On Fri, Jul 05, 2019 at 09:12:37AM +0200, Hannes Reinecke wrote: > On 7/4/19 3:19 PM, Paolo Bonzini wrote: > > On 19/06/19 12:31, Paolo Bonzini wrote: > >>> I'm a bit unsure if 'bd->last' is always set; it's quite obvious that > >>> it's present if set, but what about requests with 'bd->last == false' ? > >>> Is there a guarantee that they will _always_ be followed with a request > >>> with bd->last == true? > >>> And if so, is there a guarantee that this request is part of the same batch? > >> It's complicated. A request with bd->last == false _will_ always be > >> followed by a request with bd->last == true in the same batch. However, > >> due to e.g. errors it may be possible that the last request is not sent. > >> In that case, the block layer sends commit_rqs, as documented in the > >> comment above, to flush the requests that have been sent already. > >> > >> So, a driver that obeys bd->last (or SCMD_LAST) but does not implement > >> commit_rqs is bound to have bugs, which is why this patch was not split > >> further. > >> > >> Makes sense? > > > > Hannes, can you provide your Reviewed-by? > > > Well ... since you asked for it: > > Where is the 'commit_rqs' callback actually used? > I seem to be going blind, but I can't find it; should be somewhere in > the first patch, no? > As per description: > > * The commit_rqs function is used to trigger a hardware > * doorbell after some requests have been queued with > * queuecommand, when an error is encountered before sending > * the request with SCMD_LAST set. > > So it should be somewhere in the error path, probably scsi_error or > something. But I don't seem to be able to find it ... The block layer calls scsi_mq_ops->commit_rqs() from blk_mq_dispatch_rq_list() and blk_mq_try_issue_list_directly(). Stefan
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