linux-kernel - Re: [RFC 13/47] mtd: nand: stm_nand

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Message-ID: <20140519140249.GM4978@lee--X1>
Date:	Mon, 19 May 2014 15:02:49 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	"Gupta, Pekon" <pekon@...com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kernel@...inux.com" <kernel@...inux.com>,
	"computersforpeace@...il.com" <computersforpeace@...il.com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"angus.clark@...com" <angus.clark@...com>
Subject: Re: [RFC 13/47] mtd: nand: stm_nand_bch: provide Device Tree support

> >> >> >+	of_property_read_u32(np, "st,bch-bitflip-threshold",
> >> >> >+			     &pdata->bch_bitflip_threshold);
> >> >> >+
> >> >> mtd->bitflip_threshold is by default set to ecc.strength (unless a driver initializes it).
> >> >> And then can be re-configured for each MTD partition separately
> >> >> 	/sys/class/mtd/mtdX/bitflip_threshold
> >> >> 	Refer: $kernel/Documentation/ABI/testing/sysfs-class-mtd
> >> >> So, I don't think this is a HW parameter, and so should not be passed from DT.
> >> >
> >> >I think the bit-flip threshold is/can be chip specific, and as we know
> >> >which chip we're likely to be booting on, we can specify this via the
> >> >hardware description.  Thus, I think it's perfectly viable for an
> >> >option to pass through DT to exist.
> >> >
> >> I don't think that’s the correct interpretation of bitflip_threshold.
> >>
> >> (1) bitflip_threshold is dependent on ecc.strength (ECC scheme) of your driver.
> >> MTD layers uses bitflip_threshold to warn above layers that number of
> >> correctable bit-flips have reached a dangerous level beyond which driver's
> >> ECC scheme may not be able to correct them. So above layers should start
> >> taking additional corrective action like scrubbing.
> >> 	@@drivers/mtd/mtdcore.c: mtd_read()
> >> 		return ret_code >= mtd->bitflip_threshold ? -EUCLEAN : 0;
> >>
> >> (2) Also, user-space may control it based on how your device ages on field.
> >> A fresh silicon may not show too many bitflips. But as device ages the
> >> probability of simultaneous bitflips will increase. So user-space may lower
> >> the bitflip_threshold to avoid accumulation of bitflips in a single page.
> >>
> >> Thus, bitflip_threshold should not be passed via DT.
> >> It's neither a hardware parameter, nor it’s a static constant.
> >
> >Ah, I see.  I will fixup, thanks for the explanation.
> 
> Please wait, I'll review your [v2] series also, then you can further
> send all fixes together. I'm bit caught in other commitments for 3.16,
> so hopefully I'll be able to review your patches by next week.

Did you manage to find some time to review the driver at all last week?

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