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mmc: core: Allow setting slot index via device tree alias

Jonas Höppner requested to merge add-dt-mmc-aliasing into linux-5.4.23-guf

From mainline fa2d0aa96941e8dc347e49a04c75468e3002ee20:

As with GPIO, UART and others, allow specifying the device index via the aliases node in the device tree.

On embedded devices, there is often a combination of removable (e.g. SD card) and non-removable MMC devices (e.g. eMMC). Therefore the index might change depending on

  • host of removable device
  • removable card present or not

This makes it difficult to hardcode the root device, if it is on the non-removable device. E.g. if SD card is present eMMC will be mmcblk1, if SD card is not present at boot, eMMC will be mmcblk0.

Alternative solutions like PARTUUIDs do not cover the case where multiple mmcblk devices contain the same image. This is a common issue on devices that can boot both from eMMC (for regular boot) and SD cards (as a temporary boot medium for development). When a firmware image is installed to eMMC after a test boot via SD card, there will be no reliable way to refer to a specific device using (PART)UUIDs oder LABELs.

The demand for this feature has led to multiple attempts to implement it, dating back at least to 2012 (see https://www.spinics.net/lists/linux-mmc/msg26586.html for a previous discussion from 2014).

All indices defined in the aliases node will be reserved for use by the respective MMC device, moving the indices of devices that don't have an alias up into the non-reserved range. If the aliases node is not found, the driver will act as before.

This is a rebased and cleaned up version of https://www.spinics.net/lists/linux-mmc/msg26588.html .

Based-on-patch-by: Sascha Hauer s.hauer@pengutronix.de Link: https://lkml.org/lkml/2020/8/5/194 Signed-off-by: Matthias Schiffer matthias.schiffer@ew.tq-group.com Link: https://lore.kernel.org/r/20200901085004.2512-2-matthias.schiffer@ew.tq-group.com Signed-off-by: Ulf Hansson ulf.hansson@linaro.org

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