Re: [PATCH v4 1/4] platform/x86: intel_scu_ipc: Check status after timeout in busy_loop()

[Ilpo Järvinen]

On Wed, 13 Sep 2023, 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.
> The readl_poll_timeout() macro implements all of this, and it is
> shorter, so use that macro here to consolidate code and fix this.
> 
> There were some concerns with using readl_poll_timeout() because it uses
> timekeeping, and timekeeping isn't running early on or during the late
> stages of system suspend or early stages of system resume, but an audit
> of the code concluded that this code isn't called during those times so
> it is safe to use the macro.
> 
> Cc: Prashant Malani <pmalani@xxxxxxxxxxxx>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Reviewed-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> Fixes: e7b7ab3847c9 ("platform/x86: intel_scu_ipc: Sleeping is fine when polling")
> Signed-off-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>

-- 
 i.


> ---
>  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..4c774ee8bb1b 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;
> +	u8 status;
> +	int err;
>  
> -	do {
> -		u32 status;
> +	err = readx_poll_timeout(ipc_read_status, scu, status, !(status & IPC_STATUS_BUSY),
> +				 100, jiffies_to_usecs(IPC_TIMEOUT));
> +	if (err)
> +		return err;
>  
> -		status = ipc_read_status(scu);
> -		if (!(status & IPC_STATUS_BUSY))
> -			return (status & IPC_STATUS_ERR) ? -EIO : 0;
> -
> -		usleep_range(50, 100);
> -	} while (time_before(jiffies, end));
> -
> -	return -ETIMEDOUT;
> +	return (status & IPC_STATUS_ERR) ? -EIO : 0;
>  }
>  
>  /* Wait till ipc ioc interrupt is received or timeout in 10 HZ */
>