[PATCH RFC 1/2] irqchip/ls-scfg-msi: refactor allocation to bitmap_find_free_region()

[Alexander Wilhelm]

Replace the manual find_first_zero_bit()/__set_bit() pair with
bitmap_find_free_region(), and __clear_bit() with bitmap_release_region().

For the current single-MSI case (guarded by WARN_ON(nr_irqs != 1)) with
order 0 this is functionally equivalent: bitmap_find_free_region() walks
the bitmap in single-bit units and internally does the test-and-set
atomically under the same lock. On failure it returns -ENOMEM which is now
propagated directly, replacing the previous -ENOSPC.

While at it, release the region on iommu_dma_prepare_msi() failure instead
of leaking the allocated bit, loop over nr_irqs when calling
irq_domain_set_info(), and range-check pos + nr_irqs in the free path. All
three are no-ops for nr_irqs = 1 but prepare the driver for a subsequent
MSI_FLAG_MULTI_PCI_MSI enable without further changes to the allocation
bookkeeping.

Assisted-by: Copilot:claude-opus-4.7
Signed-off-by: Alexander Wilhelm <alexander.wilhelm@xxxxxxxxxxxx>
---
 drivers/irqchip/irq-ls-scfg-msi.c | 36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/drivers/irqchip/irq-ls-scfg-msi.c b/drivers/irqchip/irq-ls-scfg-msi.c
index 4910f364e568..bd5833ef621a 100644
--- a/drivers/irqchip/irq-ls-scfg-msi.c
+++ b/drivers/irqchip/irq-ls-scfg-msi.c
@@ -141,28 +141,32 @@ static int ls_scfg_msi_domain_irq_alloc(struct irq_domain *domain,
 {
 	msi_alloc_info_t *info = args;
 	struct ls_scfg_msi *msi_data = domain->host_data;
-	int pos, err = 0;
+	int order = get_count_order(nr_irqs);
+	int pos, err;
+	unsigned int i;
 
 	WARN_ON(nr_irqs != 1);
 
 	spin_lock(&msi_data->lock);
-	pos = find_first_zero_bit(msi_data->used, msi_data->irqs_num);
-	if (pos < msi_data->irqs_num)
-		__set_bit(pos, msi_data->used);
-	else
-		err = -ENOSPC;
+	pos = bitmap_find_free_region(msi_data->used, msi_data->irqs_num,
+				      order);
 	spin_unlock(&msi_data->lock);
 
-	if (err)
-		return err;
+	if (pos < 0)
+		return pos;
 
 	err = iommu_dma_prepare_msi(info->desc, msi_data->msiir_addr);
-	if (err)
+	if (err) {
+		spin_lock(&msi_data->lock);
+		bitmap_release_region(msi_data->used, pos, order);
+		spin_unlock(&msi_data->lock);
 		return err;
+	}
 
-	irq_domain_set_info(domain, virq, pos,
-			    &ls_scfg_msi_parent_chip, msi_data,
-			    handle_simple_irq, NULL, NULL);
+	for (i = 0; i < nr_irqs; i++)
+		irq_domain_set_info(domain, virq + i, pos + i,
+				    &ls_scfg_msi_parent_chip, msi_data,
+				    handle_simple_irq, NULL, NULL);
 
 	return 0;
 }
@@ -172,16 +176,18 @@ static void ls_scfg_msi_domain_irq_free(struct irq_domain *domain,
 {
 	struct irq_data *d = irq_domain_get_irq_data(domain, virq);
 	struct ls_scfg_msi *msi_data = irq_data_get_irq_chip_data(d);
+	int order = get_count_order(nr_irqs);
 	int pos;
 
 	pos = d->hwirq;
-	if (pos < 0 || pos >= msi_data->irqs_num) {
-		pr_err("failed to teardown msi. Invalid hwirq %d\n", pos);
+	if (pos < 0 || pos + nr_irqs > msi_data->irqs_num) {
+		pr_err("failed to teardown msi. Invalid hwirq %d nr %u\n",
+		       pos, nr_irqs);
 		return;
 	}
 
 	spin_lock(&msi_data->lock);
-	__clear_bit(pos, msi_data->used);
+	bitmap_release_region(msi_data->used, pos, order);
 	spin_unlock(&msi_data->lock);
 }
 

-- 
2.43.0