Date:   Thu, 9 Mar 2017 13:01:05 +0100
From:   Peter Rosin <peda@...ntia.se>
To:     Paul Gortmaker <paul.gortmaker@...driver.com>
CC:     <linux-kernel@...r.kernel.org>, Jonathan Cameron <jic23@...nel.org>
Subject: Re: [PATCH] mux-core: make it explicitly non-modular

On 2017-03-09 10:39, Peter Rosin wrote:

*snip* *snip*

>> Per the above use case (minimal vmlinux) the unload isn't so important,
>> and in many cases it may be racy or simply not make sense.  If that is
>> the case, you don't need to provide a module_exit, and in the past I
>> think we used to bump the module use count at a successful load to
>> prevent unloading ; there might be a more elegant method now; google is
>> your friend here.  Also, I don't think name reuse/confusion is an issue.
> 
> Ok, I thought it all boiled down to making the mux-core Kconfig a tristate
> option and changing subsys_initcall(...) to module_init(...).
> 
> But if I do that, I cannot be sure that the mux-core has been initialized
> before drivers and clients start to use it in the non-modular case (if
> things are modules, the dependencies should ensure that the mux-core is
> loaded/initialized before any users are loaded). But there is no
> topological sorting going on for ordering init calls in the non-modular
> case.
> 
> In short, I get:
> 
> WARNING: CPU: 0 PID: 1 at drivers/base/class.c:438: class_find_device+0xac/0xb8
> class_find_device called for class 'mux' before it was initialized
> CPU:0 PID: 1 Comm: swapper Not tainted 4.11-rc1+ #1243
> Hardware name: Atmel SAMA5
> [<c010d24c>] (unwind_backtrace) from [<c010b1ec>] (show_stack+0x10/0x14)
> [<c010b1ec>] (show_stack) from [<c0115290>] (__warn+0xe0/0xf8)
> [<c0115290>] (__warn) from [<c01152e0>] (warn_slowpath_fmt+0x38/0x48)
> [<c01152e0>] (warn_slowpath_fmt) from [<c034715c>] (class_find_device+0xac/0xb8)
> [<c034715c>] (class_find_device) from [<c0434230>] (mux_control_get+0xa0/0x1bc)
> [<c0434230>] (mux_control_get) from [<c0434394>] (devm_mux_control_get+0x3c/0x78)
> [<c0434394>] (devm_mux_control_get) from [<c04060f0>] (i2c_mux_probe+0x44/0x294)
> [<c04060f0>] (i2c_mux_probe) from [<c03475ac>] (platform_drv_probe+0x4c/0xb0)
> [<c03475ac>] (platform_drv_probe) from [<c0345e30>] (driver_probe_device+0x26c/0x464)
> [<c0345e30>] (driver_probe_device) from [<c0346128>] (__driver_attach+0x100/0x11c)
> [<c0346128>] (__driver_attach) from [<c034403c>] (bus_for_each_dev+0x6c/0xa0)
> [<c034403c>] (bus_for_each_dev) from [<c0344a94>] (bus_add_driver+0x1c8/0x260)
> [<c0344a94>] (bus_add_driver) from [<c0346a7c>] (driver_register+0x78/0xf8)
> [<c0346a7c>] (driver_register) from [<c0800d50>] (do_one_initcall+0xb0/0x15c)
> [<c0800d50>] (do_one_initcall) from [<c0800f38>] (kernel_init_freeable+0x13c/0x1d8)
> [<c0800f38>] (kernel_init_freeable) from [<c057ab60>] (kernel_init+0x8/0x10c)
> [<c057ab60>] (kernel_init) from [<c0107fb8>] (ret_from_fork+0x14/0x3c)
> ---[ end trace d97274a16af7ef1c ]---
> 
> So, it appears that I also have to determine if the core has been
> initialized in all its entry points and return -EPROBE_DEFER (or something)
> when not. I suppose I could instead initialize on-demand, but that seems
> more difficult the do without races...
> 
> Am I missing something?

I did some digging and e.g. drivers/uwb was moved before drivers/usb in
commit ae5d82cb8d60 ("uwb: build UWB before USB/WUSB") to resolve what
appears to be a similar situation. I suppose I could just move mux up in
drivers/Makefile so that it is before both i2c and iio.

Cheers,
peda

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