linux-kernel - Re: [PATCH] mmc: core: add auto bkops support

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Date:	Wed, 22 Jun 2016 14:57:05 +0000
From:	Alex Lemberg <Alex.Lemberg@...disk.com>
To:	Ulf Hansson <ulf.hansson@...aro.org>
CC:	Adrian Hunter <adrian.hunter@...el.com>,
	Shawn Lin <shawn.lin@...k-chips.com>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Doug Anderson <dianders@...omium.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>
Subject: Re: [PATCH] mmc: core: add auto bkops support



On 6/22/16, 5:28 PM, "Ulf Hansson" <ulf.hansson@...aro.org> wrote:

>On 22 June 2016 at 16:20, Alex Lemberg <Alex.Lemberg@...disk.com> wrote:
>>
>>
>> On 6/22/16, 1:21 PM, "Ulf Hansson" <ulf.hansson@...aro.org> wrote:
>>
>>>On 13 June 2016 at 14:25, Adrian Hunter <adrian.hunter@...el.com> wrote:
>>>> On 13/06/16 11:58, Shawn Lin wrote:
>>>>> 在 2016/6/13 16:17, Adrian Hunter 写道:
>>>>>> 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?
>>
>> The even worst case is – involve mmc-utils/userspace to DISABLE it.
>> I think this register need to be set by vendor and no need to be changed on runtime.
>
>If it is set by the Vendor, that's of course the best.

It can be set by Storage Vendor.
According to the spec, the default value of this bit is vendor specific.

>
>Are you saying that we shouldn't enable it during the card init
>sequence from the kernel, in case it is disabled?

No.
By the spec – a 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 byte 163] to 1b.

>
>Kind regards
>Uffe