linux-kernel - Re: [PATCH 3/4] auxdisplay: charlcd: fix x/y address commands

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Message-ID: <20180227051905.GA26781@1wt.eu>
Date:   Tue, 27 Feb 2018 06:19:05 +0100
From:   Willy Tarreau <w@....eu>
To:     Robert Abel <rabel@...ertabel.eu>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Subject: Re: [PATCH 3/4] auxdisplay: charlcd: fix x/y address commands

On Mon, Feb 26, 2018 at 11:43:36PM +0100, Robert Abel wrote:
> On 26 Feb 2018 17:49, Miguel Ojeda wrote:
> > On a general note, the code seems a bit convoluted for what it does,
> > specially without the comment written in the commit message :-) Isn't
> > it simpler to use a tiny array in the stack and put the numbers to be
> > converted instead of modifying the input sequence and dancing with
> > pointers?
> 
> That's what I felt at first, too. If we can drop the backwards
> compatibility of repeated xy commands, the whole affair gets much
> easier, but will unfortunately break existing use.
> 
> Ex. ^[[Lx004y002x006; --> x6y2, because repeats of x would just
> overwrite earlier values. That's what the while loop allowed in the
> first place.
> 
> I suspect the while loop to parse was just a clever way of parsing y
> followed by x and x followed by y using the same code and the
> overwriting behavior is actually an unaccounted-for side-effect.

Well actually I don't see a problem there at all. The principle is simply
to accept any sequence assigning x or y or both. If you write x4y2x6, it
simply means that you changed your mind regarding x and that the last
value (6) is the one you want. Just as if you wrote "^[[Lx4;^[[y2;^[[x6;".
The while loop doesn't even try to do anything clever, it simply parses
everything matching x and y followed by digits. I think the only reason
for having both x and y processed in the same loop was to call
charlcd_gotoxy() only once for both axes.

Regards,
Willy