[PATCH v6 12/13] firmware: qcom: scm: clarify the comment in qcom_scm_pas_init_image()

[Bartosz Golaszewski]

From: Bartosz Golaszewski <bartosz.golaszewski at linaro.org>

The "memory protection" mechanism mentioned in the comment is the SHM
Bridge. This is also the reason why we do not convert this call to using
the TZ memory allocator.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski at linaro.org>
Tested-by: Andrew Halaney <ahalaney at redhat.com> # sc8280xp-lenovo-thinkpad-x13s
---
 drivers/firmware/qcom/qcom_scm.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
index 839773270a21..7ba5cff6e4e7 100644
--- a/drivers/firmware/qcom/qcom_scm.c
+++ b/drivers/firmware/qcom/qcom_scm.c
@@ -563,9 +563,13 @@ int qcom_scm_pas_init_image(u32 peripheral, const void *metadata, size_t size,
 	struct qcom_scm_res res;
 
 	/*
-	 * During the scm call memory protection will be enabled for the meta
-	 * data blob, so make sure it's physically contiguous, 4K aligned and
-	 * non-cachable to avoid XPU violations.
+	 * During the SCM call the hypervisor will make the buffer containing
+	 * the program data into an SHM Bridge. This is why we exceptionally
+	 * must not use the TrustZone memory allocator here as - depending on
+	 * Kconfig - it may already use the SHM Bridge mechanism internally.
+	 *
+	 * If we pass a buffer that is already part of an SHM Bridge to this
+	 * call, it will fail.
 	 */
 	mdata_buf = dma_alloc_coherent(__scm->dev, size, &mdata_phys,
 				       GFP_KERNEL);
-- 
2.40.1