[PATCH] PCI: vmd: Delay interrupt handling on MTL VMD controller

[Kai-Heng Feng <kai.heng.feng@canonical.com>]

Meteor Lake VMD has a bug that the IRQ raises before the DMA region is
ready, so the requested IO is considered never completed:
[   97.343423] nvme nvme0: I/O 259 QID 2 timeout, completion polled
[   97.343446] nvme nvme0: I/O 384 QID 3 timeout, completion polled
[   97.343459] nvme nvme0: I/O 320 QID 4 timeout, completion polled
[   97.343470] nvme nvme0: I/O 707 QID 5 timeout, completion polled

The is documented as erratum MTL016 [0]. The suggested workaround is to
"The VMD MSI interrupt-handler should initially perform a dummy register
read to the MSI initiator device prior to any writes to ensure proper
PCIe ordering." which essentially is adding a delay before the interrupt
handling.

Hence add a delay before handle interrupt to workaround the erratum.

[0] https://edc.intel.com/content/www/us/en/design/products/platforms/details/meteor-lake-u-p/core-ultra-processor-specification-update/errata-details/#MTL016

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217871
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
---
 drivers/pci/controller/vmd.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
index a726de0af011..3433b3730f9c 100644
--- a/drivers/pci/controller/vmd.c
+++ b/drivers/pci/controller/vmd.c
@@ -16,6 +16,7 @@
 #include <linux/srcu.h>
 #include <linux/rculist.h>
 #include <linux/rcupdate.h>
+#include <linux/delay.h>
 
 #include <asm/irqdomain.h>
 
@@ -74,6 +75,9 @@ enum vmd_features {
 	 * proper power management of the SoC.
 	 */
 	VMD_FEAT_BIOS_PM_QUIRK		= (1 << 5),
+
+	/* Erratum MTL016 */
+	VMD_FEAT_INTERRUPT_QUIRK	= (1 << 6),
 };
 
 #define VMD_BIOS_PM_QUIRK_LTR	0x1003	/* 3145728 ns */
@@ -90,6 +94,8 @@ static DEFINE_IDA(vmd_instance_ida);
  */
 static DEFINE_RAW_SPINLOCK(list_lock);
 
+static bool interrupt_delay;
+
 /**
  * struct vmd_irq - private data to map driver IRQ to the VMD shared vector
  * @node:	list item for parent traversal.
@@ -105,6 +111,7 @@ struct vmd_irq {
 	struct vmd_irq_list	*irq;
 	bool			enabled;
 	unsigned int		virq;
+	bool			delay_irq;
 };
 
 /**
@@ -680,8 +687,11 @@ static irqreturn_t vmd_irq(int irq, void *data)
 	int idx;
 
 	idx = srcu_read_lock(&irqs->srcu);
-	list_for_each_entry_rcu(vmdirq, &irqs->irq_list, node)
+	list_for_each_entry_rcu(vmdirq, &irqs->irq_list, node) {
+		if (interrupt_delay)
+			udelay(4);
 		generic_handle_irq(vmdirq->virq);
+	}
 	srcu_read_unlock(&irqs->srcu, idx);
 
 	return IRQ_HANDLED;
@@ -1015,6 +1025,9 @@ static int vmd_probe(struct pci_dev *dev, const struct pci_device_id *id)
 	if (features & VMD_FEAT_OFFSET_FIRST_VECTOR)
 		vmd->first_vec = 1;
 
+	if (features & VMD_FEAT_INTERRUPT_QUIRK)
+		interrupt_delay = true;
+
 	spin_lock_init(&vmd->cfg_lock);
 	pci_set_drvdata(dev, vmd);
 	err = vmd_enable_domain(vmd, features);
@@ -1106,7 +1119,8 @@ static const struct pci_device_id vmd_ids[] = {
 	{PCI_VDEVICE(INTEL, 0xa77f),
 		.driver_data = VMD_FEATS_CLIENT,},
 	{PCI_VDEVICE(INTEL, 0x7d0b),
-		.driver_data = VMD_FEATS_CLIENT,},
+		.driver_data = VMD_FEATS_CLIENT |
+			       VMD_FEAT_INTERRUPT_QUIRK,},
 	{PCI_VDEVICE(INTEL, 0xad0b),
 		.driver_data = VMD_FEATS_CLIENT,},
 	{PCI_VDEVICE(INTEL, PCI_DEVICE_ID_INTEL_VMD_9A0B),
-- 
2.43.0