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
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