linux-kernel - [PATCH v9 12/13] firmware: qcom: scm: clarify the comment in qcom_scm_pas_init

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Date: Mon, 25 Mar 2024 11:03:58 +0100
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>,
	Arnd Bergmann <arnd@...db.de>
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>,
	Deepti Jaggi <quic_djaggi@...cinc.com>
Subject: [PATCH v9 12/13] 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 TZ memory allocator.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Tested-by: Andrew Halaney <ahalaney@...hat.com> # sc8280xp-lenovo-thinkpad-x13s
Tested-by: Deepti Jaggi <quic_djaggi@...cinc.com> #sa8775p-ride
Reviewed-by: Elliot Berman <quic_eberman@...cinc.com>
---
 drivers/firmware/qcom/qcom_scm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/firmware/qcom/qcom_scm.c b/drivers/firmware/qcom/qcom_scm.c
index 7a8d67c7b71c..1972757f8931 100644
--- a/drivers/firmware/qcom/qcom_scm.c
+++ b/drivers/firmware/qcom/qcom_scm.c
@@ -563,6 +563,13 @@ int qcom_scm_pas_init_image(u32 peripheral, const void *metadata, size_t size,
 	 * 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.
+	 *
+	 * For PIL calls the hypervisor creates SHM Bridges for the blob
+	 * buffers on behalf of Linus so we must not do it ourselves hence
+	 * not using the TZMem allocator here.
+	 *
+	 * 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