[PATCH v4 07/10] arm_mpam: prepare mon_sel locking for MPAM-Fb

[Andre Przywara]

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 MSCs are not accesible from every CPU. This makes an irqsave
spinlock the obvious lock to protect these registers. On systems with
MPAM-Fb mailbox MSC access it must be able to sleep, meaning a mutex must
be used. So MPAM-Fb platforms cannot support an overflow interrupt easily.
Clearly these two methods can't exist for one MSC at the same time.

Change the mon_sel locking wrapper function to only use a spinlock when
the MSC is accessed directly via MMIO. In case of MPAM-Fb, we use a
mutex, but only if we are in a sleepable context. If that's not the
case, we return an error. This should not happen, as MPAM-Fb by design
does not require an MSC access to happen from a specific CPU, so there
is no need for any IPIs or preemption disabling to satisfy CPU
constraints. And since overflow interrupts are not supported at the moment
anyway, we also wouldn't meet the other case.
Bailing out early is already happening in rare occasions today.

Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
---
 drivers/resctrl/mpam_devices.c  |  5 ++++-
 drivers/resctrl/mpam_internal.h | 32 ++++++++++++++++++++++++++------
 2 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
index 6329443c451f..e2cb884eacf4 100644
--- a/drivers/resctrl/mpam_devices.c
+++ b/drivers/resctrl/mpam_devices.c
@@ -2225,7 +2225,10 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
 	if (err)
 		return ERR_PTR(err);
 
-	mpam_mon_sel_lock_init(msc);
+	err = mpam_mon_sel_lock_init(dev, msc);
+	if (err)
+		return ERR_PTR(err);
+
 	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 0c3f6a040b20..b3a6ed9ed175 100644
--- a/drivers/resctrl/mpam_internal.h
+++ b/drivers/resctrl/mpam_internal.h
@@ -126,6 +126,7 @@ struct mpam_msc {
 	 */
 	raw_spinlock_t		_mon_sel_lock;
 	unsigned long		_mon_sel_flags;
+	struct mutex		mon_sel_mutex;
 
 	void __iomem		*mapped_hwpage;
 	size_t			mapped_hwpage_sz;
@@ -139,27 +140,46 @@ struct mpam_msc {
 /* 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)
 {
-	/* Locking will require updating to support a firmware backed interface */
-	if (WARN_ON_ONCE(msc->iface != MPAM_IFACE_MMIO))
+	if (msc->iface == MPAM_IFACE_MMIO) {
+		raw_spin_lock_irqsave(&msc->_mon_sel_lock, msc->_mon_sel_flags);
+
+		return true;
+	}
+
+	if (!preemptible())
 		return false;
 
-	raw_spin_lock_irqsave(&msc->_mon_sel_lock, msc->_mon_sel_flags);
+	mutex_lock(&msc->mon_sel_mutex);
+
 	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);
+	if (msc->iface == MPAM_IFACE_MMIO) {
+		raw_spin_unlock_irqrestore(&msc->_mon_sel_lock,
+					   msc->_mon_sel_flags);
+
+		return;
+	}
+
+	mutex_unlock(&msc->mon_sel_mutex);
 }
 
 static inline void mpam_mon_sel_lock_held(struct mpam_msc *msc)
 {
-	lockdep_assert_held_once(&msc->_mon_sel_lock);
+	if (msc->iface == MPAM_IFACE_MMIO)
+		lockdep_assert_held_once(&msc->_mon_sel_lock);
+	else
+		lockdep_assert_held_once(&msc->mon_sel_mutex);
 }
 
-static inline void mpam_mon_sel_lock_init(struct mpam_msc *msc)
+static inline int mpam_mon_sel_lock_init(struct device *dev,
+					 struct mpam_msc *msc)
 {
 	raw_spin_lock_init(&msc->_mon_sel_lock);
+
+	return devm_mutex_init(dev, &msc->mon_sel_mutex);
 }
 
 DEFINE_GUARD(mon_sel,
-- 
2.43.0