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

[Adrian Hunter]

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.