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

[Damien Le Moal]

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.

-- 
Damien Le Moal
Western Digital Research