Hello All Over the last while I have been working on getting ubifs working on omap3530 using 16-bit NAND with the latest 2.6.37 prefetch code. This is basically a tweaked Overo kernel. Here is what I found: After initial exploration I found that there were three problem: * ECC bytes not being read correctly during sub-page reads. * Disabling prefetch meant that the flash was not being read at all. * NAND access seems to have slowed down significantly. Both these boiled down to one root cause. After the prefetch changes, nand->IO_ADDR_R is set to an address that only works within the context of the prefetch code. It no longer works if prefetch is disabled. Execution of: static void omap_read_buf16(struct mtd_info *mtd, u_char *buf, int len) { struct nand_chip *nand = mtd->priv; ioread16_rep(nand->IO_ADDR_R, buf, len / 2); } Even if prefetch is enabled, subpage reads that are not 32-bit aligned call the above function which means the ECC does not read correctly - resulting in ECC errors. I managed to work around this by applying the following patch to force all buffer reads to u32 alignment: --- a/drivers/mtd/nand/omap2.c +++ b/drivers/mtd/nand/omap2.c @@ -245,6 +245,18 @@ static void omap_read_buf_pref(struct mtd_info *mtd, u_char *buf, int len) int ret = 0; u32 *p = (u32 *)buf; + /* u32 align the buffer and read */ + /* NB: This assumes the buf ptr can be aligned *down* which is a valid. + * Assumption when dealing with ecc buffers etc. + */ + u32 addr = (u32)p; + + int diff = addr & 3; + addr -= diff; + len += diff; + len = (len + 3) & ~3; + p = (u32 *)addr; + /* take care of subpage reads */ if (len % 4) { if (info->nand.options & NAND_BUSWIDTH_16) Yeah I know that is ugly, but it works! Prefetch is enabled, dma is disabled. ECC is done in software. OK, that gives me a working UBIFS, but I found something else... NAND access has got way, way slower since 2.6.34. I created a 2MB file foo then do the following: sync; date; cp foo bar; sync; date In 2.6.34 the time between the two dates was 3 seconds. In 2.6.37 this now takes 14 seconds! Now question time: 1) How well has the prefetch code been tested? 2) Does it work with prefetch disabled? 3) Has it been performance tested? 4) What should the value in nand->IO_ADDR_R be? Thanks Charles -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html