Re: [PATCH v4 4/4] platform/x86: intel_scu_ipc: Fail IPC send if still busy

[Ilpo Järvinen]

On Wed, 13 Sep 2023, 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.
> 
> Cc: Prashant Malani <pmalani@xxxxxxxxxxxx>
> Cc: 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>

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

-- 
 i.

> ---
>  drivers/platform/x86/intel_scu_ipc.c | 40 +++++++++++++++++++---------
>  1 file changed, 28 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/platform/x86/intel_scu_ipc.c b/drivers/platform/x86/intel_scu_ipc.c
> index 3271f81a9c00..a68df4133403 100644
> --- a/drivers/platform/x86/intel_scu_ipc.c
> +++ b/drivers/platform/x86/intel_scu_ipc.c
> @@ -265,6 +265,24 @@ static int intel_scu_ipc_check_status(struct intel_scu_ipc_dev *scu)
>  	return scu->irq > 0 ? ipc_wait_for_interrupt(scu) : busy_loop(scu);
>  }
>  
> +static struct intel_scu_ipc_dev *intel_scu_ipc_get(struct intel_scu_ipc_dev *scu)
> +{
> +	u8 status;
> +
> +	if (!scu)
> +		scu = ipcdev;
> +	if (!scu)
> +		return ERR_PTR(-ENODEV);
> +
> +	status = ipc_read_status(scu);
> +	if (status & IPC_STATUS_BUSY) {
> +		dev_dbg(&scu->dev, "device is busy\n");
> +		return ERR_PTR(-EBUSY);
> +	}
> +
> +	return scu;
> +}
> +
>  /* Read/Write power control(PMIC in Langwell, MSIC in PenWell) registers */
>  static int pwr_reg_rdwr(struct intel_scu_ipc_dev *scu, u16 *addr, u8 *data,
>  			u32 count, u32 op, u32 id)
> @@ -278,11 +296,10 @@ static int pwr_reg_rdwr(struct intel_scu_ipc_dev *scu, u16 *addr, u8 *data,
>  	memset(cbuf, 0, sizeof(cbuf));
>  
>  	mutex_lock(&ipclock);
> -	if (!scu)
> -		scu = ipcdev;
> -	if (!scu) {
> +	scu = intel_scu_ipc_get(scu);
> +	if (IS_ERR(scu)) {
>  		mutex_unlock(&ipclock);
> -		return -ENODEV;
> +		return PTR_ERR(scu);
>  	}
>  
>  	for (nc = 0; nc < count; nc++, offset += 2) {
> @@ -437,12 +454,12 @@ int intel_scu_ipc_dev_simple_command(struct intel_scu_ipc_dev *scu, int cmd,
>  	int err;
>  
>  	mutex_lock(&ipclock);
> -	if (!scu)
> -		scu = ipcdev;
> -	if (!scu) {
> +	scu = intel_scu_ipc_get(scu);
> +	if (IS_ERR(scu)) {
>  		mutex_unlock(&ipclock);
> -		return -ENODEV;
> +		return PTR_ERR(scu);
>  	}
> +
>  	cmdval = sub << 12 | cmd;
>  	ipc_command(scu, cmdval);
>  	err = intel_scu_ipc_check_status(scu);
> @@ -482,11 +499,10 @@ int intel_scu_ipc_dev_command_with_size(struct intel_scu_ipc_dev *scu, int cmd,
>  		return -EINVAL;
>  
>  	mutex_lock(&ipclock);
> -	if (!scu)
> -		scu = ipcdev;
> -	if (!scu) {
> +	scu = intel_scu_ipc_get(scu);
> +	if (IS_ERR(scu)) {
>  		mutex_unlock(&ipclock);
> -		return -ENODEV;
> +		return PTR_ERR(scu);
>  	}
>  
>  	memcpy(inbuf, in, inlen);
>