linux-kernel - Re: Re: [PATCH] gpu/drm/mgag200:remove break after return

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Date:   Sat, 24 Oct 2020 19:14:57 +0200
From:   Sam Ravnborg <sam@...nborg.org>
To:     Bernard <bernard@...o.com>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        opensource.kernel@...o.com, David Airlie <airlied@...ux.ie>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Dave Airlie <airlied@...hat.com>
Subject: Re: Re: [PATCH] gpu/drm/mgag200:remove break after return

Hi Bernard.

On Fri, Oct 23, 2020 at 03:48:49PM +0800, Bernard wrote:
> 
> 
> From: Thomas Zimmermann <tzimmermann@...e.de>
> Date: 2020-10-23 15:13:30
> To:  Bernard Zhao <bernard@...o.com>,Dave Airlie <airlied@...hat.com>,David Airlie <airlied@...ux.ie>,Daniel Vetter <daniel@...ll.ch>,dri-devel@...ts.freedesktop.org,linux-kernel@...r.kernel.org
> Cc:  opensource.kernel@...o.com
> Subject: Re: [PATCH] gpu/drm/mgag200:remove break after return>Hi
> >
> >On 23.10.20 09:00, Bernard Zhao wrote:
> >> In function mgag200_set_pci_regs, there are some switch cases
> >> returned, then break. These break will never run.
> >> This patch is to make the code a bit readable.
> >> 
> >> Signed-off-by: Bernard Zhao <bernard@...o.com>
> >> ---
> >>  drivers/gpu/drm/mgag200/mgag200_mode.c | 5 +----
> >>  1 file changed, 1 insertion(+), 4 deletions(-)
> >> 
> >> diff --git a/drivers/gpu/drm/mgag200/mgag200_mode.c b/drivers/gpu/drm/mgag200/mgag200_mode.c
> >> index 38672f9e5c4f..de873a5d276e 100644
> >> --- a/drivers/gpu/drm/mgag200/mgag200_mode.c
> >> +++ b/drivers/gpu/drm/mgag200/mgag200_mode.c
> >> @@ -794,20 +794,17 @@ static int mgag200_crtc_set_plls(struct mga_device *mdev, long clock)
> >>  	case G200_SE_A:
> >>  	case G200_SE_B:
> >>  		return mga_g200se_set_plls(mdev, clock);
> >> -		break;
> >>  	case G200_WB:
> >>  	case G200_EW3:
> >>  		return mga_g200wb_set_plls(mdev, clock);
> >> -		break;
> >>  	case G200_EV:
> >>  		return mga_g200ev_set_plls(mdev, clock);
> >> -		break;
> >>  	case G200_EH:
> >>  	case G200_EH3:
> >>  		return mga_g200eh_set_plls(mdev, clock);
> >> -		break;
> >>  	case G200_ER:
> >>  		return mga_g200er_set_plls(mdev, clock);
> >> +	default:
> >
> >No default case here. If one of the enum values is not handled by the
> >switch, the compiler should warn about it.
> 
> Hi
> 
> For this point I was a little confused, about this switch variable "mdev->type", my understanding is that this variable`s value can be certain only when the code is running.
> How does the compiler warn this("If one of the enum values is not handled") before the code runs?

If the switch/case does not include "G200_ER" then the compiler can see
one enum value is missing from the list and can warn.
As a test - Try to drop the default and drop G200_ER - then the
compiler (hopefully) will warn.

	Sam