linux-kernel - Re: [PATCH v2 2/5] kdb: Simplify code to fetch characters from console

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Date:   Tue, 8 Oct 2019 15:20:41 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Jason Wessel <jason.wessel@...driver.com>,
        kgdb-bugreport@...ts.sourceforge.net,
        LKML <linux-kernel@...r.kernel.org>,
        Patch Tracking <patches@...aro.org>
Subject: Re: [PATCH v2 2/5] kdb: Simplify code to fetch characters from console

Hi,

On Tue, Oct 8, 2019 at 6:21 AM Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> Currently kdb_read_get_key() contains complex control flow that, on
> close inspection, turns out to be unnecessary. In particular:
>
> 1. It is impossible to enter the branch conditioned on (escape_delay == 1)
>    except when the loop enters with (escape_delay == 2) allowing us to
>    combine the branches.
>
> 2. Most of the code conditioned on (escape_delay == 2) simply modifies
>    local data and then breaks out of the loop causing the function to
>    return escape_data[0].
>
> 3. Based on #2 there is not actually any need to ever explicitly set
>    escape_delay to 2 because we it is much simpler to directly return
>    escape_data[0] instead.
>
> 4. escape_data[0] is, for all but one exit path, known to be '\e'.
>
> Simplify the code based on these observations.
>
> There is a subtle (and harmless) change of behaviour resulting from this
> simplification: instead of letting the escape timeout after ~1998
> milliseconds we now timeout after ~2000 milliseconds
>
> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> ---
>  kernel/debug/kdb/kdb_io.c | 38 ++++++++++++++------------------------
>  1 file changed, 14 insertions(+), 24 deletions(-)

Looks great.  My only comment is that since this was the 2nd patch in
the series I spent a whole bunch of time deducing all these same
things when reviewing the first patch.  :-P

Reviewed-by: Douglas Anderson <dianders@...omium.org>