Re: [PATCH 1/9] macintosh/via-macii: Access autopoll_devs when inside lock

[Guenter Roeck <linux@roeck-us.net>]

Hi,

On Sun, Jun 28, 2020 at 02:23:12PM +1000, Finn Thain wrote:
> The interrupt handler should be excluded when accessing the autopoll_devs
> variable.
> 

I am quite baffled by this patch. Other than adding an unnecessary lock /
unlock sequence, accessing a variable (which is derived from another
variable) from inside or outside a lock does not make a difference.
If autopoll_devs = devs & 0xfffe is 0 inside the lock, it will just
as much be 0 outside the lock, and vice versa.

Can you explain this in some more detail ? Not that is matters much since
the change already made it into mainline, but I would like to understand
what if anything I am missing here.

Thanks,
Guenter

> Fixes: d95fd5fce88f0 ("m68k: Mac II ADB fixes") # v5.0+
> Tested-by: Stan Johnson <userm57@yahoo.com>
> Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
> ---
>  drivers/macintosh/via-macii.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/macintosh/via-macii.c b/drivers/macintosh/via-macii.c
> index ac824d7b2dcfc..6aa903529570d 100644
> --- a/drivers/macintosh/via-macii.c
> +++ b/drivers/macintosh/via-macii.c
> @@ -270,15 +270,12 @@ static int macii_autopoll(int devs)
>  	unsigned long flags;
>  	int err = 0;
>  
> +	local_irq_save(flags);
> +
>  	/* bit 1 == device 1, and so on. */
>  	autopoll_devs = devs & 0xFFFE;
>  
> -	if (!autopoll_devs)
> -		return 0;
> -
> -	local_irq_save(flags);
> -
> -	if (current_req == NULL) {
> +	if (autopoll_devs && !current_req) {
>  		/* Send a Talk Reg 0. The controller will repeatedly transmit
>  		 * this as long as it is idle.
>  		 */