linux-kernel - [PATCH v2] irqchip: gic-v2m: Handle Multiple MSI base IRQ Alignment

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Message-ID: <20250902091045.220847-1-christian.bruel@foss.st.com>
Date: Tue, 2 Sep 2025 11:10:45 +0200
From: Christian Bruel <christian.bruel@...s.st.com>
To: <maz@...nel.org>, <tglx@...utronix.de>
CC: <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        <fabrice.gasnier@...s.st.com>, <mani@...nel.org>,
        Christian Bruel
	<christian.bruel@...s.st.com>
Subject: [PATCH v2] irqchip: gic-v2m: Handle Multiple MSI base IRQ Alignment

The PCI Local Bus Specification 3.0 (section 6.8.1.6) allows modifying the
low-order bits of the MSI Message DATA register to encode nr_irqs interrupt
numbers in the log2(nr_irqs) bits for the domain.

The problem arises if the base vector (GICV2m base spi) is not aligned with
nr_irqs; in this case, the low-order log2(nr_irqs) bits from the base
vector conflict with the nr_irqs masking, causing the wrong MSI interrupt
to be identified.

To fix this, use bitmap_find_next_zero_area_off() instead of
bitmap_find_free_region() to align the initial base vector with nr_irqs.

Signed-off-by: Christian Bruel <christian.bruel@...s.st.com>
---
Changes in v2:
   (Marc Zyngier)
 - Move align_off definition inside the loop
 - Reworked Commit Message

Changes in v1:
   (Marc Zyngier)
 - Replace the incorrect usage of msi_attrib.multiple with nr_irqs
 - Reworked changelog
---
 drivers/irqchip/irq-gic-v2m.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/drivers/irqchip/irq-gic-v2m.c b/drivers/irqchip/irq-gic-v2m.c
index 24ef5af569fe..8a3410c2b7b5 100644
--- a/drivers/irqchip/irq-gic-v2m.c
+++ b/drivers/irqchip/irq-gic-v2m.c
@@ -153,14 +153,19 @@ static int gicv2m_irq_domain_alloc(struct irq_domain *domain, unsigned int virq,
 {
 	msi_alloc_info_t *info = args;
 	struct v2m_data *v2m = NULL, *tmp;
-	int hwirq, offset, i, err = 0;
+	int hwirq, i, err = 0;
+	unsigned long offset;
+	unsigned long align_mask = nr_irqs - 1;
 
 	spin_lock(&v2m_lock);
 	list_for_each_entry(tmp, &v2m_nodes, entry) {
-		offset = bitmap_find_free_region(tmp->bm, tmp->nr_spis,
-						 get_count_order(nr_irqs));
-		if (offset >= 0) {
+		unsigned long align_off = tmp->spi_start - (tmp->spi_start & ~align_mask);
+
+		offset = bitmap_find_next_zero_area_off(tmp->bm, tmp->nr_spis, 0,
+							nr_irqs, align_mask, align_off);
+		if (offset < tmp->nr_spis) {
 			v2m = tmp;
+			bitmap_set(v2m->bm, offset, nr_irqs);
 			break;
 		}
 	}
-- 
2.34.1