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I am really surprised there isn't a linux command out there that allows you to send low-level CMD's to an MMC controller. I thought mmc-utils did. > SD cards a

I am really surprised there isn't a linux command out there that allows you to send low-level CMD's to an MMC controller. I thought mmc-utils did. > SD cards are divided into sections, called Application Units (or AUs). The Video Speed Classes in particular require certain commands to be issued to it to put the card into Video Speed Class mode and to specify which AU the host will be writing to. The host is then supposed to write only to that AU, in a sequential fashion (skipping over any blocks that are already in use). Once the host has reached the AU, it must issue another command to specify which AU it will be writing to next This is something I didn't know about SD cards. Does SD card firmware mark blocks as belonging to a specific "Application Unit"? Seems to be some sort of "preallocation" scheme.

mikaey 1y ago
> I am really surprised there isn't a linux command out there that allows you to send low-level CMD's to an MMC controller. I thought mmc-utils did. It does -- but the problem is that there are very few readers out there that are supported by the Linux mmcblk driver. Most of them just present themselves to the system as generic USB block devices. > Does SD card firmware mark blocks as belonging to a specific "Application Unit"? Yup. Basically cards are divided up into application units, and application units are divided up into recording units. Where they're physically located on the card is opaque to the host -- so yeah, it can rearrange them as needed.