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

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

On Wed, Aug 30, 2023 at 06:14:01PM -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);
>   <long time scheduled away>
>   if (!(status & IPC_STATUS_BUSY))

How can the status bit change here as we are the only user and the SCU
access is serialized by ipclock?

> If this happens, then the status bit could change and this function
> would never test it again after checking the jiffies against the timeout
> limit. Polling code should check the condition one more time after the
> timeout in case this happens.
> 
> The read_poll_timeout() helper implements this logic, and is shorter, so
> simply use that helper here.

Yes, I agree it makes the code simpler.

> 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>
> ---
>  drivers/platform/x86/intel_scu_ipc.c | 19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/platform/x86/intel_scu_ipc.c b/drivers/platform/x86/intel_scu_ipc.c
> index 6851d10d6582..5a37becc65aa 100644
> --- a/drivers/platform/x86/intel_scu_ipc.c
> +++ b/drivers/platform/x86/intel_scu_ipc.c
> @@ -19,6 +19,7 @@
>  #include <linux/init.h>
>  #include <linux/interrupt.h>
>  #include <linux/io.h>
> +#include <linux/iopoll.h>
>  #include <linux/module.h>
>  #include <linux/slab.h>
>  
> @@ -231,19 +232,15 @@ static inline u32 ipc_data_readl(struct intel_scu_ipc_dev *scu, u32 offset)
>  /* Wait till scu status is busy */
>  static inline int busy_loop(struct intel_scu_ipc_dev *scu)
>  {
> -	unsigned long end = jiffies + IPC_TIMEOUT;
> -
> -	do {
> -		u32 status;
> -
> -		status = ipc_read_status(scu);
> -		if (!(status & IPC_STATUS_BUSY))
> -			return (status & IPC_STATUS_ERR) ? -EIO : 0;
> +	u8 status;
> +	int err;
>  
> -		usleep_range(50, 100);
> -	} while (time_before(jiffies, end));
> +	err = read_poll_timeout(ipc_read_status, status, !(status & IPC_STATUS_BUSY),
> +				100, jiffies_to_usecs(IPC_TIMEOUT), false, scu);
> +	if (err)
> +		return err;
>  
> -	return -ETIMEDOUT;
> +	return (status & IPC_STATUS_ERR) ? -EIO : 0;
>  }
>  
>  /* Wait till ipc ioc interrupt is received or timeout in 10 HZ */
> -- 
> https://chromeos.dev