linux-kernel - Re: [PATCH 04/11] hwmon: applesmc: Introduce a register lookup table (rev2)

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Message-ID: <4CD9A1DC.2000407@euromail.se>
Date:	Tue, 09 Nov 2010 20:32:44 +0100
From:	Henrik Rydberg <rydberg@...omail.se>
To:	guenter.roeck@...csson.com
CC:	Jean Delvare <khali@...ux-fr.org>,
	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/11] hwmon: applesmc: Introduce a register lookup table
 (rev2)

Hi Guenter,

>> +/*

>> + * applesmc_init_smcreg_try - Try to initialize register cache. Idempotent.
>> + */
>> +static int applesmc_init_smcreg_try(void)
>> +{
>> +       struct applesmc_registers *s = &smcreg;
>> +       int ret;
>> +
>> +       if (s->init_complete)
>> +               return 0;
>> +
>> +       mutex_init(&s->mutex);
>> +
> I am a bit concerned that mutex_init() can be called multiple times. Are
> you sure this is safe ?


mutex_destroy() is defined as a nop, so I guess the question is whether anything
could be holding the lock when entering a second init. There are no sysfs files
created at that point, so I would say no. The mutex could be put back with a
static initializer, if this is not satisfactory. The real reason to move it to
the smcreg struct was to force a rename of the mutex itself.

> 
>> +       ret = read_register_count(&s->key_count);
>> +       if (ret)
>> +               return ret;
>> +
>> +       if (!s->cache)
>> +               s->cache = kcalloc(s->key_count, sizeof(*s->cache), GFP_KERNEL);
>> +       if (!s->cache)
>> +               return -ENOMEM;
>> +
>> +       s->init_complete = true;
>> +
>> +       pr_info("key=%d\n", s->key_count);
>> +
> Hope that means more to macbook users than it does to me ;).


It means a lot from a diagnostic point of view - a normal user does not really
care about the dmesg output anyways. :-)

>> +static int applesmc_init_smcreg(void)
>> +{
>> +       int ms, ret;
>> +
>> +       for (ms = 0; ms < INIT_TIMEOUT_MSECS; ms += INIT_WAIT_MSECS) {
>> +               ret = applesmc_init_smcreg_try();
>> +               if (!ret)
>> +                       return 0;
>> +               pr_warn("slow init, retrying\n");
> 
> INIT_WAIT_MS is 50ms, so you issue this warning every 50ms for up to
> five seconds. Pretty noisy... sure that is what you want ? Also, does it
> really make sense to retry if the error is ENOMEM ?


With the empirical failure rate, it is extremely unlikely to get more than a
couple of failures in a row - information which in itself could be very useful.
A direct escape on ENOMEM makes sense, though.

Changing the place of the mutex will ripple through all patches, so I will
resend from this one onwards. I suppose you have more comments on the following
patches?

Thanks,
Henrik
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