linux-kernel - Re: [PATCH 1/3] mtd: nand: Add on-die ECC support

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Message-ID: <CAGnW=BYVn5quNKooiB44CiCYtuYckWArTkft4BQLxs188FwUpg@mail.gmail.com>
Date:	Tue, 28 Apr 2015 08:18:12 +0530
From:	punnaiah choudary kalluri <punnaia@...inx.com>
To:	Brian Norris <computersforpeace@...il.com>
Cc:	Richard Weinberger <richard@....at>,
	Ben Shelton <ben.shelton@...com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Punnaiah Choudary Kalluri 
	<punnaiah.choudary.kalluri@...inx.com>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>
Subject: Re: [PATCH 1/3] mtd: nand: Add on-die ECC support

On Tue, Apr 28, 2015 at 4:53 AM, Brian Norris
<computersforpeace@...il.com> wrote:
> On Tue, Apr 28, 2015 at 12:19:16AM +0200, Richard Weinberger wrote:
>> Am 27.04.2015 um 23:35 schrieb Ben Shelton:
>> > I tested this against the latest version of the PL353 NAND driver that Punnaiah
>> > has been working to upstream (copying her on this message).  With a few changes
>> > to that driver, I got it most of the way through initialization with on-die ECC
>> > enabled, but it segfaults here with a null pointer dereference because the
>> > PL353 driver does not implement chip->cmd_ctrl.  Instead, it implements a
>> > custom override of cmd->cmdfunc that does not call cmd_ctrl.  Looking through
>> > the other in-tree NAND drivers, it looks like most of them do implement
>> > cmd_ctrl, but quite a few of them do not (e.g. au1550nd, denali, docg4).
>> >
>> > What do you think would be the best way to handle this?  It seems like this gap
>> > could be bridged from either side -- either the PL353 driver could implement
>> > cmd_ctrl, at least as a stub version that provides the expected behavior in
>> > this case; or the on-die framework could break this out into a callback
>> > function with a default implementation that the driver could override to
>> > perform this behavior in the manner of its choosing.
>>
>> Oh, I thought every driver has to implement that function. ;-\
>
> Nope.
>
>> But you're right there is a corner case.
>
> And it's not the only one! Right now, there's no guarantee even that
> read_buf() returns raw data, unmodified by the SoC's controller. Plenty
> of drivers actually have HW-enabled ECC turned on by default, and so
> they override the chip->ecc.read_page() (and sometimes
> chip->ecc.read_page_raw() functions, if we're lucky) with something
> that pokes the appropriate hardware instead. I expect anything
> comprehensive here is probably going to have to utilize
> chip->ecc.read_page_raw(), at least if it's provided by the hardware
> driver.

Yes, overriding the chip->ecc.read_page_raw would solve this. Agree that
read_buf need not be returning raw data always including my new driver for
arasan nand flash controller.

http://lkml.iu.edu/hypermail/linux/kernel/1504.2/00313.html


Regards,
Punnaiah
>
>> What we could do is just using chip->cmdfunc() with a custom NAND command.
>> i.e. chip->cmdfunc(mtd, NAND_CMD_READMODE, -1, -1);
>>
>> Gerhard Sittig tried to introduce such a command some time ago:
>> http://lists.infradead.org/pipermail/linux-mtd/2014-April/053115.html
>
> Yikes! Please no! It's bad enough to have a ton of drivers doing
> switch/case on a bunch of real, somewhat well-known opcodes, but to add
> new fake ones? I'd rather not. We're inflicting ourselves with a
> kernel-internal version of ioctl(). What's the justification, again? I
> don't really remember the context of Gerhard's previous patch.
>
> Brian
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