Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command handling

Hannes Reinecke
2022-06-20T11:56:39Z

On 6/20/22 13:24, Damien Le Moal wrote:
> On 6/20/22 18:05, Christoph Hellwig wrote:
>> On Mon, Jun 20, 2022 at 06:02:30PM +0900, Damien Le Moal wrote:
>>> So reserving a tag/req to be able to do NCQ at the cost of max qd being 31
>>> works for that. We could keep max qd at 32 by creating one more "fake" tag
>>> and having a request for it, that is, having the fake tag visible to the
>>> block layer as a reserved tag, as John's series is doing, but for the
>>> reserved tags, we actually need to use an effective tag (qc->hw_tag) when
>>> issuing the commands. And for that, we can reuse the tag of one of the
>>> failed commands.
>>
>> Take a look at the magic flush request in blk-flush.c, which is
>> preallocated but borrows a tag from the request that wants a pre- or
>> post-flush. The logic is rather ugly, but maybe it might actually
>> become cleaner by generalizing it a bit.
>
> Thanks. Will check.
> I am also looking at scsi_unjam_host() and scsi_eh_get_sense(). These
> reuse a scsi command to do eh operations. So I could use that too, modulo
> making it work outside of eh context to keep the command flow intact.
>

Tsk. I was hoping to be able to remove it (especially
scsi_eh_get_sense()), but looks as if we actually do need it.
But it might be not a bad idea to have scsi_eh_get_sense() to run
independent on the SCSI EH stuff; returning with a sense code is not
necessary an error, so there are reasons for not always invoking SCSI EH.

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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