Re: [PATCH] mmc: core: add auto bkops support

[Ulf Hansson]

On 13 June 2016 at 14:25, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
> On 13/06/16 11:58, Shawn Lin wrote:
>> å 2016/6/13 16:17, Adrian Hunter åé:
>>> On 13/06/16 10:48, Shawn Lin wrote:
>>>> On 2016/6/13 14:29, Adrian Hunter wrote:
>>>>> On 06/06/16 06:07, Shawn Lin wrote:
>>>>>> JEDEC eMMC v5.1 introduce an autonomously initiated method
>>>>>> for background operations.
>>>>>>
>>>>>> Host that wants to enable the device to perform background
>>>>>> operations during device idle time, should signal the device
>>>>>> by setting AUTO_EN in BKOPS_EN field EXT_CSD[163] to 1b. When
>>>>>> this bit is set, the device may start or stop background operations
>>>>>> whenever it sees fit, without any notification to the host.
>>>>>>
>>>>>> When AUTO_EN bit is set, the host should keep the device power
>>>>>> active. The host may set or clear this bit at any time based on
>>>>>> its power constraints or other considerations.
>>>>>>
>>>>>> Currently the manual bkops is only be used under the async req
>>>>>> circumstances and it's a bit complicated to be controlled as the
>>>>>> perfect method is that we should do some idle monitor just as rpm
>>>>>> and send HPI each time if receiving rd/wr req. But it will impact
>>>>>> performance significantly, especially for random iops since the
>>>>>> weight of executing HPI against r/w small piece of LBAs is
>>>>>> nonnegligible.
>>>>>>
>>>>>> So we now prefer to select the auto one unconditionally if supported
>>>>>> which makes it as simple as possible. It should really good enough
>>>>>> for devices to manage its internal policy for bkops rather than the
>>>>>> host, which makes us believe that we could achieve the best
>>>>>> performance for all the devices implementing auto bkops and the only
>>>>>> thing we should do is to disable it when cutting off the power.
>>>>>
>>>>> Do you know if there is really a requirement to do that?
>>>>
>>>> Even without bkops enable, no matter for manual or auto one, FTL should
>>>> always do bkops like GC internally when needed to guarantee the
>>>> performance and balance the wear leveling. What I thought to do is to
>>>> make it more explicitly.
>>>>
>>>> Because then, what
>>>>> is the point of power off notification?
>>>>
>>>> When power off notification is sent, bkops will be stopped
>>>> in _mmc_suspend. So I don't undertand your point here?
>>>
>>> I am trying to understand why we need to do anything for auto bkops.
>>> Since AUTO_EN is persistent, we can leave the decision whether to turn it on
>>> to whomever provisions the device. Then we just leave it alone.
>>>
>>
>> Hrm..
>>
>> one possible way is to control it by mmc-utils on
>> user space?  So we should add a cmd for mmc-utils
>> there?
>
> That would be consistent with manual bkops.
>

>From my first impression I agree, as that is the policy we have been
sticking to when writing to persistent EXT_CSD registers.
Although, in this case, I am actually wondering on what is the best approach.

Is there really ever a case when we don't want auto BKOPS to be default enabled?
I think BKOPS is a fundamental feature of an FTL and I can't see a
reason to why we need to involve mmc-utils/userspace to enable it. Am
I wrong?

Kind regards
Uffe