Date:	Tue, 1 Apr 2014 12:29:26 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Brian Norris <computersforpeace@...il.com>
Cc:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	angus.clark@...com, kernel@...inux.com,
	linux-mtd@...ts.infradead.org, pekon@...com, dwmw2@...radead.org
Subject: Re: [RFC 00/47] mtd: nand: Add new driver supporting ST's BCH h/w

> > > The call to nand_scan_tail() would remove the need to export those NAND
> > > core functions, and remove the need to scan and print the bad blocks.
> > > I don't know if you have a real reason for not doing it this way, or
> > > maybe it's the way this driver was originally written.
> > > 
> > > Care to review this and re-spin the driver? You'll have a more nicer
> > > driver, and more framework-compliant.
> > 
> > A hearty +1 to this. You are avoiding much of the core of the NAND
> > framework by avoiding the nand_chip callbacks and nand_scan_tail(), and
> > by reimplementing the BBT. I will have to NAK to some of the patches
> > that EXPORT the nand_base private core (e.g., nand_get_device()), and I
> > will most likely NAK the custom BBT implementation (please improve
> > nand_bbt.c as needed).
> 
> This is a good catch. I will attempt to reimplement the driver's
> initialisation steps to utilise more of the core infrastructure in an
> attempt to mitigate the requirement for exportation of private
> routines.
> 
> The BBT requirements are somewhere more complex. To provide you with
> the complete picture, a little knowledge of driver history is
> required. When it was initially created the MTD core only supported
> OOB BBTs, but the ST BCH Controller doesn't support OOB access, so
> Angus wrote his on In-Band (IB) implementation. Unfortunately the IB
> support which _is_ now present in the kernel doesn't match the
> internal implementation. Normally this wouldn't be an issue in itself,
> but ST's boot-stack and tooling (Primary Bootloader, U-Boot, various
> Programmers, etc) are aware of the internal IB BTT and utilise it
> in varying ways. Shifting over to the Mainline version in
> one-foul-swoop _will_ cause lots of pain and will probably result in
> the disownership of driver we're trying to Mainline today. Naturally
> I'm keen to avoid this.

Just looking into this now. Can I add support for a vendor specific
signature extension? ST's flashers, bootloaders and tooling currently
use the format:

/* Extend IBBT header with some stm-nand-bch niceties */
struct nand_ibbt_bch_header {
	uint8_t signature[4];           /* "Bbt0" or "1tbB" signature */
	uint8_t version;                /* BBT version ("age") */
	uint8_t reserved[3];            /* padding */
	uint8_t baseschema[4];          /* "base" schema (x4) */
	uint8_t privschema[4];          /* "private" schema (x4) */
	uint8_t ecc_size[4];            /* ECC bytes (0, 32, 54) (x4) */
	char    author[64];             /* Arbitrary string for S/W to use */
}; __attribute__((__packed__))

It would be great if we can support this with a descriptor option or
suchlike, as it would a) save me a lot of aggravation and b) continue
to support ST with their current use-case.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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