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SubjectRe: [PATCH 2/2] mmc: core: Support packed command for eMMC4.5 device
On Fri, Nov 11, 2011 at 12:56 PM, Seungwon Jeon <tgih.jun@samsung.com> wrote:
> Maya Erez wrote:
>> On Thu, Nov 10, 2011 Maya Erez wrote:
>> > S, Venkatraman <svenkatr@ti.com> wrote:
>> >> On Thu, Nov 3, 2011 at 7:23 AM, Seungwon Jeon <tgih.jun@samsung.com>
>> wrote:
>> >> >> > +static u8 mmc_blk_chk_packable(struct mmc_queue *mq, struct
>> >> request *req)
>>
>> The function prepares the checkable list and not only checks if packing is
>> possible, therefore I think its name should change to reflect its real
>> action
> I labored at naming. Isn't it proper? :)
> Do you have any recommendation?
> group_pack_req?
>
>>
>> >> >> > +       if (!(md->flags & MMC_BLK_CMD23) &&
>> >> >> > +                       !card->ext_csd.packed_event_en)
>> >> >> > +               goto no_packed;
>>
>> Having the condition with a && can lead to cases where CMD23 is not
>> supported and we send packed commands. Therfore the condition should be
>> changed to || or be splitted to 2 separate checks.
>> Also, according to the standard the generic error flag in
>> PACKED_COMMAND_STATUS is set in case of any error and having
>> PACKED_EVENT_EN is only optional. Therefore, I don't think that setting
>> the packed_event_en should be a mandatory condition for using packed
>> coammnds.
> ... cases where CMD23 is not supported and we send packed commands?
> Packed command must not be allowed in such a case.
> It works only with predefined mode which is essential fator.
> And spec doesn't mentioned PACKED_EVENT_EN must be set.
> So Packed command can be sent regardless PACKED_EVENT_EN,
> but it's not complete without reporting of error.
> Then host driver may suffer error recovery.
> Why packed command is used without error reporting?
>
>>
>> >> >> > +       if (mmc_req_rel_wr(cur) &&
>> >> >> > +                       (md->flags & MMC_BLK_REL_WR) &&
>> >> >> > +                       !en_rel_wr) {
>> >> >> > +               goto no_packed;
>> >> >> > +       }
>>
>> Can you please explain this condition and its purpose?
>>
> In the case where reliable write is request but enhanced reliable write
> is not supported, write access must be partial according to
> reliable write sector count. Because even a single request can be split,
> packed command is not allowed in this case.
>
>> >> >> > +               phys_segments +=  next->nr_phys_segments;
>> >> >> > +               if (phys_segments > max_phys_segs) {
>> >> >> > +                       blk_requeue_request(q, next);
>> >> >> > +                       break;
>> >> >> > +               }
>> >> >> I mentioned this before - if the next request is not packable and
>> >> requeued,
>> >> >> blk_fetch_request will retrieve it again and this while loop will
>> never terminate.
>> >> >>
>> >> > If next request is not packable, it is requeued and 'break'
>> terminates
>> >> this loop.
>> >> > So it not infinite.
>> >> Right !! But that doesn't help finding the commands that are packable.
>> Ideally, you'd need to pack all neighbouring requests into one packed
>> command.
>> >> The way CFQ works, it is not necessary that the fetch would return all
>> outstanding
>> >> requests that are packable (unless you invoke a forced dispatch) It
>> would be good to see some numbers on the number of pack hits /
>> misses
>> >> that
>> >> you would encounter with this logic, on a typical usecase.
>> > Is it considered only for CFQ scheduler? How about other I/O scheduler?
>> If all requests are drained from scheduler queue forcedly,
>> > the number of candidate to be packed can be increased.
>> > However we may lose the unique CFQ's strength and MMC D/D may take the
>> CFQ's duty.
>> > Basically, this patch accommodates the origin order requests from I/O
>> scheduler.
>> >
>>
>> In order to better utilize the packed commands feature and achieve the
>> best performance improvements I think that the command packing should be
>> done in the block layer, according to the scheduler policy.
>> That is, the scheduler should be aware of the capability of the device to
>> receive a request list and its constrains (such as maximum number of
>> requests, max number of sectors etc) and use this information as a  factor
>> to its algorithm.
>> This way you keep the decision making in the hands of the scheduler while
>> the MMC layer will only have to send this list as a packed command.
>>
> Yes, it would be another interesting approach.
> Command packing you mentioned means gathering request among same direction(read/write)?
> Currently I/O scheduler may know device constrains which MMC driver informs
> with the exception of order information for packed command.
> But I think the dependency of I/O scheduler may be able to come up.
> How can MMC layer treat packed command with I/O scheduler which doesn't support this?

The very act of packing presumes some sorting and re-ordering at the
I/O scheduler level.
When no such sorting is done (ex. noop), MMC should resort to
non-packed execution, respecting the system configuration choice.

Looking deeper into this, I think a better approach would be to set
the prep_rq_fn of the request_queue, with a custom mmc function that
decides if the requests are packable or not, and return a
BLKPREP_DEFER for those that can't be packed.

>
>> >> >> > +       if (rqc)
>> >> >> > +               reqs = mmc_blk_chk_packable(mq, rqc);
>>
>> It would be best to keep all the calls to blk_fetch_request in the same
>> location. Therefore, I suggest to move the call to mmc_blk_chk_packable to
>> mmc/card/queue.c after the first request is fetched.
>
> At the first time, I considered that way.
> I'll do more, if possible.
>>
>> >> >> >  cmd_abort:
>> >> >> > -       spin_lock_irq(&md->lock);
>> >> >> > -       while (ret)
>> >> >> > -               ret = __blk_end_request(req, -EIO,
>> >> blk_rq_cur_bytes(req));
>> >> >> > -       spin_unlock_irq(&md->lock);
>> >> >> > +       if (mq_rq->packed_cmd != MMC_PACKED_NONE) {
>>
>> This should be the case for MMC_PACKED_NONE.
> Right, I missed it.
>>
>> >> >> > +               spin_lock_irq(&md->lock);
>> >> >> > +               while (ret)
>> >> >> > +                       ret = __blk_end_request(req, -EIO,
>> >> blk_rq_cur_bytes(req));
>>
>> Do we need the while or should it be an if? In other cases where
>> __blk_end_request is called there is no such while.
> This part is not only the new but also origin code which is moved in this patch.
> Maybe...,'If' case is used  for a whole of remained bytes and
> 'while' case is used for partial report of remained bytes.
>
> Thank you for review.
>
> Best regards,
> Seugwon Jeon.
>>
>> Thanks,
>> Maya Erez
>> Consultant for Qualcomm Innovation Center, Inc.
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum
>>
>>
>>
>>
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>
>
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