On Wed, 13 Sep 2023, Stephen Boyd wrote:
> It's possible for the completion in ipc_wait_for_interrupt() to timeout,
> simply because the interrupt was delayed in being processed. A timeout
> in itself is not an error. This driver should check the status register
> upon a timeout to ensure that scheduling or interrupt processing delays
> don't affect the outcome of the IPC return value.
>
> CPU0 SCU
> ---- ---
> ipc_wait_for_interrupt()
> wait_for_completion_timeout(&scu->cmd_complete)
> [TIMEOUT] status[IPC_STATUS_BUSY]=0
>
> Fix this problem by reading the status bit in all cases, regardless of
> the timeout. If the completion times out, we'll assume the problem was
> that the IPC_STATUS_BUSY bit was still set, but if the status bit is
> cleared in the meantime we know that we hit some scheduling delay and we
> should just check the error bit.
Hi,
I don't understand the intent here. What prevents IPC_STATUS_BUSY from
changing right after you've read it in ipc_read_status(scu)? Doesn't that
end you exactly into the same situation where the returned value is stale
so I cannot see how this fixes anything, at best it just plays around the
race window that seems to still be there after this fix?
--
i.
> Cc: Prashant Malani <pmalani@xxxxxxxxxxxx>
> Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Reviewed-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> Fixes: ed12f295bfd5 ("ipc: Added support for IPC interrupt mode")
> Signed-off-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
> ---
> drivers/platform/x86/intel_scu_ipc.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/platform/x86/intel_scu_ipc.c b/drivers/platform/x86/intel_scu_ipc.c
> index 4c774ee8bb1b..299c15312acb 100644
> --- a/drivers/platform/x86/intel_scu_ipc.c
> +++ b/drivers/platform/x86/intel_scu_ipc.c
> @@ -248,10 +248,12 @@ static inline int ipc_wait_for_interrupt(struct intel_scu_ipc_dev *scu)
> {
> int status;
>
> - if (!wait_for_completion_timeout(&scu->cmd_complete, IPC_TIMEOUT))
> - return -ETIMEDOUT;
> + wait_for_completion_timeout(&scu->cmd_complete, IPC_TIMEOUT);
>
> status = ipc_read_status(scu);
> + if (status & IPC_STATUS_BUSY)
> + return -ETIMEDOUT;
> +
> if (status & IPC_STATUS_ERR)
> return -EIO;
>
>
