[PATCH] mtd: gpmi: Deal with bitflips in erased regions regions

[Huang Shijie]

On Tue, Dec 17, 2013 at 10:17:38AM +0000, Gupta, Pekon wrote:
> >+/* Returns 1 if the last transaction consisted only out of ones. */
> >+int gpmi_all_ones(struct gpmi_nand_data *this)
> >+{
> >+	struct resources *r = &this->resources;
> >+	uint32_t reg = readl(r->bch_regs + HW_BCH_STATUS0);
> >+
> >+	return reg & BM_BCH_STATUS0_ALLONES_MASK;
> >+}
> >+
> >
> Just query.. 
> Is GPMI controller is able to detect number of 0s or 1s while reading
> the raw data from NAND ?

The gpmi can detect the all-one case, in other word, if the page is all
0xff, we can know this case.

Since the bit flips of erase page is very very rare, i think 
the performance is not effected by this patch.


> I'm asking because a similar implementation was used in omap2.c
> driver reference: drivers/mtd/nand/omap2.c: erased_sector_bitflips()
> But, we ended up degrading the driver performance, so even I was
> looking for alternative implementations..
> 
>  [...]
> 
> >
> >+/*
> >+ * Count the number of 0 bits in a supposed to be
> >+ * erased region and correct them. Return the number
> >+ * of bitflips or zero when the region was correct.
> >+ */
> >+static unsigned int erased_sector_bitflips(unsigned char *data,
> >+					unsigned int chunk,
> >+					struct bch_geometry *geo)
> >+{
> >+	unsigned int flip_bits = 0;
> >+	int i;
> >+	int base = geo->ecc_chunk_size * chunk;
> >+
> >+	/* Count bitflips */
> >+	for (i = 0; i < geo->ecc_chunk_size; i++)
> >+		flip_bits += hweight8(~data[base + i]);
> >+
> >+	/* Correct bitflips by 0xFF'ing this chunk. */
> >+	if (flip_bits)
> >+		memset(&data[base], 0xFF, geo->ecc_chunk_size);
of course, this function could be optimized more better.

> 
> But I don't quite understand here is.. 
> Why do you want to mask off the bit-flips in NAND, and give corrected
> data to upper layers ? Won't this lead to accumulation of bit-flips ?
> 
> Actually UBIFS checks for clean erased-pages before writing anything
> on it [1]. And if UBIFS finds an unclean page, it re-erases it without
> harming the system to avoid accumulation of bit-flips [2].
> (Artem, please correct me if I'm wrong here)..
> 
> But with your above patch you will actually fool UBIFS to write on
> unclean pages, which will increase the probability of bit-flips failures.
> Example:  your erased page had 4-bit flips, but you still reported
> data as clean. Now when UBIFS will write on this erased page, it
> has the margin of tolerating only 4 more bit-flips.
> 
if we use the 8 as the ECC strengh, we still can correct the bitflips,
am I right? I feel a little confused at this.

Btw: From Pekon's comment, i suddenly notice that we should fill the
     ERASE_THRESHOLD like this:

	int threshold = gf_len / 2;

	if (threshold > geo->ecc_strength)
		threshold = geo->ecc_strength;

	/*
	 * Set the tolerance for bitflips when reading erased blocks
	 * equal to half the gf_len.
	 */
	writel(threshold & BM_BCH_MODE_ERASE_THRESHOLD_MASK,
		r->bch_regs + HW_BCH_MODE);


thanks
Huang Shijie