The common answer on the Internet is to read the spec (they never mention it) or know the chipset (hard to know before buying) before buying. Where can I find a list of SD card readers which supports this feature and allows the use of blkdiscard?

If the device can't work with blkdiscard, which is true for all my USB based card-reader (20 of them), then you get the following:

# blkdiscard /dev/sdb
blkdiscard: /dev/sdb: BLKDISCARD ioctl failed: Operation not supported

blkdiscard works with TRIM. As far as I can see to support TRIM via USB bridge the USB bridge chip MUST support SCSI / ATA Translation(SAT)'s ATA PASS THROUGH command in order to pass the TRIM command to the disk, so this is effectively asking which USB based card reader supports ATA PASS THROUGH.

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