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

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Message-ID: <B9A5F20C-E8A0-4E81-BA07-79E60017E728@sandisk.com>
Date:	Wed, 22 Jun 2016 14:20:48 +0000
From:	Alex Lemberg <Alex.Lemberg@...disk.com>
To:	Ulf Hansson <ulf.hansson@...aro.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Shawn Lin <shawn.lin@...k-chips.com>,
	"Jaehoon Chung" <jh80.chung@...sung.com>
CC:	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, 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.

>
>Kind regards
>Uffe