linux-kernel - [PATCH v3 14/15] firmware: qcom: scm: clarify the comment in qcom_scm_pas_init

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Message-Id: <20231009153427.20951-15-brgl@bgdev.pl>
Date:   Mon,  9 Oct 2023 17:34:26 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Elliot Berman <quic_eberman@...cinc.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Guru Das Srinagesh <quic_gurus@...cinc.com>,
        Andrew Halaney <ahalaney@...hat.com>,
        Maximilian Luz <luzmaximilian@...il.com>,
        Alex Elder <elder@...aro.org>,
        Srini Kandagatla <srinivas.kandagatla@...aro.org>
Cc:     linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kernel@...cinc.com,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: [PATCH v3 14/15] firmware: qcom: scm: clarify the comment in qcom_scm_pas_init_image()

From: Bartosz Golaszewski <bartosz.golaszewski@...aro.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 TM mem allocator.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
---
 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..8a2475ced10a 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 TrustZone 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);
-- 
2.39.2