Re: Pass -EUCLEN to userspace?

[Boris Brezillon <boris.brezillon@free-electrons.com>]

On Mon, 25 Apr 2016 10:22:11 +0200
Sascha Hauer <s.hauer@pengutronix.de> wrote:

> On Mon, Apr 25, 2016 at 09:50:34AM +0200, Boris Brezillon wrote:
> > On Mon, 25 Apr 2016 07:28:57 +0200
> > Sascha Hauer <s.hauer@pengutronix.de> wrote:
> >   
> > > On Fri, Apr 22, 2016 at 05:48:35PM +0200, Richard Weinberger wrote:  
> > > > Sascha, Boris,
> > > > 
> > > > Am 22.04.2016 um 17:28 schrieb Boris Brezillon:    
> > > > >>> I am currently working on a program similar to ubihealthd, just for raw
> > > > >>> mtd pages, not UBI. Basically I want to find out in userspace if my Nand needs
> > > > >>> scrubbing. Is it possible somehow to get this information in userspace?    
> > > > >>
> > > > >> Actually we discussed that a year ago with Richard. I told him that we
> > > > >> should put the read/write/erase statistics at the MTD level so that
> > > > >> other MTD users (including userspace programs) could use the same infra
> > > > >> for non-UBI partitions (I need that for the UBOOT and SPL partitions).
> > > > >>
> > > > >> My suggestion was to store those information at the MTD level, and let
> > > > >> UBI implement its own scrubbing layer on top of that, but Richard
> > > > >> decided to go for a simpler approach for its first implementation.    
> > > > 
> > > > Yeah, I did a first implementation on UBI layer as it had everything we need
> > > > and I didn't want to replicate UBI at MTD level.
> > > > Another reason is that we were not sure how sophisticated ubihealthd needs to be.
> > > > 
> > > > Sasha, what exactly is your use case and why is the UBI approach not sufficient for you?
> > > > On Linux MTD access should only happen through UBI and UBOOT/SPL partitions stay untouched.    
> > > 
> > > On i.MX6 the Bootloader in Nand can indeed be redundant, so it's
> > > possible to scrub the pages. This is exactly our usecase, we want to be
> > > able to detect bitflips in the bootloader area.
> > > Note that on i.MX6 the first page in the first n blocks on Nand contains
> > > a structure called FCB (flash control block). This is not encoded with
> > > the standard ECC algorithm used on the other areas in Nand. Reading
> > > these pages will always return -EBABDMSG, they have to be read in raw
> > > mode. That just to say that a "maximum bitflips per block" might not be
> > > sufficient.  
> > 
> > Okay, pretty much the same use-case we have on sunxi platforms: the SPL
> > partition is written in raw mode because the page layout (in-band/ECC
> > data disposition) is not the one we're using for the rest of the NAND.
> > For this specific partition, I see 2 solutions that you can implement
> > in userspace to count the number of bitflips:
> > 
> > 1/ read the partition page by page in raw mode and compare each
> >    page to a reference file. This implies having a reference file stored
> >    on your FS.
> > 2/ if you know the ECC algorithm (and the platform specific config,
> >    like the polynomial for a BCH engine) then you can create a tool
> >    doing the ECC error detection in userspace.  
> 
> I can do the ECC calculation for the FBCs in userspace, that's no
> problem. I was referring to:
> 
> > Fair enough. So all we'll need is a way to retrieve the maximum number
> > of bitflips for a given block  
> 
> When a block contains both FCB and data protected with regular ECC in
> other pages, an algorithm retrieving the maximum number of bitlfips for
> a given block might stumble over the bad data in the page containing the
> FCB. So I think we need the maximum number of bitflips per page, not per
> block.

Oh, so you mix 2 different page layout in the same partition (we
try to avoid that, even if this means loosing some space) :-/.

Regarding the maximum number of bitflips per chunk, maybe we can make it
part of the ioctl request instead of saving the statistics at the MTD
level.

How about creating a new ioctl taking a pointer to this struct as a
parameter:

struct mtd_extended_read_ops {
	/* Existing params */
	unsigned int mode;
	size_t len;
	size_t retlen;
	size_t ooblen;
	size_t oobretlen;
	uint32_t ooboffs;
	void *datbuf;
	void *oobbuf;

	/*
	 * Param containing the maximum number of bitflips for this
	 * read request.
	 */
	unsigned int max_bitflips;
};

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com