linux-kernel - Re: [PATCH] platform/x86: acer-wmi: Mark expected switch fall-throughs

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Message-ID: <b1869c7f-0927-9a30-a195-a188719c748c@embeddedor.com>
Date:   Wed, 8 May 2019 20:48:43 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Darren Hart <dvhart@...radead.org>
Cc:     "Lee, Chun-Yi" <jlee@...e.com>,
        Andy Shevchenko <andy@...radead.org>,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] platform/x86: acer-wmi: Mark expected switch
 fall-throughs

Darren,

Please, see my comments below...

On 5/8/19 6:06 PM, Darren Hart wrote:
> On Wed, May 08, 2019 at 11:49:34AM -0500, Gustavo A. R. Silva wrote:
>> In preparation to enabling -Wimplicit-fallthrough, mark switch
>> cases where we are expecting to fall through.
>>
>> This patch fixes the following warnings:
>>
>> drivers/platform/x86/acer-wmi.c: In function ‘set_u32’:
>> drivers/platform/x86/acer-wmi.c:1378:33: warning: this statement may fall through [-Wimplicit-fallthrough=]
>>     if (cap == ACER_CAP_WIRELESS ||
>>                                  ^
>> drivers/platform/x86/acer-wmi.c:1386:3: note: here
>>    case ACER_WMID:
>>    ^~~~
>> drivers/platform/x86/acer-wmi.c:1393:12: warning: this statement may fall through [-Wimplicit-fallthrough=]
>>     else if (wmi_has_guid(WMID_GUID2))
>>             ^
>> drivers/platform/x86/acer-wmi.c:1395:3: note: here
>>    default:
>>    ^~~~~~~
>> drivers/platform/x86/acer-wmi.c: In function ‘get_u32’:
>> drivers/platform/x86/acer-wmi.c:1340:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
>>    if (cap == ACER_CAP_MAILLED) {
>>       ^
>> drivers/platform/x86/acer-wmi.c:1344:2: note: here
>>   case ACER_WMID:
>>   ^~~~
>> drivers/platform/x86/acer-wmi.c: In function ‘WMID_get_u32’:
>> drivers/platform/x86/acer-wmi.c:1013:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
>>    if (quirks->mailled == 1) {
>>       ^
>> drivers/platform/x86/acer-wmi.c:1018:2: note: here
>>   default:
>>   ^~~~~~~
>>
>> Warning level 3 was used: -Wimplicit-fallthrough=3
>>
>> This patch is part of the ongoing efforts to enable
>> -Wimplicit-fallthrough.
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
>> ---
>>  drivers/platform/x86/acer-wmi.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/platform/x86/acer-wmi.c b/drivers/platform/x86/acer-wmi.c
>> index fcfeadd1301f..bd87f9037f95 100644
>> --- a/drivers/platform/x86/acer-wmi.c
>> +++ b/drivers/platform/x86/acer-wmi.c
>> @@ -1015,6 +1015,7 @@ static acpi_status WMID_get_u32(u32 *value, u32 cap)
>>  			*value = tmp & 0x1;
>>  			return 0;
>>  		}
>> +		/* fall through */
>>  	default:
>>  		return AE_ERROR;
>>  	}
>> @@ -1341,6 +1342,7 @@ static acpi_status get_u32(u32 *value, u32 cap)
>>  			status = AMW0_get_u32(value, cap);
>>  			break;
>>  		}
>> +		/* fall through */
> 
> This doesn't strike me as obviously the right thing to do here. If the interface
> type is AMW0_V2, why is it the right thing to do to use WMID_get_u32 if the cap
> isn't ACER_CAP_MAILLED?
> 
In commit commit 745a5d2126926808295742932d0e36d485efa485 case ACER_AMW0_V2 falls
through to case ACER_WMID deliberately in function set_u32(), without reporting
any error or warning. So, I thought it was fair to assume that the fall-through
is intentional in both functions get_u32() and set_u32(). Otherwise I would
expect to see a message indicating that interface ACER_AMW0_V2 is unavailable
in function set_u32().

This is also complemented by the following...

>>  	case ACER_WMID:
>>  		status = WMID_get_u32(value, cap);
>>  		break;
>> @@ -1383,6 +1385,7 @@ static acpi_status set_u32(u32 value, u32 cap)
>>  
>>  				return AMW0_set_u32(value, cap);
>>  			}
>> +			/* fall through */
> 
> Similarly here.
> 
> Are we documenting intended behavior, or covering up a bug.
> 

Commit 5c742b45dd5fbbb6cf74d3378341704f4b23c5e8 mentions that "This was fixed
in acer_acpi some time ago, but I forgot to port the patch over to acer-wmi
when it was merged." Notice that this driver (acer-wmi) is based on the
no-longer existing acer_acpi driver. But after googling for a while I could
found the fix the original author talks about:

https://repo.or.cz/acer_acpi.git/commitdiff/74c08a38875ffa9989c3100947650ac8a388c189

So, the fix is indeed similar and contains the same fall-throughs from case
ACER_AMW0_V2 to case ACER_WMID in both functions get_u32() and set_u32().

Thanks
--
Gustavo