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SubjectRe: [PATCH 1/2] scsi_host: add support for request batching
On 7/5/19 9:44 AM, Stefan Hajnoczi wrote:
> 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().
Ah, right.

Now, then:

Reviewed-by: Hannes Reinecke <>


Dr. Hannes Reinecke Teamlead Storage & Networking +49 911 74053 688
SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: Felix Imendörffer, Mary Higgins, Sri Rasiah
HRB 21284 (AG Nürnberg)

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