Re: [PATCH v1 4/9] blk-mq: introduce init_flush_rq_fn callback in 'blk_mq_ops'

[Jens Axboe <axboe@kernel.dk>]

On 2014-08-16 01:49, Ming Lei wrote:
> On 8/16/14, Jens Axboe <axboe@kernel.dk> wrote:
>> On 08/14/2014 09:50 AM, Ming Lei wrote:
>>> Currently pdu of the flush rq is simlpy copied from another rq,
>>> it isn't enough to initialize pointer field well, so introduce
>>> the callback for driver to handle the case easily.
>>
>> This is the only patch I don't really like. Can't we make do with
>> calling ->init_request() for this instead of having to add another
>> (weird) hook?
>
> I considered ->init_request() before, but looks there are some problems:
>
> - from API view, both 'hctx_idx' and 'request_idx' parameter don't make
> sense for flush rq since it beongs to request queue instead of any one
> of hctx
>
> - init_request()/exit_request() are weird too, since they will be called
> for queuing every flush req, and they should have been called one
> shot
>
> - it is called before queuing each flush req, and might introduce a bit cost
> unnecessarily if init_request does lots of stuff
>
> - using init_request may break some current drivers(like scsi)
>
> Now I feel ->init_flush_rq() isn't good too, how about introducing
> prepare_flush_rq() and unprepare_flush_rq()? And they can be
> lightweight and have document benefit at least.

I think that's a better approach, lets do that.

-- 
Jens Axboe

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