Re: [PATCH v4 2/4] platform/x86: intel_scu_ipc: Check status upon timeout in ipc_wait_for_interrupt()

[Ilpo Järvinen]

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;
>  
>