media: camss: csiphy-3ph: avoid undefined behavior

  From: Arnd Bergmann <arnd@arndb.de>

Marking a case of the switch statement as unreachable means the
compiler treats it as undefined behavior, which is then caught by
an objtool warning:

drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.o: warning: objtool: csiphy_lanes_enable() falls through to next function csiphy_lanes_disable()

Instead of simply continuing execution at a random place of the
driver, print a warning and return from to the caller, which
makes it possible to understand what happens and avoids the
warning.

Fixes: 53655d2a0ff2 ("media: camss: csiphy-3ph: add support for SM8250 CSI DPHY")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
  

On Thu, 15 Dec 2022 at 17:29, Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> Marking a case of the switch statement as unreachable means the
> compiler treats it as undefined behavior, which is then caught by
> an objtool warning:
>
> drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.o: warning: objtool: csiphy_lanes_enable() falls through to next function csiphy_lanes_disable()
>
> Instead of simply continuing execution at a random place of the
> driver, print a warning and return from to the caller, which
> makes it possible to understand what happens and avoids the
> warning.
>
> Fixes: 53655d2a0ff2 ("media: camss: csiphy-3ph: add support for SM8250 CSI DPHY")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c b/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
> index 451a4c9b3d30..04baa80494c6 100644
> --- a/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
> +++ b/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
> @@ -429,7 +429,8 @@ static void csiphy_gen2_config_lanes(struct csiphy_device *csiphy,
>                 array_size = ARRAY_SIZE(lane_regs_sm8250[0]);
>                 break;
>         default:
> -               unreachable();
> +               WARN(1, "unknown cspi version\n");
> +               return;
>         }
>
>         for (l = 0; l < 5; l++) {
> --
> 2.35.1
>

Reviewed-by: Robert Foss <robert.foss@linaro.org>
  

On Thu, Dec 15, 2022 at 05:28:46PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> Marking a case of the switch statement as unreachable means the
> compiler treats it as undefined behavior, which is then caught by
> an objtool warning:
> 
> drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.o: warning: objtool: csiphy_lanes_enable() falls through to next function csiphy_lanes_disable()
> 
> Instead of simply continuing execution at a random place of the
> driver, print a warning and return from to the caller, which
> makes it possible to understand what happens and avoids the
> warning.
> 
> Fixes: 53655d2a0ff2 ("media: camss: csiphy-3ph: add support for SM8250 CSI DPHY")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c b/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
> index 451a4c9b3d30..04baa80494c6 100644
> --- a/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
> +++ b/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
> @@ -429,7 +429,8 @@ static void csiphy_gen2_config_lanes(struct csiphy_device *csiphy,
>  		array_size = ARRAY_SIZE(lane_regs_sm8250[0]);
>  		break;
>  	default:
> -		unreachable();
> +		WARN(1, "unknown cspi version\n");
> +		return;
>  	}

So no real objection here; but unreachable() does have an objtool
annotation inside, so clearly the compiler managed to defeat that --
perhaps we should look at that too.
  

On Fri, Dec 16, 2022, at 12:23, Peter Zijlstra wrote:
> On Thu, Dec 15, 2022 at 05:28:46PM +0100, Arnd Bergmann wrote:
>> diff --git a/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c b/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
>> index 451a4c9b3d30..04baa80494c6 100644
>> --- a/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
>> +++ b/drivers/media/platform/qcom/camss/camss-csiphy-3ph-1-0.c
>> @@ -429,7 +429,8 @@ static void csiphy_gen2_config_lanes(struct csiphy_device *csiphy,
>>  		array_size = ARRAY_SIZE(lane_regs_sm8250[0]);
>>  		break;
>>  	default:
>> -		unreachable();
>> +		WARN(1, "unknown cspi version\n");
>> +		return;
>>  	}
>
> So no real objection here; but unreachable() does have an objtool
> annotation inside, so clearly the compiler managed to defeat that --
> perhaps we should look at that too.

Ah, I forgot this annotation existed. I see that this
particular objtool warning only happens with clang (I used
version 14.0.6), but it does not happen with gcc-12.2.

I see the function ends in

	jmp	.LBB3_45
.LBB3_54:
	#APP
.Ltmp0:
	.section	.discard.unreachable,"",@progbits
.Ltmp1:
	.long	.Ltmp0-.Ltmp1
	.text
 	#NO_APP
.Lfunc_end3:
	.size	csiphy_lanes_enable, .Lfunc_end3-csiphy_lanes_enable
                                        # -- End function


full assembler output at https://pastebin.com/F8HGCUvk, object file
attached.

    Arnd