MMC_BLOCK_MINORS

1.static int perdev_minors = CONFIG_MMC_BLOCK_MINORS

config MMC_BLOCK_MINORS
	int "Number of minors per block device"
	depends on MMC_BLOCK
	range 4 256
	default 8
	help
	  Number of minors per block device. One is needed for every
	  partition on the disk (plus one for the whole disk).

	  Number of total MMC minors available is 256, so your number
	  of supported block devices will be limited to 256 divided
	  by this number.

	  Default is 8 to be backwards compatible with previous
	  hardwired device numbering.

	  If unsure, say 8 here.

MMC_BLOCK_MINORS：一个block设备能支持的次设备的数量。可以理解为一个emmc支持的分区数量。

2.max_devices = min(MAX_DEVICES, (1 << MINORBITS) / perdev_minors);
max_devices即为能够支持的block设备最大的数量。