linux-kernel - Re: [PATCH v2 6/6] firmware: arm_scmi: Check if platform has released shmem before using

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Date:   Mon, 5 Aug 2019 14:33:53 +0200
From:   Etienne Carriere <etienne.carriere@...aro.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, Peng Fan <peng.fan@....com>,
        linux-kernel@...r.kernel.org,
        Bo Zhang <bozhang.zhang@...adcom.com>,
        Jim Quinlan <james.quinlan@...adcom.com>,
        Volodymyr Babchuk <volodymyr_babchuk@...m.com>,
        Gaku Inami <gaku.inami.xh@...esas.com>
Subject: Re: [PATCH v2 6/6] firmware: arm_scmi: Check if platform has released
 shmem before using

Hello Sudeep,

On Fri, 26 Jul 2019 at 15:46, Sudeep Holla <sudeep.holla@....com> wrote:
>
> Sometimes platfom may take too long to respond to the command and OS
> might timeout before platform transfer the ownership of the shared
> memory region to the OS with the response.
>
> Since the mailbox channel associated with the channel is freed and new
> commands are dispatch on the same channel, OS needs to wait until it
> gets back the ownership. If not, either OS may end up overwriting the
> platform response for the last command(which is fine as OS timed out
> that command) or platform might overwrite the payload for the next
> command with the response for the old.
>
> The latter is problematic as platform may end up interpretting the
> response as the payload. In order to avoid such race, let's wait until
> the OS gets back the ownership before we prepare the shared memory with
> the payload for the next command.
>
> Reported-by: Jim Quinlan <james.quinlan@...adcom.com>
> Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> ---
>  drivers/firmware/arm_scmi/driver.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
> index 69bf85fea967..765573756987 100644
> --- a/drivers/firmware/arm_scmi/driver.c
> +++ b/drivers/firmware/arm_scmi/driver.c
> @@ -265,6 +265,14 @@ static void scmi_tx_prepare(struct mbox_client *cl, void *m)
>         struct scmi_chan_info *cinfo = client_to_scmi_chan_info(cl);
>         struct scmi_shared_mem __iomem *mem = cinfo->payload;
>
> +       /*
> +        * Ideally channel must be free by now unless OS timeout last
> +        * request and platform continued to process the same, wait
> +        * until it releases the shared memory, otherwise we may endup
> +        * overwriting it's response with new command payload or vice-versa

minor typo: s/it's/its/
maybe also s/command/message/

regards,
etienne


> +        */
> +       spin_until_cond(ioread32(&mem->channel_status) &
> +                       SCMI_SHMEM_CHAN_STAT_CHANNEL_FREE);
>         /* Mark channel busy + clear error */
>         iowrite32(0x0, &mem->channel_status);
>         iowrite32(t->hdr.poll_completion ? 0 : SCMI_SHMEM_FLAG_INTR_ENABLED,
> --
> 2.17.1
>