Re: [PATCH] drm/amdgpu/display: add fallthrough comment

[Harry Wentland]

On 2019-12-05 5:16 p.m., Liu, Zhan wrote:
> 
> 
>> -Original Message-
>> From: Alex Deucher 
>> Sent: 2019/December/05, Thursday 5:13 PM
>> To: Liu, Zhan 
>> Cc: amd-gfx@lists.freedesktop.org; Deucher, Alexander
>> 
>> Subject: Re: [PATCH] drm/amdgpu/display: add fallthrough comment
>>
>> On Thu, Dec 5, 2019 at 5:10 PM Liu, Zhan  wrote:
>>>
>>>
>>>
>>>> -Original Message-
>>>> From: amd-gfx  On Behalf Of
>>>> Alex Deucher
>>>> Sent: 2019/December/05, Thursday 4:39 PM
>>>> To: amd-gfx@lists.freedesktop.org
>>>> Cc: Deucher, Alexander 
>>>> Subject: [PATCH] drm/amdgpu/display: add fallthrough comment
>>>>
>>>> To avoid a compiler warning.
>>>>
>>>> Signed-off-by: Alex Deucher 
>>>> ---
>>>>  drivers/gpu/drm/amd/display/dc/dce/dce_aux.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
>>>> b/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
>>>> index 191b68b8163a..f1a5d2c6aa37 100644
>>>> --- a/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
>>>> +++ b/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
>>>> @@ -645,6 +645,7 @@ bool dce_aux_transfer_with_retries(struct
>>>> ddc_service *ddc,
>>>>   case AUX_TRANSACTION_REPLY_AUX_DEFER:
>>>>   case
>>>> AUX_TRANSACTION_REPLY_I2C_OVER_AUX_DEFER:
>>>>   retry_on_defer = true;
>>>> + /* fall through */
>>>
>>> I am a bit confusing here. Why a comment can avoid a compiler warning?
>>
>> The kernel enables fall through warnings, so unless there is a comment
>> mentioning that are are expecting to fall through here, we get a warning.
>> The idea is to find missing breaks in switch code.
>>
>> Alex
> 
> Got you. Thank you for your explanation.
> 
> Reviewed-by: Zhan Liu 
> 

Interesting. It's described at [1] (search for
-Wimplicit-fallthrough=3). Never knew that.

This is also
Reviewed-by: Harry Wentland 

[1]
https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/

Harry

>>
>>>
>>>>   case
>>>> AUX_TRANSACTION_REPLY_I2C_OVER_AUX_NACK:
>>>>   if (++aux_defer_retries >=
>>>> AUX_MAX_DEFER_RETRIES) {
>>>>   goto fail;
>>>> --
>>>> 2.23.0
>>>>
>>>> ___
>>>> amd-gfx mailing list
>>>> amd-gfx@lists.freedesktop.org
>>>>
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.
>>>> freedesktop.org%2Fmailman%2Flistinfo%2Famd-
>>>>
>> gfxdata=02%7C01%7Czhan.liu%40amd.com%7C77d85a0275aa4c0a78
>>>>
>> 2f08d779cb9716%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C6
>>>>
>> 37111788208702340sdata=3m%2FuNuNIgq82z%2BAeXJ9y7wn81kV%
>>>> 2FiDt3AyC3CDiTRrc%3Dreserved=0
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RE: [PATCH] drm/amdgpu/display: add fallthrough comment

[Liu, Zhan]

> -Original Message-
> From: Alex Deucher 
> Sent: 2019/December/05, Thursday 5:13 PM
> To: Liu, Zhan 
> Cc: amd-gfx@lists.freedesktop.org; Deucher, Alexander
> 
> Subject: Re: [PATCH] drm/amdgpu/display: add fallthrough comment
> 
> On Thu, Dec 5, 2019 at 5:10 PM Liu, Zhan  wrote:
> >
> >
> >
> > > -Original Message-
> > > From: amd-gfx  On Behalf Of
> > > Alex Deucher
> > > Sent: 2019/December/05, Thursday 4:39 PM
> > > To: amd-gfx@lists.freedesktop.org
> > > Cc: Deucher, Alexander 
> > > Subject: [PATCH] drm/amdgpu/display: add fallthrough comment
> > >
> > > To avoid a compiler warning.
> > >
> > > Signed-off-by: Alex Deucher 
> > > ---
> > >  drivers/gpu/drm/amd/display/dc/dce/dce_aux.c | 1 +
> > >  1 file changed, 1 insertion(+)
> > >
> > > diff --git a/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> > > b/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> > > index 191b68b8163a..f1a5d2c6aa37 100644
> > > --- a/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> > > +++ b/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> > > @@ -645,6 +645,7 @@ bool dce_aux_transfer_with_retries(struct
> > > ddc_service *ddc,
> > >   case AUX_TRANSACTION_REPLY_AUX_DEFER:
> > >   case
> > > AUX_TRANSACTION_REPLY_I2C_OVER_AUX_DEFER:
> > >   retry_on_defer = true;
> > > + /* fall through */
> >
> > I am a bit confusing here. Why a comment can avoid a compiler warning?
> 
> The kernel enables fall through warnings, so unless there is a comment
> mentioning that are are expecting to fall through here, we get a warning.
> The idea is to find missing breaks in switch code.
> 
> Alex

Got you. Thank you for your explanation.

Reviewed-by: Zhan Liu 

> 
> >
> > >   case
> > > AUX_TRANSACTION_REPLY_I2C_OVER_AUX_NACK:
> > >   if (++aux_defer_retries >=
> > > AUX_MAX_DEFER_RETRIES) {
> > >   goto fail;
> > > --
> > > 2.23.0
> > >
> > > ___
> > > amd-gfx mailing list
> > > amd-gfx@lists.freedesktop.org
> > >
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.
> > > freedesktop.org%2Fmailman%2Flistinfo%2Famd-
> > >
> gfxdata=02%7C01%7Czhan.liu%40amd.com%7C77d85a0275aa4c0a78
> > >
> 2f08d779cb9716%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C6
> > >
> 37111788208702340sdata=3m%2FuNuNIgq82z%2BAeXJ9y7wn81kV%
> > > 2FiDt3AyC3CDiTRrc%3Dreserved=0
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Re: [PATCH] drm/amdgpu/display: add fallthrough comment

[Alex Deucher]

On Thu, Dec 5, 2019 at 5:10 PM Liu, Zhan  wrote:
>
>
>
> > -Original Message-
> > From: amd-gfx  On Behalf Of Alex
> > Deucher
> > Sent: 2019/December/05, Thursday 4:39 PM
> > To: amd-gfx@lists.freedesktop.org
> > Cc: Deucher, Alexander 
> > Subject: [PATCH] drm/amdgpu/display: add fallthrough comment
> >
> > To avoid a compiler warning.
> >
> > Signed-off-by: Alex Deucher 
> > ---
> >  drivers/gpu/drm/amd/display/dc/dce/dce_aux.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> > b/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> > index 191b68b8163a..f1a5d2c6aa37 100644
> > --- a/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> > +++ b/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> > @@ -645,6 +645,7 @@ bool dce_aux_transfer_with_retries(struct
> > ddc_service *ddc,
> >   case AUX_TRANSACTION_REPLY_AUX_DEFER:
> >   case
> > AUX_TRANSACTION_REPLY_I2C_OVER_AUX_DEFER:
> >   retry_on_defer = true;
> > + /* fall through */
>
> I am a bit confusing here. Why a comment can avoid a compiler warning?

The kernel enables fall through warnings, so unless there is a comment
mentioning that are are expecting to fall through here, we get a
warning.  The idea is to find missing breaks in switch code.

Alex

>
> >   case
> > AUX_TRANSACTION_REPLY_I2C_OVER_AUX_NACK:
> >   if (++aux_defer_retries >=
> > AUX_MAX_DEFER_RETRIES) {
> >   goto fail;
> > --
> > 2.23.0
> >
> > ___
> > amd-gfx mailing list
> > amd-gfx@lists.freedesktop.org
> > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.
> > freedesktop.org%2Fmailman%2Flistinfo%2Famd-
> > gfxdata=02%7C01%7Czhan.liu%40amd.com%7C77d85a0275aa4c0a78
> > 2f08d779cb9716%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C6
> > 37111788208702340sdata=3m%2FuNuNIgq82z%2BAeXJ9y7wn81kV%
> > 2FiDt3AyC3CDiTRrc%3Dreserved=0
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RE: [PATCH] drm/amdgpu/display: add fallthrough comment

[Liu, Zhan]

> -Original Message-
> From: amd-gfx  On Behalf Of Alex
> Deucher
> Sent: 2019/December/05, Thursday 4:39 PM
> To: amd-gfx@lists.freedesktop.org
> Cc: Deucher, Alexander 
> Subject: [PATCH] drm/amdgpu/display: add fallthrough comment
> 
> To avoid a compiler warning.
> 
> Signed-off-by: Alex Deucher 
> ---
>  drivers/gpu/drm/amd/display/dc/dce/dce_aux.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> b/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> index 191b68b8163a..f1a5d2c6aa37 100644
> --- a/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> +++ b/drivers/gpu/drm/amd/display/dc/dce/dce_aux.c
> @@ -645,6 +645,7 @@ bool dce_aux_transfer_with_retries(struct
> ddc_service *ddc,
>   case AUX_TRANSACTION_REPLY_AUX_DEFER:
>   case
> AUX_TRANSACTION_REPLY_I2C_OVER_AUX_DEFER:
>   retry_on_defer = true;
> + /* fall through */

I am a bit confusing here. Why a comment can avoid a compiler warning?

>   case
> AUX_TRANSACTION_REPLY_I2C_OVER_AUX_NACK:
>   if (++aux_defer_retries >=
> AUX_MAX_DEFER_RETRIES) {
>   goto fail;
> --
> 2.23.0
> 
> ___
> amd-gfx mailing list
> amd-gfx@lists.freedesktop.org
> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.
> freedesktop.org%2Fmailman%2Flistinfo%2Famd-
> gfxdata=02%7C01%7Czhan.liu%40amd.com%7C77d85a0275aa4c0a78
> 2f08d779cb9716%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C6
> 37111788208702340sdata=3m%2FuNuNIgq82z%2BAeXJ9y7wn81kV%
> 2FiDt3AyC3CDiTRrc%3Dreserved=0
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