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

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Date:	Tue, 28 Apr 2015 00:19:16 +0200
From:	Richard Weinberger <richard@....at>
To:	Ben Shelton <ben.shelton@...com>
CC:	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
	dwmw2@...radead.org, computersforpeace@...il.com,
	punnaiah.choudary.kalluri@...inx.com
Subject: Re: [PATCH 1/3] mtd: nand: Add on-die ECC support

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. ;-\
But you're right there is a corner case.

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

Maybe Brian can bring some light into that too...

> When I build this without CONFIG_MTD_NAND_ECC_ON_DIE enabled, I get the
> following warning here:
> 
> In file included from drivers/mtd/nand/nand_base.c:46:0:
> include/linux/mtd/nand_ondie.h: In function 'nand_read_subpage_on_die':
> include/linux/mtd/nand_ondie.h:28:1: warning: no return statement in function returning non-void [-Wreturn-type]
> include/linux/mtd/nand_ondie.h: In function 'nand_read_page_on_die':
> include/linux/mtd/nand_ondie.h:34:1: warning: no return statement in function returning non-void [-Wreturn-type]
> 
> Perhaps return an error code here, even though you'll never get past the BUG()?

What gcc is this?
gcc 4.8 here does not warn, I thought it is smart enough that this function does never
return. Can it be that your .config has CONFIG_BUG=n?
Anyway, this functions clearly needs a return statement. :)

Thanks,
//richard
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