linux-kernel - Re: [PATCH v2 13/16] mtd/docg3: add ECC correction code

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Message-ID: <4EC0794B.3010604@newsguy.com>
Date:	Sun, 13 Nov 2011 18:13:31 -0800
From:	Mike Dunn <mikedunn@...sguy.com>
To:	Robert Jarzmik <robert.jarzmik@...e.fr>
CC:	linux-mtd@...ts.infradead.org, dwmw2@...radead.org,
	linux-kernel@...r.kernel.org, dedekind1@...il.com
Subject: Re: [PATCH v2 13/16] mtd/docg3: add ECC correction code

On 11/13/2011 02:35 AM, Robert Jarzmik wrote:
> Mike Dunn <mikedunn@...sguy.com> writes:
>
>>
>> Where do you check for reads of a blank page?
> On stack frame above. Look at doc_read_oob():
> 		if ((block0 >= DOC_LAYOUT_BLOCK_FIRST_DATA) &&
> 		    (eccconf1 & DOC_ECCCONF1_BCH_SYNDROM_ERR) &&
> 		    (eccconf1 & DOC_ECCCONF1_PAGE_IS_WRITTEN) &&
>                                 \---> this is the key
> 		    (ops->mode != MTD_OPS_RAW) &&
> 		    (nbdata == DOC_LAYOUT_PAGE_SIZE)) {
> 			ret = doc_ecc_bch_fix_data(docg3, buf, hwecc);
>
> Here you see that I'll make the error correction only if the page was written
> before. If it's blank, I continue reading without attempting ECC correction.


G4 probably has this capability too.  If so, I'll be improving my blank page
detection.  I *really* have to go through your driver in its entirety to see
what other insights you have.  Currently getting pulled in multiple directions.


>> Not specifically related to this patch, but... are you sure you want to
>> initialize the ecc on every read?  I'm sure it's not necessary; you can just
>> leave it on; maybe turn it off if doing raw reads.  I know this is the case for
>> both the P3 and G4 when running under PalmOS / TrueFFS library.  I notice that
>> this function has delays and polls the status register in between calls to
>> cpu_relax(), so the performance hit is probably not insignificant, especiallu
>> when done for every 512 byte page.
> Well, that's some info.
> And yes, it adds some latency.
> Now for the necessity, I'm not fully convinced. I know that the ECC register is
> set up differently for reads and writes (that's the
> DOC_ECCCONF0_READ_MODE). When doc_read_oob() is called, I don't know if the
> previous action was a read or a write ...
>
> What I could do to improve performance would be to store in the docg3 private
> data if last action was a read or a write, and perform the doc_*_page_ecc_init()
> only if action changes. What do you think ?
>


Never mind.  Sorry.  I was thinking DOC_ECCCONF1, which I write to during
initialization to (I believe) enable ecc.  Anyway, you have a better handle on
the internals of the chip than I do.  I'm still looking forward to comparing
your code to my old P3 test code (which mostly just parrots what I observed
during operation).  Until then I'll keep my opinions on register sequences, etc,
to myself!'

Good job!

Mike

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