Hi folks, I earlier left some unanswered questions in my MMC to MQ conversion series but I figured it is better if I collect them and ask the blk-mq maintainers directly how to deal with the following situations that occur in the MMC block layer: 1. The current MMC code locks the host when the first request comes in from blk_fetch_request() and unlocks it when blk_fetch_request() returns NULL twice in a row. Then the polling thread terminated and is not restarted until we get called by the mmc_request_fn. Host locking means that we will not send other commands to the MMC card from i.e. userspace, which sometimes can send spurious stuff orthogonal to the block layer. If the block layer has locked the host, userspace has to wait and vice versa. It is not a common contention point but it still happens. In MQ, I have simply locked the host on the first request and then I never release it. Clearly this does not work. I am uncertain on how to handle this and whether MQ has a way to tell us that the queue is empty so we may release the host. I toyed with the idea to just set up a timer, but a "queue empty" callback from the block layer is what would be ideal. 2. When MMC cards are ejected a serious error condition occurs. So for this reason we spool out the queue with req->rq_flags |= RQF_QUIET; blk_end_request_all(req, -EIO); This will shut up a huge amount of console errors for example. I have no clue on how to manage this with MQ. I am currently using blk_mq_complete_request(mq_rq->req, -EIO); and nothing else, and it will hit all requests for the ejected card coming in from this point. Is this the right solution? Or is there some medium eject handling I'm not aware of inside the MQ layer? It seems like something that can happen on pluggable harddrives and CDROMS and what not. 3. Sometimes a read or write gets partially completed. Say we read 12 out of 15 sectors or somthing like that. I have no idea how often this occurs in practice. With the old block layer we did this: blk_end_request(req, 0, bytes_xfered); It seems MQ cannot handle partially completed transfers. I currently do this: blk_mq_requeue_request(req, false); I do not feel that is the right thing to do either, and would like recommendations on how to proceed with this. Yours, Linus Walleij
