specifying order of /dev/mmcblk devices via device-tree?

[stefan@agner.ch (Stefan Agner)]

On 2016-11-15 15:55, Russell King - ARM Linux wrote:
> On Tue, Nov 15, 2016 at 10:10:02PM +0000, Russell King - ARM Linux wrote:
>> On Tue, Nov 15, 2016 at 01:39:42PM -0800, Tim Harvey wrote:
>> > On Tue, Nov 15, 2016 at 1:35 PM, Russell King - ARM Linux
>> > <linux@armlinux.org.uk> wrote:
>> > > On Tue, Nov 15, 2016 at 12:27:53PM -0800, Tim Harvey wrote:
>> > >> On Mon, Nov 14, 2016 at 11:08 AM, Russell King - ARM Linux
>> > >> <linux@armlinux.org.uk> wrote:
>> > >> > So, someone merged a patch which makes mmcblk devices follow the
>> > >> > host controller numbering.
>> > >> >
>> > >> > Now my cubox-i fails to boot correctly because the SD card in the
>> > >> > _only_ SD card slot now gets called "mmcblk1" and not "mmcblk0".
>> > >> >
>> > >> > USDHC1 is wired to the on-microsom WiFi, and never has anything
>> > >> > remotely near a SD card or eMMC present.  So, this change is
>> > >> > confusing on these platforms.
>> > >> >
>> > >> > Moreover, this is _going_ to break SolidRun distros if people upgrade
>> > >> > their kernels.
>> > >> >
>> > >> > It may be appropriate for eMMC, but it's not appropriate everywhere.
>> > >> >
>> > >> > This is a user visible _regression_ in 4.9-rc.  Whoever did this,
>> > >> > please revert whatever change caused this, and next time limit it
>> > >> > to only eMMC.
>> > >> >
>> > >> > Thanks.
>> > >>
>> > >> I see the same thing on newer kernels, which is why I asked the
>> > >> question. I didn't expect (or even want honestly) a non mmcblk0 boot
>> > >> device and was looking for a way to control that via dt. Now I'm
>> > >> understanding that to avoid this kind of bootloader/kernel dependence
>> > >> issue I should be using UUID's to identify the boot device.
>> > >>
>> > >> >From my testing it looks like the change your looking for occurred
>> > >> some time ago and is somewhere between 4.5 and 4.6 and not a 4.9
>> > >> regression specifically.
>> > >
>> > > That depends how you look at it.  Yes, there's a change in 4.5 to 4.6
>> > > which ties the block device number to the host device index, but that's
>> > > really only part of the story here.
>> > >
>> > > 4.8 definitely identifies the SD card in iMX6 usdhc2 as "mmcblk0".
>> > > 4.9-rc identifies the SD card as "mmcblk1".  This makes it a 4.9 change
>> > > of behaviour - there can be no argument about that.
>> > >
>> > > Now, digging further into this today, it appears that:
>> > >
>> > > v4.8: usdhc2 was probed first, and is given mmc0.
>> > >       usdhc1 is probed second, and is given mmc1.
>> > >
>> > > v4.9-rc: usdhc1 is probed first, and is given mmc0.
>> > >          usdhc2 is probed second, and is given mmc1.
>> > >
>> > > I haven't yet been able to figure out why there's been this change
>> > > of probe order.  There's no change that I can see in the iMX6 DT
>> > > files that would account for this.
>> > >
>> >
>> > I bisected it and the commit your looking for is
>> > 9aaf3437aa72ed5370bf32c99580a3fa2c330e3d
>>
>> No it's not.
>>
>> Let me try and put it plainer:
>>
>> * Commit 9aaf3437aa72ed5370bf32c99580a3fa2c330e3d ties the mmc block
>>   device number (mmcblkN) to the mmc host interface number (mmcN).
>>   This change happened between 4.5 and 4.6.
>>
>> * The change I'm seeing happened between 4.8 and 4.9-rc.  I'm not
>>   seeing a change of behaviour between 4.5 and 4.6.
>>
>> * The change I'm seeing changes the order of the physical device
>>   associated with the hosts named mmc0 and mmc1 in the kernel.
>>
>> * Because physical devices associated with the mmc0 and mmc1 hosts
>>   swap over, the mmcblkN number changes due to the commit you point
>>   out.
>>
>> * So, the change that I'm seeing between 4.8 and 4.9-rc is not caused
>>   by commit 9aaf3437aa72ed5370bf32c99580a3fa2c330e3d, but by something
>>   else changing the order in which the two usdhc physical hardware
>>   blocks get probed.
>>
>> Does this make it clearer?
> 
> It turns out to be this commit:
> 
> commit 6eb1c9496b81680f2cd2e0eda06c531317e2e28d
> Author: Masahiro Yamada <yamada.masahiro@socionext.com>
> Date:   Mon Sep 19 01:16:44 2016 +0900
> 
>     clk: probe common clock drivers earlier
> 
>     Several SoCs implement platform drivers for clocks rather than
>     CLK_OF_DECLARE().  Clocks should come earlier because they are
>     prerequisites for many of other drivers.  It will help to mitigate
>     EPROBE_DEFER issues.
> 
>     Also, drop the comment since it does not carry much value.
> 
>     Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
>     Acked-by: Michael Turquette <mturquette@baylibre.com>
>     Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> which changes the init order.  In 4.8, we get:

Afaik, init order is not guaranteed, it never was. Usually the order
ends up to be in some order of the device tree, but one could also parse
the device tree in reverse order (which would be an interesting
experiment).

If we want to rely on ordering, we need to add alias support.

--
Stefan