Re: [PATCH] platform/x86: drop unneeded assignment in host_control_smi()

From: Hans de Goede <hdegoede@redhat.com>
To: Yang Li <yang.lee@linux.alibaba.com>, stuart.w.hayes@gmail.com
Cc: mgross@linux.intel.com, platform-driver-x86@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] platform/x86: drop unneeded assignment in host_control_smi()
Date: Tue, 11 May 2021 13:10:55 +0200	[thread overview]
Message-ID: <7c959c70-dc9f-c34c-e7fb-8b3c1c5fe2ab@redhat.com> (raw)
In-Reply-To: <1619523119-112342-1-git-send-email-yang.lee@linux.alibaba.com>

Hi Yang Li,

On 4/27/21 1:31 PM, Yang Li wrote:
> Making '==' operation with ESM_STATUS_CMD_UNSUCCESSFUL directly
> after calling the function inb() is more efficient, so assignment
> to 'cmd_status' is redundant.
> 
> Eliminate the following clang_analyzer warning:
> drivers/platform/x86/dell/dcdbas.c:397:11: warning: Although the value
> stored to 'cmd_status' is used in the enclosing expression, the value is
> never actually read from 'cmd_status'
> 
> No functional change.
> 
> Reported-by: Abaci Robot <abaci@linux.alibaba.com>
> Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>

Thank you for your patch, but as the "kernel test robot <lkp@intel.com>"
reported, this actually breaks the check in the while loop.

cmd_status is a s8 and ESM_STATUS_CMD_UNSUCCESSFUL is defined as -1.

By dropping the intermediate step of storing the inb() value into the
s8, we end up comparing the inb() unsigned result directly to -1 which
is never true.

A possible way to fix this (without reworking the rest of the code) would
be to either cast the inb() result to a s8, so that you end up with this:

		while ((s8)inb(PCAT_APM_STATUS_PORT) == ESM_STATUS_CMD_UNSUCCESSFUL) {

Also while at it please change the condition to a single line as I did
above.

Thanks & Regards,

Hans

> ---
>  drivers/platform/x86/dell/dcdbas.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/platform/x86/dell/dcdbas.c b/drivers/platform/x86/dell/dcdbas.c
> index d513a59..a9e8a88 100644
> --- a/drivers/platform/x86/dell/dcdbas.c
> +++ b/drivers/platform/x86/dell/dcdbas.c
> @@ -394,7 +394,7 @@ static int host_control_smi(void)
>  
>  		/* wait a few to see if it executed */
>  		num_ticks = TIMEOUT_USEC_SHORT_SEMA_BLOCKING;
> -		while ((cmd_status = inb(PCAT_APM_STATUS_PORT))
> +		while (inb(PCAT_APM_STATUS_PORT)
>  		       == ESM_STATUS_CMD_UNSUCCESSFUL) {
>  			num_ticks--;
>  			if (num_ticks == EXPIRED_TIMER)
> 