Re: [RFC 13/47] mtd: nand: stm_nand_bch: provide Device Tree support

Lee Jones
2014-05-19T14:03:22Z

> >> >> >+ 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
Follow Linaro: Facebook | Twitter | Blog