linux-kernel - Re: [PATCH v4 2/2] firmware: arm_scmi: Augment SMC/HVC to allow optional interrupt

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Date:   Tue, 5 Jan 2021 17:35:26 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Jim Quinlan <jim2101024@...il.com>,
        Sudeep Holla <sudeep.holla@....com>,
        bcm-kernel-feedback-list@...adcom.com, james.quinlan@...adcom.com,
        "open list:SYSTEM CONTROL & POWER/MANAGEMENT INTERFACE Mes..." 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/2] firmware: arm_scmi: Augment SMC/HVC to allow
 optional interrupt

On Tue, Dec 22, 2020 at 07:37:22PM -0800, Florian Fainelli wrote:
> 
> 
> On 12/22/2020 6:56 AM, Jim Quinlan wrote:
> > The SMC/HVC SCMI transport is modified to allow the completion of an SCMI
> > message to be indicated by an interrupt rather than the return of the smc
> > call.  This accommodates the existing behavior of the BrcmSTB SCMI
> > "platform" whose SW is already out in the field and cannot be changed.
> > 
> > Signed-off-by: Jim Quinlan <jim2101024@...il.com>
> 
> This looks good to me, just one question below:
> 
> [snip]
> 
> > @@ -111,6 +145,8 @@ static int smc_send_message(struct scmi_chan_info *cinfo,
> >  	shmem_tx_prepare(scmi_info->shmem, xfer);
> >  
> >  	arm_smccc_1_1_invoke(scmi_info->func_id, 0, 0, 0, 0, 0, 0, 0, &res);
> > +	if (scmi_info->irq)
> > +		wait_for_completion(&scmi_info->tx_complete);
> 
> Do we need this to have a preceding call to reinit_completion()? It does
> not look like this is going to make any practical difference but there
> are drivers doing that for correctness.

Why do you think that might not cause any issue ? After first message
is completed and ISR is executed, the completion flag remains done for
ever. So practically 2nd message onwards won't block in wait_for_completion
which means return from smc/hvc is actually completion too which is clearly
wrong or am I missing something ?

Jim, please confirm either way. If you agree I can add the below snippet,
no need to repost.

Regards,
Sudeep

--
diff --git i/drivers/firmware/arm_scmi/smc.c w/drivers/firmware/arm_scmi/smc.c
index fd41d436e34b..86eac0831d3c 100644
--- i/drivers/firmware/arm_scmi/smc.c
+++ w/drivers/firmware/arm_scmi/smc.c
@@ -144,6 +145,8 @@ static int smc_send_message(struct scmi_chan_info *cinfo,

        shmem_tx_prepare(scmi_info->shmem, xfer);

+       if (scmi_info->irq)
+               reinit_completion(&scmi_info->tx_complete);
        arm_smccc_1_1_invoke(scmi_info->func_id, 0, 0, 0, 0, 0, 0, 0, &res);
        if (scmi_info->irq)
                wait_for_completion(&scmi_info->tx_complete);