Re: [PATCH v2 1/3] platform/x86: intel_scu_ipc: Check status after timeout in busy_loop()

[Mika Westerberg <mika.westerberg@linux.intel.com>]

On Wed, Sep 06, 2023 at 11:09:41AM -0700, Stephen Boyd wrote:
> It's possible for the polling loop in busy_loop() to get scheduled away
> for a long time.
> 
>   status = ipc_read_status(scu); // status = IPC_STATUS_BUSY
>   <long time scheduled away>
>   if (!(status & IPC_STATUS_BUSY))
> 
> If this happens, then the status bit could change while the task is
> scheduled away and this function would never read the status again after
> timing out. Instead, the function will return -ETIMEDOUT when it's
> possible that scheduling didn't work out and the status bit was cleared.
> Bit polling code should always check the bit being polled one more time
> after the timeout in case this happens.
> 
> Fix this by reading the status once more after the while loop breaks.
> 
> Cc: Prashant Malani <pmalani@chromium.org>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Fixes: e7b7ab3847c9 ("platform/x86: intel_scu_ipc: Sleeping is fine when polling")
> Signed-off-by: Stephen Boyd <swboyd@chromium.org>
> ---
> 
> This is sufficiently busy so I didn't add any tags from previous round.
> 
>  drivers/platform/x86/intel_scu_ipc.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/platform/x86/intel_scu_ipc.c b/drivers/platform/x86/intel_scu_ipc.c
> index 6851d10d6582..b2a2de22b8ff 100644
> --- a/drivers/platform/x86/intel_scu_ipc.c
> +++ b/drivers/platform/x86/intel_scu_ipc.c
> @@ -232,18 +232,21 @@ static inline u32 ipc_data_readl(struct intel_scu_ipc_dev *scu, u32 offset)
>  static inline int busy_loop(struct intel_scu_ipc_dev *scu)
>  {
>  	unsigned long end = jiffies + IPC_TIMEOUT;
> +	u32 status;
>  
>  	do {
> -		u32 status;
> -
>  		status = ipc_read_status(scu);
>  		if (!(status & IPC_STATUS_BUSY))
> -			return (status & IPC_STATUS_ERR) ? -EIO : 0;
> +			goto not_busy;
>  
>  		usleep_range(50, 100);
>  	} while (time_before(jiffies, end));
>  
> -	return -ETIMEDOUT;
> +	status = ipc_read_status(scu);

Does the issue happen again if we get scheduled away here for a long
time? ;-)

Regardless, I'm fine with this as is but if you make any changes, I
would prefer see readl_busy_timeout() used here instead (as was in the
previous version).

> +	if (status & IPC_STATUS_BUSY)
> +		return -ETIMEDOUT;
> +not_busy:
> +	return (status & IPC_STATUS_ERR) ? -EIO : 0;
>  }
>  
>  /* Wait till ipc ioc interrupt is received or timeout in 10 HZ */
> -- 
> https://chromeos.dev