linux-kernel - Re: [Intel-gfx] [PATCH] drm/i915: mark expected switch fall-through

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Date:   Thu, 21 Jun 2018 11:03:14 +0300
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>
Cc:     Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [Intel-gfx] [PATCH] drm/i915: mark expected switch fall-through

On Wed, 20 Jun 2018, "Gustavo A. R. Silva" <gustavo@...eddedor.com> wrote:
> On 06/20/2018 02:06 PM, Rodrigo Vivi wrote:
>> On Wed, Jun 20, 2018 at 08:31:00AM -0500, Gustavo A. R. Silva wrote:
>>> In preparation to enabling -Wimplicit-fallthrough, mark switch cases
>>> where we are expecting to fall through.
>>>
>>> Addresses-Coverity-ID: 1470102 ("Missing break in switch")
>> 
>> Any other advantage besides coverity?
>> Can't we address it by marking as "Intentional" on the tool?
>> 
>
> Yes. The advantage of this is that it will eventually allows to enable 
> -Wimplicit-fallthrough, hence, enabling the compiler to trigger a 
> warning, which will force us to double check if we are actually missing 
> a break before committing the code.

I applaud the efforts. Since you're doing the comment changes, do you
have an idea what -Wimplicit-fallthrough=N level is being considered for
kernel?

>> I'm afraid there will be so many more places to add fallthrough
>> marks....
>> 
>
> Oh yeah, there are around 1000 similar places in the whole codebase. 
> There is an ongoing effort to review each case. Months ago, it used to 
> be around 1500 of these cases.

We use our own MISSING_CASE() to indicate stuff that's not supposed to
happen, or to be implemented, etc. and in many cases the fallthrough is
normal. I wonder if we could embed __attribute__ ((fallthrough)) in
there to tackle all of these without a comment.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center