Re: [PATCH v2 3/3] platform/x86: intel_scu_ipc: Fail IPC send if still busy

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Wed, Sep 06, 2023 at 11:09:43AM -0700, Stephen Boyd wrote:
> It's possible for interrupts to get significantly delayed to the point
> that callers of intel_scu_ipc_dev_command() and friends can call the
> function once, hit a timeout, and call it again while the interrupt
> still hasn't been processed. This driver will get seriously confused if
> the interrupt is finally processed after the second IPC has been sent
> with ipc_command(). It won't know which IPC has been completed. This
> could be quite disastrous if calling code assumes something has happened
> upon return from intel_scu_ipc_dev_simple_command() when it actually
> hasn't.
> 
> Let's avoid this scenario by simply returning -EBUSY in this case.
> Hopefully higher layers will know to back off or fail gracefully when
> this happens. It's all highly unlikely anyway, but it's better to be
> correct here as we have no way to know which IPC the status register is
> telling us about if we send a second IPC while the previous IPC is still
> processing.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Also see below.

...

> @@ -450,6 +468,12 @@ int intel_scu_ipc_dev_simple_command(struct intel_scu_ipc_dev *scu, int cmd,
>  		return -ENODEV;
>  	}

>  	scu = ipcdev;

Side observation: Isn't this a bug? We should not override the supplied parameter.

> +	err = intel_scu_ipc_busy(scu);
> +	if (err) {
> +		mutex_unlock(&ipclock);
> +		return err;
> +	}
> +
>  	cmdval = sub << 12 | cmd;
>  	ipc_command(scu, cmdval);
>  	err = intel_scu_ipc_check_status(scu);

-- 
With Best Regards,
Andy Shevchenko