現象

板子上插入SD卡啟動Linux系統後，emmc的裝置名稱是/dev/mmcblck1;板子上沒有插入SD卡，Linux啟動後，emmc的裝置名稱是/dev/mmcblck0.

uboot傳遞給核心的cmdline引數bootargs 中root=/dev/mmcblck0p14，這個配置在沒有插SD卡的時候工作正常，但是插入SD卡後，就無法正常啟動Linux系統，因為無法找到root裝置。

思考解決方案

要麼改核心，使得emmc的裝置名稱固定；要麼改bootargs，使root不依賴裝置名稱。

過程

root=       [KNL] Root filesystem
            See name_to_dev_t comment in
 
 init/do_mounts.c.

    rootdelay=  [KNL] Delay (in seconds) to pause before attempting to
            mount the root filesystem

    rootflags=  [KNL] Set root filesystem mount option string

    rootfstype= [KNL] Set root filesystem type

    rootwait    [KNL] Wait (indefinitely) for root device to
 
 show up.
            Useful for devices that are detected asynchronously
            (e.g. USB and MMC devices).

183 *   Convert a name into device number.  We accept the following variants:
184 *
185 *   1) <hex_major><hex_minor> device number in hexadecimal represents itself
186 *         no leading 0
 
x, for example b302.
187 *   2) /dev/nfs represents Root_NFS (0xff)
188 *   3) /dev/<disk_name> represents the device number of disk
189 *   4) /dev/<disk_name><decimal> represents the device number
190 *         of partition - device number of disk plus the partition number
191 *   5) /dev/<disk_name>p<decimal> - same as the above, that form is
192 *      used when disk name of partitioned disk ends on a digit.
193 *   6) PARTUUID=00112233-4455-6677-8899-AABBCCDDEEFF representing the
194 *      unique id of a partition if the partition table provides it.
195 *      The UUID may be either an EFI/GPT UUID, or refer to an MSDOS
196 *      partition using the format SSSSSSSS-PP, where SSSSSSSS is a zero-
197 *      filled hex representation of the 32-bit "NT disk signature", and PP
198 *      is a zero-filled hex representation of the 1-based partition number.
199 *   7) PARTUUID=<UUID>/PARTNROFF=<int> to select a partition in relation to
200 *      a partition with a known unique id.
201 *   8) <major>:<minor> major and minor number of the device separated by
202 *      a colon.
203 *
204 *   If name doesn't have fall into the categories above, we return (0,0).
205 *   block_class is used to check if something is a disk name. If the disk
206 *   name contains slashes, the device name has them replaced with
207 *   bangs.
208 */
209
210dev_t name_to_dev_t(char *name)

嘗試

嘗試使用<hex_major><hex_minor> device number方式。插SD卡和不插SD卡進入系統後，分別檢視mmcblck0p14的裝置號對應什麼。

• 插入SD卡的情況

[email protected]:/dev/block # ls -l mmc*
brw------- root     root     179,   8 1970-01-02 07:06 mmcblk0
brw------- root     root     179,  16 1970-01-02 07:06 mmcblk0boot0
brw------- root     root     179,  24 1970-01-02 07:06 mmcblk0boot1
brw------- root     root     179,   9 1970-01-02 07:06 mmcblk0p1
brw------- root     root     179,  10 1970-01-02 07:06 mmcblk0p2
brw------- root     root     179,  11 1970-01-02 07:06 mmcblk0p3
brw------- root     root     179,  12 1970-01-02 07:06 mmcblk0p4
brw------- root     root     179,  13 1970-01-02 07:06 mmcblk0p5
brw------- root     root     179,  14 1970-01-02 07:06 mmcblk0p6
brw------- root     root     179,  15 1970-01-02 07:06 mmcblk0p7
brw------- root     root     259,   0 1970-01-02 07:06 mmcblk0p8
brw------- root     root     259,   1 1970-01-02 07:06 mmcblk0p9
brw------- root     root     179,  32 1970-01-02 07:06 mmcblk0rpmb
brw------- root     root     179,   0 1970-01-02 07:06 mmcblk1
brw------- root     root     179,   1 1970-01-02 07:06 mmcblk1p1

• 未插入SD卡時候的情況

[email protected]:/dev/block # ls -l mmc*                                       
brw------- root     root     179,   0 1970-01-02 07:16 mmcblk0
brw------- root     root     179,   8 1970-01-02 07:16 mmcblk0boot0
brw------- root     root     179,  16 1970-01-02 07:16 mmcblk0boot1
brw------- root     root     179,   1 1970-01-02 07:16 mmcblk0p1
brw------- root     root     179,   2 1970-01-02 07:16 mmcblk0p2
brw------- root     root     179,   3 1970-01-02 07:16 mmcblk0p3
brw------- root     root     179,   4 1970-01-02 07:16 mmcblk0p4
brw------- root     root     179,   5 1970-01-02 07:16 mmcblk0p5
brw------- root     root     179,   6 1970-01-02 07:16 mmcblk0p6
brw------- root     root     179,   7 1970-01-02 07:16 mmcblk0p7
brw------- root     root     259,   0 1970-01-02 07:16 mmcblk0p8
brw------- root     root     259,   1 1970-01-02 07:16 mmcblk0p9
brw------- root     root     179,  24 1970-01-02 07:16 mmcblk0rpmb

可以看到，同樣地分割槽，major和minor並不固定。當然有些是固定的，比如p8，p9.
碰巧我們使用的分割槽在兩種情況下Major和Minor是固定沒有變化的。所以我們設定bootargs root=259:1，然後成功兩種情況下都啟動了。

附錄