwu_overflow = read_msmon_mbwu_is_overflow(msc);
 	}

 	/*

@@ -1103,8 +1114,10 @@ static void __ris_msmon_read(void *arg)
 	config_mismatch = cur_flt != flt_val ||
 			  cur_ctl != (ctl_val | MSMON_CFG_x_CTL_EN);

-	if (config_mismatch || reset_on_next_read)
+	if (config_mismatch || reset_on_next_read) {
 		write_msmon_ctl_flt_vals(m, ctl_val, flt_val);
+		mbwu_overflow = false;
+	}

 	switch (m->type) {
 	case mpam_feat_msmon_csu:

@@ -1138,10 +1151,9 @@ static void __ris_msmon_read(void *arg)
 		mbwu_state = &ris->mbwu_state[ctx->mon];

 		/* Add any pre-overflow value to the mbwu_state->val */
-		if (mbwu_state->prev_val > now)
-			overflow_val = mpam_msmon_overflow_val(m->type) - mbwu_state->prev_val;

+		if (mbwu_overflow)
+			overflow_val = mpam_msmon_overflow_val(m->type);

-		mbwu_state->prev_val = now;
 		mbwu_state->correction += overflow_val;

 		/* Include bandwidth consumed before the last hardware reset */

diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
index 4f25681b56ab..8837c0cd7b0c 100644
--- a/drivers/resctrl/mpam_internal.h
+++ b/drivers/resctrl/mpam_internal.h

@@ -226,9 +226,6 @@ struct msmon_mbwu_state {
 	bool		reset_on_next_read;
 	struct mon_cfg	cfg;

-	/* The value last read from the hardware. Used to detect overflow. */
-	u64		prev_val;
-
 	/*
 	 * The value to add to the new reading to account for power management,
 	 * and shifts to trigger the overflow interrupt.
--

2.25.1