linux-kernel - Re: [PATCH v6 2/3] drm: bridge: Add thc63lvd1024 LVDS decoder driver

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Date:   Tue, 27 Mar 2018 11:06:03 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Andrzej Hajda <a.hajda@...sung.com>
Cc:     Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>,
        Jacopo Mondi <jacopo+renesas@...ndi.org>,
        Archit Taneja <architt@...eaurora.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        David Airlie <airlied@...ux.ie>,
        Simon Horman <horms@...ge.net.au>,
        Magnus Damm <magnus.damm@...il.com>,
        Niklas Söderlund <niklas.soderlund@...natech.se>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 2/3] drm: bridge: Add thc63lvd1024 LVDS decoder driver

Hi Andrezj,

On Tue, Mar 27, 2018 at 10:36 AM, Andrzej Hajda <a.hajda@...sung.com> wrote:
> On 27.03.2018 09:36, Geert Uytterhoeven wrote:
>> On Tue, Mar 27, 2018 at 9:28 AM, Andrzej Hajda <a.hajda@...sung.com> wrote:
>>>>> --- /dev/null
>>>>> +++ b/drivers/gpu/drm/bridge/thc63lvd1024.c
>>>>> +static void thc63_enable(struct drm_bridge *bridge)
>>>>> +{
>>>>> +    struct thc63_dev *thc63 = to_thc63(bridge);
>>>>> +    struct regulator *vcc;
>>>>> +    int i;
>>>> unsigned int i;
>>> Why? You are introducing silly bug this way, see few lines below.
>>>
>>>>> +
>>>>> +    for (i = 0; i < ARRAY_SIZE(thc63->vccs); i++) {
>>>>> +            vcc = thc63->vccs[i];
>>>>> +            if (!vcc)
>>>>> +                    continue;
>>>>> +
>>>>> +            if (regulator_enable(vcc))
>>>>> +                    goto error_vcc_enable;
>>>>> +    }
>>>>> +
>>>>> +    if (thc63->pdwn)
>>>>> +            gpiod_set_value(thc63->pdwn, 0);
>>>>> +
>>>>> +    if (thc63->oe)
>>>>> +            gpiod_set_value(thc63->oe, 1);
>>>>> +
>>>>> +    return;
>>>>> +
>>>>> +error_vcc_enable:
>>>>> +    dev_err(thc63->dev, "Failed to enable regulator %s\n",
>>>>> +            thc63_reg_names[i]);
>>>>> +
>>>>> +    for (i = i - 1; i >= 0; i--) {
>>> Here, the loop will not end if you define i unsigned.
>> True.
>>
>>> I know one can change the loop, to make it working with unsigned. But
>>> this clearly shows that using unsigned is more risky.
>>> What are advantages of unsigned vs int in this case. Are there some
>>> guidelines/discussions about it?
>> Some people consider signed integers harmful, as they may be converted
>> silently by the compiler to the "larger" unsigned type when needed.
>
> Wow, it sounds crazy, shall we expect gigantic patchsets, converting all
> occurrences of int to "unsigned int" ? :)

No we shall not.

> I know both types have their pros and cons and can behave unexpectedly
> in corner cases, but I do not see why unsigned should be preferred over
> signed in general, or in this particular case.

When looping over array indices, and comparing with ARRAY_SIZE (which
is unsigned), using "unsigned int" is preferred.

However, in this case the error code relies on the index becoming negative,
so a signed integer should be used.

> I guess there were somewhere discussion about it, could you point me to
> it if possible, to avoid unnecessary noise in this thread.

Not here, but Google pointed me to
http://blog.robertelder.org/signed-or-unsigned/

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds