getting rid of subsys_initcall usage? (was: Re: [PATCH RESEND] i2c: designware: use module_platform_driver)

[Wolfram Sang]

On Fri, Aug 30, 2013 at 01:27:13AM -0700, Tony Lindgren wrote:
> * zhangfei gao <zhangfei.gao at gmail.com> [130829 23:36]:
> > What about concerns from Wolfram:
> > " Other people might be
> > depending on subsys_initcall to get I2C active before they want to
> > activate, say, PMICs. So, I fear regressions, since deferred probing
> > might not be available in the needed places to avoid these regressions."
> 
> There should not be any reason to get a PMIC activated
> early on. The system should be booting already at that point,
> and the PMIC related init can be done later on.

Okay, here is a more concrete example:

Consider the amplifier driver 'sound/soc/codecs/max9768.c'. Back then, unaware
of deferred probing, I wrote the following code to get the GPIOs (which are
optional):

	err = gpio_request_one(pdata->mute_gpio, GPIOF_INIT_HIGH, "MAX9768 Mute");
	max9768->mute_gpio = err ?: pdata->mute_gpio;

And later in the process:

        if (gpio_is_valid(max9768->mute_gpio)) {
                ret = snd_soc_add_codec_controls(codec, max9768_mute,
                                ARRAY_SIZE(max9768_mute));
                if (ret)
                        return ret;
        }

So, the mute control will only be added if the gpio_request succeeded. On that
particular board, the mute GPIO was wired to an I2C GPIO controller. If I now
change the I2C (or GPIO) driver from subsys_initcall to module_init, then the
gpio_request in the amplifier driver could hit -EPROBE_DEFER and the mute
control will then disappear. Yes, the driver can be fixed easily, yet I fear a
number of regressions like this. Instead of people digging into why things
disappear after a kernel update, I wonder if there is a way to guide users if
this happens. I didn't have time for that, though, sadly. Still, it makes me
wonder how easily we could shift from subsys_initcall to module_init, although
I'd really love to get away from subsys_initcall in device drivers.

Regards,

   Wolfram

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