RE: [PATCH V1] mmc: core: fix still flush cache when eMMC cache off

["Bean Huo (beanhuo)" <beanhuo@micron.com>]

Hi, Uffe

>>>On 19 March 2017 at 01:45, Bean Huo (beanhuo) <beanhuo@micron.com>
>wrote:
>>>> This patch fixes the issue that mmc_blk_issue_rq still flushes cache
>>>> when eMMC cache has already been off through user space tool, such
>>>> as mmc-utils.
>>>> The reason is that card->ext_csd.cache_ctrl isn't reset.
>>>
>>>First, why do you want to turn of the cache ctrl? Is the eMMC device
>>>having issues with it? Then we should invent a card quirk instead.
>>
>>
>> Why I turn off it? because I did power loss testing and validation, I should
>switch it off and on.
>> When I do some performance and power loss case validation on several
>> Linux release versions, I need to switch off or on cache through user space tool.
>> I can't confirm every user that likes me, But I think at least it is
>> not reasonable to flush eMMC cache, when internal eMMC cache is off.
>
>Ah, I see. Your use-case seems reasonable while validating robustness of the
>eMMC!
>
>>
>>>Second, what errors do you encounter when the mmc core tries to flush
>>>the cache when it has been turned off? Can you please elaborate on this?
>>
>>
>> No error found, but firstly, please think about overhead introduced by
>> useless flush cache, Unless you Don't care this tiny time. second,
>> under the condition of cache off, additional flush cache request still has impact
>>on Internal eMMC logic. I don't know What and how impact, but at least it is
>>really exist.
>
>Got it!
>
>However I still don't like the mmc ioctls API to compensate and deal with all
>"crazy-ness" that user-space may cause. Cache-ctrl is only one case out of many.

Question is that not every Linux-mmc user really understand that shouldn't use "mmc ioctls API 
to compensate and deal with all crazy-ness that user-space may cause ".
no matter we like or dislike, Issue is already there; I think that most of eMMC user is now 
configuring eMMC through mmc-utils and mmc ioctls. This is a real condition.
Some automotive applications, Cache would not be enabled. 

Maybe I am shallow, mmc ioctls likes backdoor that let user access and configure eMMC some feature,
Even not too often. I still suggest to fix this in mmc ioctls, unless mmc iotcls being deleted, and mmc-utils
doesn't support Cache off/on.
Because eMMC internal cache has been disabled, but Linux-mmc still issue flush cache request, this is indeed
Not reasonable. It definitely increases system level overhead.

>I see two viable options to solve your problem.
>1) Extend mmc_test with a new test(s) for cache ctrl and perhaps suspend/resume.
>Isn't this actually exactly what you want?

>2) Extend debugfs to be able to turn cache ctrl on/off.

I can try, but this patch is just to fix mmc ctrls backdoor on eMMC flush cache.

>[...]
>
>Kind regards
>Uffe
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