[PATCH] accel/amdxdna: document trusted mailbox ring geometry and drop the power-of-two check

[Eva Crystal]

The mailbox ring buffer geometry comes from AMD signed firmware, through
the management or mailbox information block, or through the CREATE_CONTEXT
response, and the driver trusts it by design.

The power-of-two test on rb_size was a firmware debugging aid. Nothing in
the driver derives a mask, a shift or a modulo from rb_size, so no code
depends on the property it tested. The ring index wrap is handled by the
explicit comparisons in mailbox_send_msg() and mailbox_get_msg().

Replace the test with a comment that records the design, so that
automated review does not flag the absence of a bound here again.

Suggested-by: Lizhi Hou <lizhi.hou@xxxxxxx>
Link: https://lore.kernel.org/all/67698952-6394-44a8-01fe-3e3558b7cb4c@xxxxxxx/
Signed-off-by: Eva Crystal <0xiviel@xxxxxxxxx>
---
The pkg_size check in xdna_mailbox_send_msg() is left in place on
purpose: it bounds a driver-supplied size, and with Max Zhen's pool
patch (20261002161122.1350075-1-max.zhen@xxxxxxx) it is the only bound
on the write into a pre-allocated message slot.

 drivers/accel/amdxdna/amdxdna_mailbox.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/accel/amdxdna/amdxdna_mailbox.c b/drivers/accel/amdxdna/amdxdna_mailbox.c
index 05c3786de135..c4a36858668f 100644
--- a/drivers/accel/amdxdna/amdxdna_mailbox.c
+++ b/drivers/accel/amdxdna/amdxdna_mailbox.c
@@ -513,11 +513,13 @@ xdna_mailbox_start_channel(struct mailbox_channel *mb_chann,
 {
 	int ret;
 
-	if (!is_power_of_2(x2i->rb_size) || !is_power_of_2(i2x->rb_size)) {
-		pr_err("Ring buf size must be power of 2\n");
-		return -EINVAL;
-	}
-
+	/*
+	 * The ring buffer geometry, rb_start_addr and rb_size for both the
+	 * x2i and the i2x channel, comes from AMD signed firmware, through
+	 * the management or mailbox information block, or through the
+	 * CREATE_CONTEXT response. The driver trusts those values by design
+	 * and does not bound them against the mapped region.
+	 */
 	mb_chann->msix_irq = mb_irq;
 	mb_chann->iohub_int_addr = iohub_int_addr;
 	memcpy(&mb_chann->res[CHAN_RES_X2I], x2i, sizeof(*x2i));