linux-kernel - [PATCH 3/6] of: parse #<name>-cells property to get the cell count

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Message-Id: <d2caaf822fde3565e4e67158da751fc97afbaa92.1762235099.git.charan.kalla@oss.qualcomm.com>
Date: Tue,  4 Nov 2025 14:21:02 +0530
From: Charan Teja Kalla <charan.kalla@....qualcomm.com>
To: robin.murphy@....com, will@...nel.org, joro@...tes.org, robh@...nel.org,
        dmitry.baryshkov@....qualcomm.com, konrad.dybcio@....qualcomm.com,
        bjorn.andersson@....qualcomm.com, bod@...nel.org, conor+dt@...nel.org,
        krzk+dt@...nel.org, saravanak@...gle.com,
        prakash.gupta@....qualcomm.com, vikash.garodia@....qualcomm.com
Cc: iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org,
        Charan Teja Kalla <charan.kalla@....qualcomm.com>
Subject: [PATCH 3/6] of: parse #<name>-cells property to get the cell count

Implement the generic function, of_map_id_cell_count(), that can
parse and return the number of cells represented by #<name>-cells,
eg: #msi-cells, #iommu-cells.

Since the second argument of a property that is parsed by the
of_map_id() is always phandle, that represents a controller, probe
it to get the #<name>-cells property.

Further patches properly consume cell count value returned by this
wrapper and this patch just returns 1 irrespective of cells value, thus
no functional changes.

Signed-off-by: Charan Teja Kalla <charan.kalla@....qualcomm.com>
---
 drivers/of/base.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/drivers/of/base.c b/drivers/of/base.c
index 5e76abcc7940..e5ba8a318769 100644
--- a/drivers/of/base.c
+++ b/drivers/of/base.c
@@ -2045,9 +2045,43 @@ int of_find_last_cache_level(unsigned int cpu)
 	return cache_level;
 }
 
+/**
+ * of_map_id_cell_count - parse the cell count.
+ *
+ * @map: pointer to the property data that needs to be translated, eg: msi-map/iommu-map.
+ * @map_name: name to identify the details of @map.
+ * @map_len: length of @map in bytes.
+ *
+ * Return: number of cells that the caller should be considered while parsing
+ * the @map. It is > 0 for success, 0 for failure.
+ */
 static int of_map_id_cell_count(const __be32 *map, const char *map_name,
 				int map_len)
 {
+	const char *cells_prop_name __free(kfree) = NULL;
+	struct device_node *node __free(device_node) = NULL;
+	u32 cells;
+	int ret;
+
+	/* map + 1 is a controller. */
+	node = of_find_node_by_phandle(be32_to_cpup(map + 1));
+	if (!node) {
+		pr_err("Failed to find controller node from phandle\n");
+		return 0;
+	}
+
+	/* get #<name>-cells property from #<name>-map */
+	cells_prop_name = kasprintf(GFP_KERNEL, "#%.*scells",
+			(int)strlen(map_name) - 3, map_name);
+	if (!cells_prop_name)
+		return 0;
+
+	ret = of_property_read_u32(node, cells_prop_name, &cells);
+	if (ret) {
+		pr_err("%pOF: Failed to read %s property\n", node, cells_prop_name);
+		return 0;
+	}
+
 	return 1;
 }
 
-- 
2.34.1