[PATCH v5] power: reset: nvmem-reboot-mode: support smaller magic

[Jennifer Berringer <jberring@redhat.com>]

Some devices, such as Qualcomm sa8775p, have an nvmem reboot mode cell
that is only 1 byte, which resulted in nvmem_reboot_mode_write() failing
when it attempts to write a 4-byte magic. Checking the nvmem cell size
and writing only the lower bits of the reboot mode magic is needed for
these devices.

Signed-off-by: Jennifer Berringer <jberring@redhat.com>
---
This patch depends on [1].

[1] https://lore.kernel.org/all/20250318094716.3053546-5-o.rempel@pengutronix.de/

Changes v5:
- Switch to using nvmem_cell_get_size() from the above linked patch
  instead of introducing another function with the same purpose.
---
 drivers/power/reset/nvmem-reboot-mode.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/power/reset/nvmem-reboot-mode.c b/drivers/power/reset/nvmem-reboot-mode.c
index 41530b70cfc4..6be178be4a58 100644
--- a/drivers/power/reset/nvmem-reboot-mode.c
+++ b/drivers/power/reset/nvmem-reboot-mode.c
@@ -20,11 +20,23 @@ static int nvmem_reboot_mode_write(struct reboot_mode_driver *reboot,
 				    unsigned int magic)
 {
 	int ret;
+	u8 *magic_ptr = (u8 *) &magic;
+	size_t cell_size;
 	struct nvmem_reboot_mode *nvmem_rbm;
 
 	nvmem_rbm = container_of(reboot, struct nvmem_reboot_mode, reboot);
 
-	ret = nvmem_cell_write(nvmem_rbm->cell, &magic, sizeof(magic));
+	ret = nvmem_cell_get_size(nvmem_rbm->cell, &cell_size, NULL);
+	if (ret < 0) {
+		dev_err(reboot->dev, "failed to get reboot mode nvmem cell size\n");
+		return ret;
+	}
+
+	/* Use magic's low-order bytes when writing to a smaller cell. */
+	if (IS_ENABLED(CONFIG_CPU_BIG_ENDIAN) && cell_size < sizeof(magic))
+		magic_ptr += sizeof(magic) - cell_size;
+
+	ret = nvmem_cell_write(nvmem_rbm->cell, magic_ptr, MIN(cell_size, sizeof(magic)));
 	if (ret < 0)
 		dev_err(reboot->dev, "update reboot mode bits failed\n");
 
-- 
2.48.1