Messages in this thread | | | Date | Mon, 13 Jun 2022 11:05:33 +0100 | Subject | Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command handling | From | John Garry <> |
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On 13/06/2022 10:43, Damien Le Moal wrote: >>> Currently, that is not possible to do cleanly as there are no guarantees >>> we can get a free tag (there is a race between block layer tag allocation >>> and libata internal tag counting). So a reserved tag for that would be >>> nice. We would end up with 31 IO tags at most + 1 reserved tag for NCQ >>> commands + ATA_TAG_INTERNAL for non-NCQ. That last one would be rendered >>> rather useless. But that also means that we kind-of go back to the days >>> when Linux showed ATA drives max QD of 31... >> So must the ATA_TAG_INTERNAL qc always be available for non-NCQ action >> like EH, and that is why you cannot reuse for this internal NCQ >> (queuable) command? > Currently, ATA_TAG_INTERNAL is always used for non-NCQ commands. Seeing a > qc with that tag means it is*not* NCQ. > > I am trying to see if I can reuse the tag from one of the commands that > completed with that weird good status/sense data available. The problem > though is that this all needs to be done*before* calling > qc->complete_fn() which will free the tag. So we endup with 2 qcs that > have the same tag, the second one (for the read log command) temporarily > using the tag but still going through the same completion path without the > original command fully completed yet. It is a real mess. >
Reusing tags seems really messy, but then reserving an NCQ command seems wasteful.
Thanks, John
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