On 01/13/2015 04:17 PM, Richard Weinberger wrote: > Am 13.01.2015 um 23:54 schrieb Jens Axboe: >>>> 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. > > Sorry, I don't fully understand. > > Currently the driver does: > to_read = blk_rq_bytes(req); > Then it fills pdu->usgl.sg up to to_read bytes > and calls blk_mq_end_request(). > > If I understand you correctly it can happen that blk_rq_bytes() returns > more bytes than blk_rq_map_sg() allocated, right?
No, the number of bytes will be the same, no magic is involved :-) But lets say the initial request has 4 bios, with each 2 pages, for a total of 8 segments. Lets further assume that the pages in each bio are contiguous, so that blk_rq_map_sg() will map this to 4 sg elements, each 2xpages long. Now, this may already be handled just fine, and you don't need to update/store the actual sg count. I just looked at the source, and I'm assuming it'll do the right thing (ubi_read_sg() will bump the active sg element, when that size has been consumed), but I don't have ubi_read_sg() in my tree to verify. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/