Re: [PATCH v3 1/2] firmware: qcom: scm: Introduce new locking mechanism for SCM driver

[Pavan Kondeti]

On Thu, Oct 01, 2026 at 06:16:48PM +0200, Albert Esteve wrote:
> From: Ninad Naik <quic_ninanaik@xxxxxxxxxxx>
> 
> qcom_scm holds its global mutex across WAITQ_SLEEP and
> wait_for_completion(). Firmware waitqs allow multiple SMCs in
> flight (wq_cnt). If one call is parked on a waitq while holding
> the mutex, a second call (e.g., SMCInvoke) cannot enter firmware
> and both stall on the waitq.
> 
> Replace the global mutex with a counting semaphore sized from wq_cnt,
> with at least 1 wait queue.
> 
> Fixes: ccd207ec848e ("firmware: qcom_scm: Support multiple waitq contexts")
> Signed-off-by: Murali Nalajala <quic_mnalajal@xxxxxxxxxxx>
> Co-developed-by: Guru Das Srinagesh <quic_gurus@xxxxxxxxxxx>
> Signed-off-by: Guru Das Srinagesh <quic_gurus@xxxxxxxxxxx>
> Signed-off-by: Venkatakrishnaiah Pari <quic_vpari@xxxxxxxxxxx>
> Signed-off-by: Jian Shu <quic_jianshu@xxxxxxxxxxx>
> Signed-off-by: Ninad Naik <quic_ninanaik@xxxxxxxxxxx>
> Signed-off-by: Albert Esteve <aesteve@xxxxxxxxxx>
> ---
>  drivers/firmware/qcom/qcom_scm-legacy.c | 8 ++------
>  drivers/firmware/qcom/qcom_scm-smc.c    | 7 ++-----
>  drivers/firmware/qcom/qcom_scm.c        | 6 +++++-
>  drivers/firmware/qcom/qcom_scm.h        | 3 +++
>  4 files changed, 12 insertions(+), 12 deletions(-)
> 
I don't know all the history, but in downstream [1] selective calls only
allowed to sleep w/o mutex.

Mukesh, do you know if it is safe to allow all slow calls w/o mutex?

Thanks,
Pavan

[1] https://git.codelinaro.org/clo/la/kernel/qcom/-/blob/KERNEL.PLATFORM.5.0.r35-02100-kernel.0/drivers/firmware/qcom/qcom_scm.c?ref_type=tags#L3057