Re: [PATCH] brcmnand: Clear EXT_ADDR error registers in PIO mode

[Brian Norris <computersforpeace@gmail.com>]

Hi,

On Fri, Nov 20, 2015 at 06:42:08PM +0000, Simon Arlott wrote:
> On 17/11/15 17:55, Brian Norris wrote:
> > On Tue, Nov 17, 2015 at 07:41:21AM +0000, Simon Arlott wrote:
> >> On 17/11/15 00:40, Brian Norris wrote:
> >> > + bcm-kernel-feedback-list
> >> > 
> >> > On Mon, Nov 16, 2015 at 10:05:39PM +0000, Simon Arlott wrote:
> >> >> If an error occurs in flash above 4GB in PIO mode then the EXT_ADDR
> >> >> registers will be set to the location of the error and never cleared.
> >> >> 
> >> >> Reset them to 0 before reading.
> >> >> 
> >> >> Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
> >> > 
> >> > Patch looks OK. Did you see this problem in practice, or is this just
> >> > theoretical? I thought the documentation seemed to suggest these
> >> > registers were cleared together with their non-_EXT counterparts. But
> >> > implementation definitely trumps documentation for HW.
> >> 
> >> It's theoretical (I don't have 4GB+ flash), but the Broadcom version of
> >> the NAND driver does this.
> > 
> > That's a funny thing to say :) There never really was a single "Broadcom
> > version" until we settled on upstreaming this one. It's a direct
> > descendant of this [1], which also does not do these writes, and was
> > tested on >4GB flash, though not extensively.
> > 
> > Which product line did you get your driver from, then?
> 
> I have a file called bcm963xx_4.12L.06B_consumer/kernel/linux/drivers/mtd/brcmnand/brcmnand_base.c:
> 
>         /* Clear ECC registers */
>         chip->ctrl_write(BCHP_NAND_ECC_CORR_ADDR, 0);
>         chip->ctrl_write(BCHP_NAND_ECC_UNC_ADDR, 0);
> #if CONFIG_MTD_BRCMNAND_VERSION >= CONFIG_MTD_BRCMNAND_VERS_1_0
>         chip->ctrl_write(BCHP_NAND_ECC_CORR_EXT_ADDR, 0);
>         chip->ctrl_write(BCHP_NAND_ECC_UNC_EXT_ADDR, 0);
> #endif

I'd bet it was done without ever actually testing or observing the
behavior. The version 1.0 controller is extremely old and most
definitely never was used with >4GB NAND.

But since this should be harmless and has some small chance of fixing a
bug, I'll take it anyway.

> There's also a workaround (brcmnand_handle_false_read_ecc_unc_errors)
> that I'm going to write a patch for as it affects my v4.0 device. It'd
> be useful to know which version of the controller fixes the issue:
> 
>        /* Flash chip returns errors 
>         || There is a bug in the controller, where if one reads from an erased block that has NOT been written to,
>         || this error is raised.  
>         || (Writing to OOB area does not have any effect on this bug)
>         || The workaround is to also look into the OOB area, to see if they are all 0xFF
>         */

That one's pretty ugly... but that's a different story.

Brian