On 2018/5/28 15:57, Christoph Hellwig wrote:
On Mon, May 28, 2018 at 03:49:26PM +0800, Shawn Lin wrote:
card->erased_byte read from SCR(for SD cards) or EXT_CSD[181](for eMMC)
indicates whether the TRIM or ERASE make the erased data content zeros,
but DISCARD doesn't. Use the fact to implement REQ_OP_WRITE_ZEROES.
I don't have the mmc spec in front of my, but the ATA/SCSI and NVMe
equivalents all have the problem that if the device decideds to not
deallocate the blocks it can leave the old data in place. Can you
confirm that MMC gurantees that this is not the case?
Yes. Quoting from JESD85-B51 spec for eMMC, section 6.6.10 TRIM,
"The contents of a write block where the trim function has been applied
shall be '0' or '1' depending on different memory technology. This value
is defined in the EXT_CSD.". But 6.6.12 Discard says "The
contents of a write block where the discard function has been applied
shall be ‘don’t care’After discard operation, the original data may
be remained partially or fully accessible to the host dependent on
device. The portions of data that are no longer accessible by the host
may be removed or unmapped just as in the case of TRIM. The device will
decide the contents of discarded write block."
Apart from the mandatory requirement in spec, I confirmed this with
several device vendors as well, in the past.
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