[PATCH v4] mtd: nand: sunxi: fix OOB handling in ->write_xxx() functions

On Monday 14 September 2015 11:41:13 Boris Brezillon wrote:

Hi Arnd,

On Mon, 14 Sep 2015 10:59:02 +0200
Arnd Bergmann [off-list ref] wrote:

quoted

On Monday 14 September 2015 10:41:03 Boris Brezillon wrote:

quoted

                /* Fill OOB data in */
-               if (oob_required) {
-                       tmp = 0xffffffff;
-                       memcpy_toio(nfc->regs + NFC_REG_USER_DATA_BASE, &tmp,
-                                   4);
-               } else {
-                       memcpy_toio(nfc->regs + NFC_REG_USER_DATA_BASE,
-                                   chip->oob_poi + offset - mtd->writesize,
-                                   4);
-               }
+               writel(NFC_BUF_TO_USER_DATA(chip->oob_poi +
+                                           layout->oobfree[i].offset),
+                      nfc->regs + NFC_REG_USER_DATA_BASE);

This looks like you are changing the endianess of the data that gets written.
Is that intentional?

Hm, the real goal of this patch was to avoid accessing the
NFC_REG_USER_DATA_BASE register using byte accessors (writeb()).
The first version of this series was directly copying data from the
buffer into a temporary u32 variable, thus forcing the data to be stored
in little endian (tell me if I'm wrong), and then changing endianness
using le32_to_cpu().
Brian suggested to use __raw_writel() (as you seem to suggest too), but
I was worried about the missing mem barrier in this function.
That's why I made my own macro doing the little endian to CPU conversion
manually, but still using the standard writel() accessor (which will
do the conversion in reverse order).

D'oh, I totally missed your open-coded le32_to_cpu macro.
So your code does look correct to me, it's just a little inefficient
on big-endian machines because you end up swapping twice.

Maybe I should use __raw_writel() and add an explicit memory barrier.

That would work, and avoid the double swapping, yes. Or you could
use writesl() with a length of one, which should have all the
necessary barriers but no byteswap.

I don't think you need a barrier on ARM here (no DMA that can interfere),
but it's better write the code architecture independent (as you did
above).

The memcpy_toio() is definitely overkill as it has a barrier after every
single byte, where you need at most one for this kind of driver.

quoted

memcpy_toio() uses the same endianess for source and destination, while writel()
assumes that the destination is a little-endian register, and that could break
if the kernel is built to run as big-endian. I also see that sunxi_nfc_write_buf()
uses memcpy_toio() for writing the actual data, and you are not changing that.

AFAIU the peripheral is always in little endian, and only the CPU can
be switched to big endian, right?

Correct.

Are you saying that memcpy_toio() uses writel? Because according to
this implementation [2] it uses writeb, which should be safe (accessing
the internal SRAM using byte accessors is authorized).

No, what I meant is that you are replacing some memcpy_toio() with
writel(), but don't replace some of the others that should/could also
be replaced.

As mentioned, I was under the impression that you changed the endianess
for the OOB data but did not change the endianess for the user data,
which would be inconsistent.

quoted

If all hardware can do 32-bit accesses here and the size is guaranteed to be a
multiple of four bytes, you can probably improve performance by using a
__raw_writel() loop there. Using __raw_writel() in general is almost always
a bug, but here it actually makes sense. See also the powerpc implementation
of _memcpy_toio().

AFAICT, buffer passed to ->write_bu() are not necessarily aligned on
32bits, so using writel here might require copying data in temporary
buffers :-/.

Don't hesitate to point where I'm wrong ;-).

Brian or Dwmw2 should be able to know for sure. I think it's definitely
worth trying as the potential performance gains could be huge, if you
replace

	for (p = start; p < start + length; data++, p++) {
		writeb(*data, p);
		wmb();
	}

with

	for (p = start; p < start + length; data++, p+=4) {
		writel(*data, p);
	};
	wmb();

	Arnd