[PATCH 4/5] arm_mpam: prevent MPAM-Fb accesses inside IRQ handler

[Andre Przywara]

When an MPAM MSC gets into an error condition, it can trigger an error
IRQ. We cannot really do much about those errors, but we at least query
and log the error, then disable MPAM functionality.

This error report relies on reading the MSC's error status register
(ESR) in the IRQ handler, which is not possible for MPAM-Fb based
MSC accesses, since they involve mailbox routines that might sleep.

For simplicity just skip the ESR read when the MSC is not using direct
MMIO accesses. We will wrap up MPAM functionality regardless, knowing
the exact error value will not change that.

Signed-off-by: Andre Przywara <andre.przywara at arm.com>
---
 drivers/resctrl/mpam_devices.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
index 90b49bfd5caa..62aa04cb6905 100644
--- a/drivers/resctrl/mpam_devices.c
+++ b/drivers/resctrl/mpam_devices.c
@@ -2472,7 +2472,7 @@ static int mpam_disable_msc_ecr(void *_msc)
 
 static irqreturn_t __mpam_irq_handler(int irq, struct mpam_msc *msc)
 {
-	u64 reg;
+	u64 reg = 0;
 	u16 partid;
 	u8 errcode, pmg, ris;
 
@@ -2481,22 +2481,30 @@ static irqreturn_t __mpam_irq_handler(int irq, struct mpam_msc *msc)
 					   &msc->accessibility)))
 		return IRQ_NONE;
 
-	reg = mpam_msc_read_esr(msc);
+	/* MPAM-Fb MSC accesses cannot be done in atomic context. */
+	if (msc->iface == MPAM_IFACE_MMIO) {
+		reg = mpam_msc_read_esr(msc);
 
-	errcode = FIELD_GET(MPAMF_ESR_ERRCODE, reg);
-	if (!errcode)
-		return IRQ_NONE;
+		errcode = FIELD_GET(MPAMF_ESR_ERRCODE, reg);
+		if (!errcode)
+			return IRQ_NONE;
+	}
 
 	/* Clear level triggered irq */
 	mpam_msc_clear_esr(msc);
 
-	partid = FIELD_GET(MPAMF_ESR_PARTID_MON, reg);
-	pmg = FIELD_GET(MPAMF_ESR_PMG, reg);
-	ris = FIELD_GET(MPAMF_ESR_RIS, reg);
+	if (reg) {
+		partid = FIELD_GET(MPAMF_ESR_PARTID_MON, reg);
+		pmg = FIELD_GET(MPAMF_ESR_PMG, reg);
+		ris = FIELD_GET(MPAMF_ESR_RIS, reg);
+
+		pr_err_ratelimited("error irq from msc:%u '%s', partid:%u, pmg: %u, ris: %u\n",
+				   msc->id, mpam_errcode_names[errcode], partid,
+				   pmg, ris);
+	} else {
+		pr_err_ratelimited("unknown error irq from msc:%u\n", msc->id);
+	}
 
-	pr_err_ratelimited("error irq from msc:%u '%s', partid:%u, pmg: %u, ris: %u\n",
-			   msc->id, mpam_errcode_names[errcode], partid, pmg,
-			   ris);
 
 	/* Disable this interrupt. */
 	mpam_disable_msc_ecr(msc);
-- 
2.43.0