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SubjectRe: [PATCH v7] mtd: rawnand: use bit-wise majority to recover the contents of ONFI parameter
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Hi,

Le 15/05/2018 à 09:34, Boris Brezillon a écrit :
> On Tue, 15 May 2018 06:45:51 +0200
> Chris Moore <moore@free.fr> wrote:
>
>> Hi,
>>
>> Le 13/05/2018 à 06:30, Wan, Jane (Nokia - US/Sunnyvale) a écrit :
>>> Per ONFI specification (Rev. 4.0), if all parameter pages have invalid CRC values, the bit-wise majority may be used to recover the contents of the parameter pages from the parameter page copies present.
>>>
>>> Signed-off-by: Jane Wan <Jane.Wan@nokia.com>
>>> ---
>>> v7: change debug print messages
>>> v6: support the cases that srcbufs are not contiguous
>>> v5: make the bit-wise majority functon generic
>>> v4: move the bit-wise majority code in a separate function
>>> v3: fix warning message detected by kbuild test robot
>>> v2: rebase the changes on top of v4.17-rc1
>>>
>>> drivers/mtd/nand/raw/nand_base.c | 50 ++++++++++++++++++++++++++++++++++----
>>> 1 file changed, 45 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/mtd/nand/raw/nand_base.c b/drivers/mtd/nand/raw/nand_base.c
>>> index 72f3a89..b43b784 100644
>>> --- a/drivers/mtd/nand/raw/nand_base.c
>>> +++ b/drivers/mtd/nand/raw/nand_base.c
>>> @@ -5087,6 +5087,35 @@ static int nand_flash_detect_ext_param_page(struct nand_chip *chip,
>>> }
>>>
>>> /*
>>> + * Recover data with bit-wise majority
>>> + */
>>> +static void nand_bit_wise_majority(const void **srcbufs,
>>> + unsigned int nsrcbufs,
>>> + void *dstbuf,
>>> + unsigned int bufsize)
>>> +{
>>> + int i, j, k;
>>> +
>>> + for (i = 0; i < bufsize; i++) {
>>> + u8 cnt, val;
>>> +
>>> + val = 0;
>>> + for (j = 0; j < 8; j++) {
>>> + cnt = 0;
>>> + for (k = 0; k < nsrcbufs; k++) {
>>> + const u8 *srcbuf = srcbufs[k];
>>> +
>>> + if (srcbuf[i] & BIT(j))
>>> + cnt++;
>>> + }
>>> + if (cnt > nsrcbufs / 2)
>>> + val |= BIT(j);
>>> + }
>>> + ((u8 *)dstbuf)[i] = val;
>>> + }
>>> +}
>>> +
>>> +/*
>>> * Check if the NAND chip is ONFI compliant, returns 1 if it is, 0 otherwise.
>>> */
>>> static int nand_flash_detect_onfi(struct nand_chip *chip)
>>> @@ -5102,7 +5131,7 @@ static int nand_flash_detect_onfi(struct nand_chip *chip)
>>> return 0;
>>>
>>> /* ONFI chip: allocate a buffer to hold its parameter page */
>>> - p = kzalloc(sizeof(*p), GFP_KERNEL);
>>> + p = kzalloc((sizeof(*p) * 3), GFP_KERNEL);
>>> if (!p)
>>> return -ENOMEM;
>>>
>>> @@ -5113,21 +5142,32 @@ static int nand_flash_detect_onfi(struct nand_chip *chip)
>>> }
>>>
>>> for (i = 0; i < 3; i++) {
>>> - ret = nand_read_data_op(chip, p, sizeof(*p), true);
>>> + ret = nand_read_data_op(chip, &p[i], sizeof(*p), true);
>>> if (ret) {
>>> ret = 0;
>>> goto free_onfi_param_page;
>>> }
>>>
>>> - if (onfi_crc16(ONFI_CRC_BASE, (uint8_t *)p, 254) ==
>>> + if (onfi_crc16(ONFI_CRC_BASE, (u8 *)&p[i], 254) ==
>>> le16_to_cpu(p->crc)) {
>>> + if (i)
>>> + memcpy(p, &p[i], sizeof(*p));
>>> break;
>>> }
>>> }
>>>
>>> if (i == 3) {
>>> - pr_err("Could not find valid ONFI parameter page; aborting\n");
>>> - goto free_onfi_param_page;
>>> + const void *srcbufs[3] = {p, p + 1, p + 2};
>>> +
>>> + pr_warn("Could not find a valid ONFI parameter page, trying bit-wise majority to recover it\n");
>>> + nand_bit_wise_majority(srcbufs, ARRAY_SIZE(srcbufs), p,
>>> + sizeof(*p));
>>> +
>>> + if (onfi_crc16(ONFI_CRC_BASE, (u8 *)p, 254) !=
>>> + le16_to_cpu(p->crc)) {
>>> + pr_err("ONFI parameter recovery failed, aborting\n");
>>> + goto free_onfi_param_page;
>>> + }
>>> }
>>>
>>> /* Check version */
>> This version is still hard coded for a three sample bitwise majority vote.
>> So why not use the method which I suggested previously for v2 and which
>> I repeat below?
> Because I want the nand_bit_wise_majority() function to work with
> nsrcbufs > 3 (the ONFI spec says there's at least 3 copy of the param
> page, but NAND vendor can decide to put more). Also, if the X copies of
> the PARAM are corrupted (which is rather unlikely), that means we
> already spent quite a lot of time reading the different copies and
> calculating the CRC, so I think we don't care about perf optimizations
> when doing bit-wise majority.
>
>> The three sample bitwise majority can be implemented without bit level
>> manipulation using the identity:
>> majority3(a, b, c) = (a & b) | (a & c) | (b & c)
>> This can be factorized slightly to (a & (b | c)) | (b & c)
>> This enables the operation to be performed 8, 16, 32 or even 64 bits at
>> a time depending on the hardware.
>>
>> This method is not only faster and but also more compact.
>>

I do understand that the ONFI specifications permit more than 3 copies.
However elsewhere the proposed code shows no intention of handling other
cases.
The constant 3 is hard coded in the following lines extracted from the
proposed code:
...
+    p = kzalloc((sizeof(*p) * 3), GFP_KERNEL);
...
     for (i = 0; i < 3; i++) {
...
     if (i == 3) {
...
+        const void *srcbufs[3] = {p, p + 1, p + 2};

Moreover the last of these is difficult to generalize.

Cheers,
Chris

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