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SubjectRe: [PATCH] barrier patch set
Bartlomiej Zolnierkiewicz wrote:
> The fact that spec says "supported" not "enabled" in description of word86
> makes me wonder - can they be disabled? (FLUSH CACHE is mandatory for General
> feature set and FLUSH CACHE EXT is mandatory if 48-bit LBA is supported)

Yes, that's why there are separate 'supported' and 'enabled' bits for
each feature.

Words 82-84 are 'supported' bits. Words 85-87 are 'enabled' bits.
These bits mirror each other, i.e. Word 83 and Word 86 have basically
the same bits, except that Word 86 definitions change _slightly_ since
the only bits that are relevant are the ones for features that can be

You use set-features command to enable and disable these features, and
then the result shows up in subsequent identify-device command output.

If the driver is testing for a capability but does not enable it, then
always use the 'enabled' set of bits, not the 'supported' set of bits.

> Jeff, please note that these bits were introduced by ATA-6 spec
> and take a look at ATA-5 spec:
> ...
> General feature set
> - Mandatory for all devices
> ...
> and ATA-4 spec:
> ...
> General feature set
> - Optional for all devices
> ...
> IMO to test if FLUSH CACHE works we should just issue it during disk setup
> and check result. This way we can use FLUSH CACHE also on < ATA-6 devices
> (there is a lot of them).

I disagree. "just issue it" is how those LG cdrom drives got cooked.

LG cdrom drives indicated in their identify-packet-device page that
flush-cache was not supported... and then re-used the flush-cache ATA
opcode for their vendor-specific download-firmware command. Combine
that with a Linux patch that didn't properly check for flush-cache
support. Result: brick.

All drives that support flush-cache list the relevant bits in
identify-device, even on pre-ATA-6 devices. Whether the feature was
optional or mandantory, we can check the feature bits.


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