linux-kernel - [PATCH v10 13/15] firmware: qcom: scm: clarify the comment in qcom_scm_pas_init

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Message-Id: <20240527-shm-bridge-v10-13-ce7afaa58d3a@linaro.org>
Date: Mon, 27 May 2024 14:55:03 +0200
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Bjorn Andersson <andersson@...nel.org>, 
 Konrad Dybcio <konrad.dybcio@...aro.org>, Rob Herring <robh@...nel.org>, 
 Krzysztof Kozlowski <krzk+dt@...nel.org>, 
 Conor Dooley <conor+dt@...nel.org>, Robert Marko <robimarko@...il.com>, 
 Das Srinagesh <quic_gurus@...cinc.com>, 
 Bartosz Golaszewski <bartosz.golaszewski@...aro.org>, 
 Maximilian Luz <luzmaximilian@...il.com>, 
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
 Srini Kandagatla <srinivas.kandagatla@...aro.org>, 
 Arnd Bergmann <arnd@...db.de>, Elliot Berman <quic_eberman@...cinc.com>, 
 Alex Elder <elder@...nel.org>
Cc: linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org, 
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
 kernel@...cinc.com, Andrew Halaney <ahalaney@...hat.com>, 
 Deepti Jaggi <quic_djaggi@...cinc.com>
Subject: [PATCH v10 13/15] 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 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 c82957727650..86e26f17ca19 100644
--- a/drivers/firmware/qcom/qcom_scm.c
+++ b/drivers/firmware/qcom/qcom_scm.c
@@ -583,6 +583,13 @@ 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);

-- 
2.43.0