Comment # 7 on bug 1196357 from Chester Lin

Checked the DT and found that the uSD device is MMC_SPI:

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localhost:~ # hexdump -C
/sys/firmware/devicetree/base/soc/spi@10050000/mmc@0/compatible 
00000000  6d 6d 63 2d 73 70 69 2d  73 6c 6f 74 00           |mmc-spi-slot.|
0000000d


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Kernel log while probing mmc:

localhost:~ # dmesg | grep SD
[   16.791410] mmc_spi spi1.0: SD/MMC host mmc0, no DMA, no WP, no poweroff
[   16.849477] mmc0: new SDHC card on SPI

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It looks like a hardware limitation since Unmatched relies on SPI interface to
access cards and there's no dedicated SD/SDIO controller on the board.