nand_read_subpage vs. S3C244x NAND: non-word reads

In recent kernels, the optimized OOB reads in nand_read_subpage often make requests which aren't an exact multiple of a word, which caused nand/s3c2410.c to do a partial read, yielding chaos and mayhem.

I'm not entirely sure whether nand_read_subpage is to blame for making non-word requests, or whether nand/s3c2410.c is to blame for not supporting them, but here's a patch for the latter.

Note that s3c2410_nand_write_buf has a similar issue. RNDIN is used in nand_write_oob_syndrome, so this may also be a real problems.

- Werner

---------------------------------- cut here -----------------------------------

fix-s3c-nand-read-bytes.patch

With the introduction of optimized OOB reads in nand_read_subpage, the length of the data requested may not be a multiple of four bytes.

This caused a partial read on the 2440, leading to false ECC errors and, worse, attempts to "correct" them.

Note that there is a similar issue in s3c2440_nand_write_buf, which doesn't seem to cause trouble yet.

Signed-off-by Werner Almesberger <[email protected]>

---

Index: ktrack/drivers/mtd/nand/s3c2410.c
===================================================================

drivers/mtd/nand/s3c2410.c +0 -0

@@ -530,7 +530,14 @@
 static void s3c2440_nand_read_buf(struct mtd_info *mtd, u_char *buf, int len)

drivers/mtd/nand/s3c2410.c +0 -0

 }
 
 static void s3c2410_nand_write_buf(struct mtd_info *mtd, const u_char *buf, int len)

______________________________________________________
Linux MTD discussion mailing list
http://lists.infradead.org/mailman/listinfo/linux-mtd/

As noted on the openmoko list,

I think we can do 256byte subpage reads
as long as they are aligned to 256bytes. We could make the ECC code deal with non-256 byte power-of-two aligned blocks without huge changes but my belief is that we cannot support anything that isn't a power of two.

I think the best thing to do is to either force the caller to read a power of two (pref. >4 bytes), so either we need some form of flag to say this, or change the behaviour of the callers to never try this.

Quoted 1 line · Ben Dooks

As noted on the openmoko list,

Sorry for starting two threads on the same topic. The joy of trying to do the right thing with lists that don't let you cross-post unless you're subscribed ...

Quoted 5 lines

I think we can do 256byte subpage reads
as long as they are aligned to 256bytes. We could make the ECC code deal with non-256 byte power-of-two aligned blocks without huge changes but my belief is that we cannot support anything that isn't a power of two.

In this case, the problem is a bit more subtle: the data blocks retrieved are perfectly normal, i.e., 256 bytes in size and properly aligned.

However, nand_read_subpage optimizes retrieval of the OOB data. So instead of retrieving, say, 64 bytes, it only retrieves 24 (for a 2048 bytes page). Sometimes, not the entire page is retrieved, and then we get those accesses with an odd size.

Quoted 3 lines

I think the best thing to do is to either force the caller to read a power of two (pref. >4 bytes), so either we need some form of flag to say this, or change the behaviour of the callers to never try this.

If it's considered generally objectionable to call read_buf for an amount of data that isn't a whole number of words, those two approaches would work as well. Making nand_read_subpage align to 4 bytes instead of 1/2 would be fairly simple. A flag would be a bit messier.