linux-kernel - Re: [PATCH 2/2 v2] UBI: Block: Add blk-mq support

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Message-ID: <54B5A213.1090502@fb.com>
Date:	Tue, 13 Jan 2015 15:54:11 -0700
From:	Jens Axboe <axboe@...com>
To:	Richard Weinberger <richard@....at>,
	Christoph Hellwig <hch@...radead.org>
CC:	<dedekind1@...il.com>, <dwmw2@...radead.org>,
	<computersforpeace@...il.com>, <linux-mtd@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <tom.leiming@...il.com>
Subject: Re: [PATCH 2/2 v2] UBI: Block: Add blk-mq support

On 01/13/2015 03:51 PM, Richard Weinberger wrote:
> Am 13.01.2015 um 17:25 schrieb Christoph Hellwig:
>>> +	struct ubi_sgl usgl;
>>
>> Btw, what's in struct ubi_sgl?  Can't find that in my tree.
> 
> "[PATCH 1/2] UBI: Add initial support for scatter gather" adds it.
> 
> /**
>  * struct ubi_sgl - UBI scatter gather list data structure.
>  * @list_pos: current position in @sg[]
>  * @page_pos: current position in @sg[@list_pos]
>  * @sg: the scatter gather list itself
>  *
>  * ubi_sgl is a wrapper around a scatter list which keeps track of the
>  * current position in the list and the current list item such that
>  * it can be used across multiple ubi_leb_read_sg() calls.
>  */
> struct ubi_sgl {
>         int list_pos;
>         int page_pos;
>         struct scatterlist sg[UBI_MAX_SG_COUNT];
> };
> 
>>> +static void ubiblock_do_work(struct work_struct *work)
>>> +{
>>> +	int ret;
>>> +	struct ubiblock_pdu *pdu = container_of(work, struct ubiblock_pdu, work);
>>> +	struct request *req = blk_mq_rq_from_pdu(pdu);
>>> +
>>> +	blk_mq_start_request(req);
>>> +	blk_rq_map_sg(req->q, req, pdu->usgl.sg);
>>
>> blk_rq_map_sg returns the number of entries actually mapped, which
>> might be smaller than the number passed in due to merging.
> 
> Yep, but the ubi_sql has a fixed number of scatterlist entries, UBI_MAX_SG_COUNT.
> And I limit it also to that using: blk_queue_max_segments(dev->rq, UBI_MAX_SG_COUNT);
> 
> Is there another reason why I should use the return value of blk_rq_map_sg()?
> Please also note that the UBI block driver is read-only.

It can return less than what you asked for, if segments are coalesced.
Read/write, doesn't matter. You should always use the returned value as
the indication of how many segments to access in pdu->usgl.sg for data
transfer.

-- 
Jens Axboe

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