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

[Tim Harvey]

On Tue, Nov 15, 2016 at 1:35 PM, Russell King - ARM Linux
<linux at 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 at 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

Tim