Re: [PATCH 02/14] firmware: qcom: Add a generic PAS service

Krzysztof Kozlowski
2026-03-06T11:15:13Z

On 06/03/2026 11:50, Sumit Garg wrote:
> From: Sumit Garg <sumit.garg@oss.qualcomm.com>
>
> Qcom platforms has the legacy of using non-standard SCM calls
> splintered over the various kernel drivers. These SCM calls aren't
> compliant with the standard SMC calling conventions which is a
> prerequisite to enable migration to the FF-A specifications from
> Arm.
>
> OP-TEE as an alternative trusted OS to QTEE can't support these non-
> standard SCM calls. And even for newer architectures QTEE won't be able
> to support SCM calls either with FF-A requirements coming in. And with
> both OP-TEE and QTEE drivers well integrated in the TEE subsystem, it
> makes further sense to reuse the TEE bus client drivers infrastructure.
>
> The added benefit of TEE bus infrastructure is that there is support
> for discoverable/enumerable services. With that client drivers don't
> have to manually invoke a special SCM call to know the service status.
>
> So enable the generic Peripheral Authentication Service (PAS) provided
> by the firmware. It acts as the common layer with different TZ
> backends plugged in whether it's an SCM implementation or a proper
> TEE bus based PAS service implementation.
>
> Signed-off-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
> ---
> drivers/firmware/qcom/Kconfig | 8 +
> drivers/firmware/qcom/Makefile | 1 +
> drivers/firmware/qcom/qcom_pas.c | 295 +++++++++++++++++++++++++
> drivers/firmware/qcom/qcom_pas.h | 53 +++++
> include/linux/firmware/qcom/qcom_pas.h | 41 ++++
> 5 files changed, 398 insertions(+)
> create mode 100644 drivers/firmware/qcom/qcom_pas.c
> create mode 100644 drivers/firmware/qcom/qcom_pas.h
> create mode 100644 include/linux/firmware/qcom/qcom_pas.h
>
> diff --git a/drivers/firmware/qcom/Kconfig b/drivers/firmware/qcom/Kconfig
> index b477d54b495a..8653639d06db 100644
> --- a/drivers/firmware/qcom/Kconfig
> +++ b/drivers/firmware/qcom/Kconfig
> @@ -6,6 +6,14 @@
>
> menu "Qualcomm firmware drivers"
>
> +config QCOM_PAS
> + tristate
> + help
> + Enable the generic Peripheral Authentication Service (PAS) provided
> + by the firmware. It acts as the common layer with different TZ
> + backends plugged in whether it's an SCM implementation or a proper
> + TEE bus based PAS service implementation.
> +
> config QCOM_SCM
> select QCOM_TZMEM
> tristate
> diff --git a/drivers/firmware/qcom/Makefile b/drivers/firmware/qcom/Makefile
> index 0be40a1abc13..dc5ab45f906a 100644
> --- a/drivers/firmware/qcom/Makefile
> +++ b/drivers/firmware/qcom/Makefile
> @@ -8,3 +8,4 @@ qcom-scm-objs += qcom_scm.o qcom_scm-smc.o qcom_scm-legacy.o
> obj-$(CONFIG_QCOM_TZMEM) += qcom_tzmem.o
> obj-$(CONFIG_QCOM_QSEECOM) += qcom_qseecom.o
> obj-$(CONFIG_QCOM_QSEECOM_UEFISECAPP) += qcom_qseecom_uefisecapp.o
> +obj-$(CONFIG_QCOM_PAS) += qcom_pas.o
> diff --git a/drivers/firmware/qcom/qcom_pas.c b/drivers/firmware/qcom/qcom_pas.c
> new file mode 100644
> index 000000000000..dc04ff1b6be0
> --- /dev/null
> +++ b/drivers/firmware/qcom/qcom_pas.c
> @@ -0,0 +1,295 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) Qualcomm Technologies, Inc. and/or its subsidiaries.
> + */
> +
> +#include <linux/delay.h>
> +#include <linux/device/devres.h>
> +#include <linux/firmware/qcom/qcom_pas.h>
> +#include <linux/of.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +
> +#include "qcom_pas.h"
> +#include "qcom_scm.h"
> +
> +static struct qcom_pas_ops *ops_ptr;

I really dislike this singleton design. And it is not even needed! If
you were storing here some allocated instance of SCM/PAS I could
understand, but singleton for only ops? Just implement one driver (so
SCM + whatever you have here) which will decide which ops to use,
through the probe. Really, this is neither needed nor beneficial.

It actually leads to more problems with this barrier handling, see
further comments.
...

> +
> +/**
> + * qcom_pas_shutdown() - Shut down the remote processor
> + * @pas_id: peripheral authentication service id
> + *
> + * Returns 0 on success.
> + */
> +int qcom_pas_shutdown(u32 pas_id)
> +{
> + if (ops_ptr)
> + return ops_ptr->shutdown(ops_ptr->dev, pas_id);
> +
> + return -ENODEV;
> +}
> +EXPORT_SYMBOL_GPL(qcom_pas_shutdown);
> +
> +/**
> + * qcom_pas_supported() - Check if the peripheral authentication service is
> + * available for the given peripheral
> + * @pas_id: peripheral authentication service id
> + *
> + * Returns true if PAS is supported for this peripheral, otherwise false.
> + */
> +bool qcom_pas_supported(u32 pas_id)
> +{
> + if (ops_ptr)

Lack of barriers here is not looking right. Existing/old code is not a
good example, I fixed only the obvious issue, but new code should be
correct from the beginning.

Barriers should normally be always paired, unless you have some clear
path no concurrent execution can happen here, but such explanation is
missing, look:

> + return ops_ptr->supported(ops_ptr->dev, pas_id);
> +
> + return false;
> +}
> +EXPORT_SYMBOL_GPL(qcom_pas_supported);
> +
> +/**
> + * qcom_pas_is_available() - Check for PAS service
> + *
> + * Returns true on success.
> + */
> +bool qcom_pas_is_available(void)
> +{
> + /* The barrier is needed to synchronize with client drivers. */

Here. This is pretty pointless/redundant comment. Obviously barriers are
to synchronize with whoever is calling this and only clients are calling.

You must say something useful, not just barrier is a barrier... It's
like documenting mutex "to synchronize".

> + return !!smp_load_acquire(&ops_ptr);

Best regards,
Krzysztof