Re: [RFC PATCH 2/2] mtd: nand: use nand_check_erased_ecc_chunk in default ECC read functions

[Andrea Scian <rnd4@dave-tech.it>]

Dear Boris,

Il 31/07/2015 18:27, Boris Brezillon ha scritto:
> On Fri, 31 Jul 2015 18:19:30 +0200
> Andrea Scian <rnd4@dave-tech.it> wrote:
>
>> Il 31/07/2015 16:10, Boris Brezillon ha scritto:
>>> On Fri, 31 Jul 2015 15:40:13 +0200
>>> Andrea Scian <rnd4@dave-tech.it> wrote:
>>>
>>>> Boris,
>>>>
>>>> Il 31/07/2015 12:32, Boris Brezillon ha scritto:
>>>>> Hi Andrea,
>>>>>
>>>>> Adding Han in Cc.
>>>>>
>>>>> On Fri, 31 Jul 2015 12:07:21 +0200
>>>>> Andrea Scian <rnd4@dave-tech.it> wrote:
>>>>>
>>>>>> Dear Boris,
>>>>>>
>>>>>>
>>>>>> Il 30/07/2015 19:34, Boris Brezillon ha scritto:
>>>>>>> The default NAND read functions are relying on an underlying controller
>>>>>>> to correct bitflips, but some of those controller cannot properly fix
>>>>>>> bitflips in erased pages.
>>>>>>> In case of ECC failures, check if the page of subpage is empty before
>>>>>>> reporting an ECC failure.
>>>>>> I'm still wondering if chip->ecc.strength is the right threshold.
>>>>>>
>>>>>> Did you see my comments here [1]? WDYT?
>>>>> Yes I've read it, and decided to go for ecc->strength as a first
>>>>> step (I'm more interested in discussing the approach than the threshold
>>>>> value right now ;-)).
>>>> I perfectly understand, that's the reason why I ask if you want to move
>>>> to another thread ;-)
>>>>
>>>>> Anyway, as you pointed out in the thread, writing data on an erased
>>>>> page already containing some bitflips might generate even more
>>>>> bitflips, so using a different threshold for the erased page check
>>>>> makes sense. This threshold should definitely be correlated to the ECC
>>>>> strength, but how, that's the question.
>>>>>
>>>>> How about taking a rather conservative value like 10% of the specified
>>>>> ECC strength, and see how it goes.
>>>> Yes, I think that there's no real way to get the right value, other than
>>>> feedbacks from on-field testing with various devices.
>>>>
>>>> I'm also thinking about changing how a NAND page is written on the
>>>> device, now that we know that even erased page may have (too many!)
>>>> bitflips if they has not been so-freshly erased.
>>>>
>>>> Read on NAND device is lot's faster that write, so maybe we can:
>>>>
>>>> a) read the page before write it, check for bitflips on erased area and
>>>> write it only if it fit our threshold
>>>>
>>>> b) read the page after write it and check if the bitflips are lower that
>>>> a give value
>>>>
>>>> In this way:
>>>> - we can use ecc_strength as read threshold, because it fits all the
>>>> other NAND read
>>>>
>>>> - we can use "something a bit lower than" mtd->bitflip_threshold on
>>>> read-before-write or read-after-write. If we don't do so the block will
>>>> be scrubbed next time we read it again (if we are lucky.. if we are
>>>> unlucky the block will have bitflip > ecc_strength!): IOW we did a write
>>>> that will trigger another erase/write cycle.
>>>>
>>>> Am I misunderstanding something?
>>> Nope, but this implies doing an extra read after each write :-/
>>>
>> Let's wait what the others says about this, but I would like to put some
>> numbers in it.
> Sure.
>
>> My micron MLC device says
>> - read page max 75 uS
>> - write page typ 1300uS, max 2600uS
>>
>> If we implement read-before-write (which is, IMO, the best approach), in
>> the worst overhead we have is 1375uS vs 1300uS, which is ~6%.
>> Please note that, if you read a page that "is not suitable" for write,
>> you avoid the write time, schedule it for scrubbing, and use another
>> free page.
> Indeed, that's not such a big overhead.
>
>> Probably I'm a bit optimistic because we also need to take in account
>> other latencies (DMA setup, ECC engine, buffer copies and so on) but
>> it's a starting point ;-)
> Yep, if you test it, could you provide some speed test results (with
> and without this solution).

I think I can find some time to do some performance tests on real hardware.
Can you please help me in finding:
- which benchmark to use (currently I'm using bonnie++ on UBIFS, maybe I 
can you just mtd_speedtest)
- where to implement those read

For the second point I think we can implement it a UBI or MTD level.
I think the former will allow us to easily schedule scrubbing and choose 
another block to issue the write to. However I don't really know how to 
implement it (I don't really know so much about the UBI code).

The latter, at least for me, is easier to implement: I think I can find 
the place to add the page read on my own, anyway any clue is welcome ;-)
But I think it will be harder (or impossible) to choose where to issue 
the write, unless UBI already do so when it saw an MTD write failure.

> And I wonder if we shouldn't do it the other way around, write then
> read-back and check the content.
> Of course this implies doing the extra write even when the erased page
> contains too many bitflips, but at least your sure that the data you
> put in the page were correct at that time.
>

You're right, I think this is something that once we can find inside the 
MTD code (something like "check NAND written page" kconfig option) but I 
cannot find this option anymore on latest kernel.

You're approach will also have another advantage, currently nearly all 
platform will use software implementation for ECC check on erased 
blocks, while nearly all of them has hardware ECC once programmed. Using 
hardware ECC will remove CPU load and, maybe, be faster than call 
nand_check_erased_ecc_chunk()
I also think that the situation of having failure on write is very 
unlikely, unless you have a very "used" NAND device or you're not using 
Richard's bitrot check. So we'll have a performance impact (issuing 
another write) only when it's really needed.

Kind Regards,

-- 

Andrea SCIAN

DAVE Embedded Systems