linux-kernel - Re: [PATCH] firmware: arm_scmi: fix SMCCC_RET_NOT

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Date:   Thu, 14 May 2020 17:06:22 +0200
From:   Etienne Carriere <etienne.carriere@...aro.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-kernel@...r.kernel.org,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] firmware: arm_scmi: fix SMCCC_RET_NOT_SUPPORTED management

On Thu, 14 May 2020 at 16:29, Sudeep Holla <sudeep.holla@....com> wrote:
>
> On Thu, May 14, 2020 at 10:24:28AM +0200, Etienne Carriere wrote:
> > Fix management of argument a0 output value of arm_smccc_1_1_invoke() that
> > should consider only SMCCC_RET_NOT_SUPPORTED as reporting an unsupported
> > function ID as correctly stated in the inline comment.
> >
>
> I agree on the comment part, but ...
>
> > Signed-off-by: Etienne Carriere <etienne.carriere@...aro.org>
> > ---
> >  drivers/firmware/arm_scmi/smc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/firmware/arm_scmi/smc.c b/drivers/firmware/arm_scmi/smc.c
> > index 49bc4b0e8428..637ad439545f 100644
> > --- a/drivers/firmware/arm_scmi/smc.c
> > +++ b/drivers/firmware/arm_scmi/smc.c
> > @@ -115,7 +115,7 @@ static int smc_send_message(struct scmi_chan_info *cinfo,
> >       mutex_unlock(&scmi_info->shmem_lock);
> >
> >       /* Only SMCCC_RET_NOT_SUPPORTED is valid error code */
> > -     if (res.a0)
> > +     if (res.a0 == SMCCC_RET_NOT_SUPPORTED)
> >               return -EOPNOTSUPP;
>
> Now this will return 0 for all values other than SMCCC_RET_NOT_SUPPORTED.
> Is that what we need ? Or do you see non-zero res.a0 for a success case ?
> If later, we need some fixing, otherwise it is safer to leave it as is
> IMO.

Firmware following SMCCC v1.x for some OEM/SiP invocation may simply
not modify invocation register argument a0 on invocation with a
SCMI-SMC transport function ID.
Resulting in res.a0 == scmi_info->func_id here. Which is, by SMCCC
v1.x not an error.

>From SMCCC v1.x only SMCCC_RET_NOT_SUPPORTED (-1 signed extended is a
reserved ) is a generic return error whatever function ID value.

Or consider part of the SCMI-SMC transport API that output arg a0
shall be 0 on success,
SMCCC_RET_NOT_SUPPORTED if function ID is not supported
and any non-zero value for non-generic **error** codes.

etienne

>
> --
> Regards,
> Sudeep