linux-kernel - Re: [PATCH v6] mfd: syscon: Add hardware spinlock support

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Date:   Mon, 18 Dec 2017 13:44:10 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Lee Jones <lee.jones@...aro.org>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>
Subject: Re: [PATCH v6] mfd: syscon: Add hardware spinlock support

On Mon, Dec 18, 2017 at 7:54 AM, Baolin Wang <baolin.wang@...aro.org> wrote:
> On 15 December 2017 at 21:13, Arnd Bergmann <arnd@...db.de> wrote:
>> On Fri, Dec 15, 2017 at 11:42 AM, Lee Jones <lee.jones@...aro.org> wrote:
>>
>>>> @@ -87,6 +88,30 @@ static struct syscon *of_syscon_register(struct device_node *np)
>>>>       if (ret)
>>>>               reg_io_width = 4;
>>>>
>>>> +     ret = of_hwspin_lock_get_id(np, 0);
>>>> +     if (ret > 0) {
>>>> +             syscon_config.hwlock_id = ret;
>>>> +             syscon_config.hwlock_mode = HWLOCK_IRQSTATE;
>>>> +     } else {
>>>> +             switch (ret) {
>>>> +             case -ENOENT:
>>>> +                     /* Ignore missing hwlock, it's optional. */
>>>> +                     break;
>>>> +             case 0:
>>>> +                     /* In case of the HWSPINLOCK is not enabled. */
>>>> +                     if (!IS_ENABLED(CONFIG_HWSPINLOCK))
>>>> +                             break;
>>>> +
>>>> +                     ret = -EINVAL;
>>>> +                     /* fall-through */
>>>> +             default:
>>>> +                     pr_err("Failed to retrieve valid hwlock: %d\n", ret);
>>>> +                     /* fall-through */
>>>> +             case -EPROBE_DEFER:
>>>> +                     goto err_regmap;
>>>> +             }
>>
>> The 'case 0' seems odd here, are we sure that this is always a failure?
>> From the of_hwspin_lock_get_id() definition it looks like zero might
>> be valid, and the !CONFIG_HWSPINLOCK implementation appears
>> to be written so that we should consider '0' valid but unused and
>> silently continue with that. If that is generally not the intended
>> use, it should probably return -EINVAL or something like that.
>
> Yes, 0 is valid for of_hwspin_lock_get_id(), but if we pass 'hwlock id
> = 0' to regmap, the regmap core will not regard it as a valid hwlock
> id to request the hwlock and will use default mutex lock instead of
> hwlock, which will cause problems. Meanwhile if we silently continue
> with case 0, users will not realize that they set one invalid hwlock
> id to regmap core, so here we regarded case 0 as one invalid id to
> print error messages for users.

Something else still seems wrong then: If regmap doesn't accept a zero
lock-id, then of_hwspin_lock_get_id() should never return that as a
valid ID, right?

        Arnd