RE: [RFC PATCH] mmc: core: Disable REQ_FUA if the eMMC supports an internal cache

["Christian Löhle" <CLoehle@hyperstone.com>]

>> 
>> REQ_FUA is in general supported for eMMC cards, which translates into so called "reliable writes". To support these write operations, the CMD23 (MMC_CAP_CMD23), needs to be supported by the mmc host too, which is common but not always the case.
>> 
>> For some eMMC devices, it has been reported that reliable writes are quite costly, leading to performance degradations.
>> 
>> In a way to improve the situation, let's avoid announcing REQ_FUA support if the eMMC supports an internal cache, as that allows us to rely solely on flush-requests (REQ_OP_FLUSH) instead, which seems to be a lot cheaper.
>> Note that, those mmc hosts that lacks CMD23 support are already using this type of configuration, whatever that could mean.
> 
> Just note that reliable write is strictly weaker than turning cache off/flushing, if card loses power during cache off/flush programming / busy, sector-wise atomicity is not mandated by the spec.
> (And that is assuming cache off/flush is actually respected by the card as intended by the spec, should some cards be checked?) Maybe some FS people can also chime in?

Nevermind, the sector-wise atomicity should not matter on 5.1 cards or if the block length isn't being played with, which it isn't in our case.
If reliable write is implemented only according to spec, I don't see why the cache flushing should be less expensive, which would only make sense if
a) < sector chunks are committed to flash
b) reliable write is implemented much stricter than the spec, ensuring atomicity for the entire write.

I guess the cards which increase performance do b)? Or something else?
Anyway regarding FUA i don't have any concerns regarding reliability with cache flush.
I can add some performance comparisons with some eMMCs I have around though.

Regards,
Christian

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