Re: [PATCH 1/2] scsi_host: add support for request batching

Hannes Reinecke
2019-07-05T07:12:45Z

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 ...

Cheers,

Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
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