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

[Richard Weinberger <richard@nod.at>]

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.

Thanks,
//richard