[PATCH 15/33] arm_mpam: Add helpers for managing the locking around the mon_sel registers

Ben Horgan
2025-11-07T12:36:46Z

From: James Morse <james.morse@arm.com>

The MSC MON_SEL register needs to be accessed from hardirq for the overflow
interrupt, and when taking an IPI to access these registers on platforms
where MSC are not accessible from every CPU. This makes an irqsave
spinlock the obvious lock to protect these registers. On systems with SCMI
or PCC mailboxes it must be able to sleep, meaning a mutex must be used.
The SCMI or PCC platforms can't support an overflow interrupt, and
can't access the registers from hardirq context.

Clearly these two can't exist for one MSC at the same time.

Add helpers for the MON_SEL locking. For now, use a irqsave spinlock and
only support 'real' MMIO platforms.

In the future this lock will be split in two allowing SCMI/PCC platforms
to take a mutex. Because there are contexts where the SCMI/PCC platforms
can't make an access, mpam_mon_sel_lock() needs to be able to fail. Do
this now, so that all the error handling on these paths is present. This
allows the relevant paths to fail if they are needed on a platform where
this isn't possible, instead of having to make explicit checks of the
interface type.

Tested-by: Fenghua Yu <fenghuay@nvidia.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Tested-by: Peter Newman <peternewman@google.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes since v3:
use devm_mutex_init()
include tiying
stray comma (Jonathan)
---
drivers/resctrl/mpam_devices.c | 2 ++
drivers/resctrl/mpam_internal.h | 39 +++++++++++++++++++++++++++++++++
2 files changed, 41 insertions(+)

diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
index e1e26cb350f7..3556299a8605 100644
--- a/drivers/resctrl/mpam_devices.c
+++ b/drivers/resctrl/mpam_devices.c
@@ -19,6 +19,7 @@
#include <linux/platform_device.h>
#include <linux/printk.h>
#include <linux/srcu.h>
+#include <linux/spinlock.h>
#include <linux/types.h>
#include <linux/workqueue.h>

@@ -746,6 +747,7 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
err = devm_mutex_init(dev, &msc->part_sel_lock);
if (err)
return ERR_PTR(err);
+ mpam_mon_sel_lock_init(msc);
msc->id = pdev->id;
msc->pdev = pdev;
INIT_LIST_HEAD_RCU(&msc->all_msc_list);
diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
index 768a58a3ab27..b62ee55e1ed5 100644
--- a/drivers/resctrl/mpam_internal.h
+++ b/drivers/resctrl/mpam_internal.h
@@ -10,6 +10,7 @@
#include <linux/llist.h>
#include <linux/mutex.h>
#include <linux/srcu.h>
+#include <linux/spinlock.h>
#include <linux/types.h>

#define MPAM_MSC_MAX_NUM_RIS 16
@@ -65,12 +66,50 @@ struct mpam_msc {
*/
struct mutex part_sel_lock;

+ /*
+ * mon_sel_lock protects access to the MSC hardware registers that are
+ * affected by MPAMCFG_MON_SEL, and the mbwu_state.
+ * Access to mon_sel is needed from both process and interrupt contexts,
+ * but is complicated by firmware-backed platforms that can't make any
+ * access unless they can sleep.
+ * Always use the mpam_mon_sel_lock() helpers.
+ * Accesses to mon_sel need to be able to fail if they occur in the wrong
+ * context.
+ * If needed, take msc->probe_lock first.
+ */
+ raw_spinlock_t _mon_sel_lock;
+ unsigned long _mon_sel_flags;
+
void __iomem *mapped_hwpage;
size_t mapped_hwpage_sz;

struct mpam_garbage garbage;
};

+/* Returning false here means accesses to mon_sel must fail and report an error. */
+static inline bool __must_check mpam_mon_sel_lock(struct mpam_msc *msc)
+{
+ WARN_ON_ONCE(msc->iface != MPAM_IFACE_MMIO);
+
+ raw_spin_lock_irqsave(&msc->_mon_sel_lock, msc->_mon_sel_flags);
+ return true;
+}
+
+static inline void mpam_mon_sel_unlock(struct mpam_msc *msc)
+{
+ raw_spin_unlock_irqrestore(&msc->_mon_sel_lock, msc->_mon_sel_flags);
+}
+
+static inline void mpam_mon_sel_lock_held(struct mpam_msc *msc)
+{
+ lockdep_assert_held_once(&msc->_mon_sel_lock);
+}
+
+static inline void mpam_mon_sel_lock_init(struct mpam_msc *msc)
+{
+ raw_spin_lock_init(&msc->_mon_sel_lock);
+}
+
struct mpam_class {
/* mpam_components in this class */
struct list_head components;
--
2.43.0