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mmc: block: Convert RPMB to a character device
The RPMB partition on the eMMC devices is a special area used for storing cryptographically safe information signed by a special secret key. To write and read records from this special area, authentication is needed. The RPMB area is *only* and *exclusively* accessed using ioctl():s from userspace. It is not really a block device, as blocks cannot be read or written from the device, also the signed chunks that can be stored on the RPMB are actually 256 bytes, not 512 making a block device a real bad fit. Currently the RPMB partition spawns a separate block device named /dev/mmcblkNrpmb for each device with an RPMB partition, including the creation of a block queue with its own kernel thread and all overhead associated with this. On the Ux500 HREFv60 platform, for example, the two eMMCs means that two block queues with separate threads are created for no use whatsoever. I have concluded that this block device design for RPMB is actually pretty wrong. The RPMB area should have been designed to be accessed from /dev/mmcblkN directly, using ioctl()s on the main block device. It is however way too late to change that, since userspace expects to open an RPMB device in /dev/mmcblkNrpmb and we cannot break userspace. This patch tries to amend the situation using the following strategy: - Stop creating a block device for the RPMB partition/area - Instead create a custom, dynamic character device with the same name. - Make this new character device support exactly the same set of ioctl()s as the old block device. - Wrap the requests back to the same ioctl() handlers, but issue them on the block queue of the main partition/area, i.e. /dev/mmcblkN We need to create a special "rpmb" bus type in order to get udev and/or busybox hot/coldplug to instantiate the device node properly. Before the patch, this appears in 'ps aux': 101 root 0:00 [mmcqd/2rpmb] 123 root 0:00 [mmcqd/3rpmb] After applying the patch these surplus block queue threads are gone, but RPMB is as usable as ever using the userspace MMC tools, such as 'mmc rpmb read-counter'. We get instead those dynamice devices in /dev: brw-rw---- 1 root root 179, 0 Jan 1 2000 mmcblk0 brw-rw---- 1 root root 179, 1 Jan 1 2000 mmcblk0p1 brw-rw---- 1 root root 179, 2 Jan 1 2000 mmcblk0p2 brw-rw---- 1 root root 179, 5 Jan 1 2000 mmcblk0p5 brw-rw---- 1 root root 179, 8 Jan 1 2000 mmcblk2 brw-rw---- 1 root root 179, 16 Jan 1 2000 mmcblk2boot0 brw-rw---- 1 root root 179, 24 Jan 1 2000 mmcblk2boot1 crw-rw---- 1 root root 248, 0 Jan 1 2000 mmcblk2rpmb brw-rw---- 1 root root 179, 32 Jan 1 2000 mmcblk3 brw-rw---- 1 root root 179, 40 Jan 1 2000 mmcblk3boot0 brw-rw---- 1 root root 179, 48 Jan 1 2000 mmcblk3boot1 brw-rw---- 1 root root 179, 33 Jan 1 2000 mmcblk3p1 crw-rw---- 1 root root 248, 1 Jan 1 2000 mmcblk3rpmb Notice the (248,0) and (248,1) character devices for RPMB. Cc: Tomas Winkler <tomas.winkler@intel.com> Signed-off-by:Linus Walleij <linus.walleij@linaro.org> Signed-off-by: