Subject: Devicetree: Initialization order of mmc block devices?

From: [email-masked] (Knut Wohlrab)
Date: Wed, 18 Jul 2012 17:16:49 +0200

On 07/18/2012 04:54 PM, Eric Nelson wrote:
> On 07/18/2012 07:11 AM, Knut Wohlrab wrote:
>> On 07/18/2012 03:47 PM, Jassi Brar wrote:
>>> On 18 July 2012 15:19, Knut Wohlrab <[email-masked]> wrote:
>>>
>>>> If a SD card is inserted at boot time, its "mmcblk0", the embedded
>>>> MMC (eMMC) device "mmcblk1". This makes it difficult to give the kernel the
>>>> correct device for the eMMC root file system ("root=/dev/mmcblk?p1 ...").
>>>>
>>> How about root=UUID=<eMMC-partition> ?
>> Because we are talking about an embedded device, it is very difficult to get a
>> UUID of a eMMC partition into kernel command line with U-Boot. Handling of UUID
>> is also a big effort at board manufacturing.
>>
>> Startup speed is very important for us. Searching for UUID on several MMC/SD
>> devices will significant slow down the boot process.
>>
>> This problem can occur on many devices with embedded MMC and removable SD, e.g.
>> smart phones. So I think we should find an solution to define MMC scan order or
>> device number/name in a device tree.
>>
> Isn't /dev/disk/by-path what you're after?
>
> ~$ ls -l /dev/disk/by-path/
> lrwxrwxrwx    1 root     root           13 Jan  1 15:22 platform-mmc0:b368 ->
> ../../mmcblk0
> lrwxrwxrwx    1 root     root           15 Jan  1 15:22 platform-mmc0:b368-part1
> -> ../../mmcblk0p1
> lrwxrwxrwx    1 root     root           15 Jan  1 15:22 platform-mmc0:b368-part2
> -> ../../mmcblk0p2
> lrwxrwxrwx    1 root     root           15 Jan  1 15:22 platform-mmc0:b368-part3
> -> ../../mmcblk0p3
No, because we have to know which will be the name of the root device 
_before_ we start the kernel. In our embedded device we start the kernel 
with a bootloader (U-Boot) and pass the root device with the kernel 
command line parameter "root=/dev/mmcblk0p1". That is not possible if we 
do not know what will be the name of the root device (mmcblk0 or 
mmcblk1) when kernel try to mount it.

Thanks and regards

Knut