[PATCH] mtd: cfi_cmdset_0002: also clamp the write buffer of an x16 part strapped to x8

[Orgad Shaneh <orgads@gmail.com>]

Commit cfc5ebc9540e ("mtd: cfi_cmdset_0002: cap the write-buffer chunk
at 256 bytes on an x8 device") limits a Write to Buffer to 256 bytes
when cfi->device_type is CFI_DEVICETYPE_X8, because do_write_buffer()
sends the word count as a single bus word and an 8-bit data lane
truncates it.

The M29EW that commit was written for is an x16 part strapped to x8.
The probe reports it as

  phys_mapped_flash: Found 1 x16 devices at 0x0 in 8-bit bank.
  Manufacturer ID 0x000089 Chip ID 0x00007e

so its device_type is CFI_DEVICETYPE_X16, the clamp does not apply, and
every full 512-byte chunk still times out:

  MTD do_write_buffer_wait(): software timeout, address:0x002203ff.

What limits the count is the width of each device's data lane, not the
part's type. Test that instead: map_bankwidth(map) / cfi_interleave(cfi)
is 1 for an x8 part, for x8 parts interleaved on a wider bus, and for an
x16 part in an 8-bit bank.

On the board (Octeon CN6335, the same M29EW) writes of 1, 2, 32, 64 and
256 bytes to an erased partition read back correctly and 512, 1024,
2048 and 4096 failed with the timeout above; with this patch every size
up to 64 KiB reads back correctly and the u-boot environment can be
saved from Linux again.

Fixes: cfc5ebc9540e ("mtd: cfi_cmdset_0002: cap the write-buffer chunk at 256 bytes on an x8 device")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Orgad Shaneh <orgads@gmail.com>
---
diff --git a/drivers/mtd/chips/cfi_cmdset_0002.c b/drivers/mtd/chips/cfi_cmdset_0002.c
--- a/drivers/mtd/chips/cfi_cmdset_0002.c
+++ b/drivers/mtd/chips/cfi_cmdset_0002.c
@@ -286,13 +286,14 @@ static void fixup_use_write_buffers(struct mtd_info *mtd)
 
 	/*
 	 * The word count of the Write to Buffer command is a single bus
-	 * word per device, so an x8 device can be told to program at most
-	 * 256 bytes however large a buffer it advertises - including when
-	 * several of them are interleaved on a wider bus, where CMD()
-	 * replicates the count into each device's lane and it is truncated
-	 * there.
+	 * word per device, so a device on an 8-bit data lane can be told
+	 * to program at most 256 bytes however large a buffer it
+	 * advertises: an x8 part, x8 parts interleaved on a wider bus
+	 * (CMD() replicates the count into each device's lane and it is
+	 * truncated there), and an x16 part strapped to x8, which the
+	 * probe reports as an x16 device in an 8-bit bank.
 	 */
-	if (cfi->device_type == CFI_DEVICETYPE_X8 &&
+	if (map_bankwidth(map) / cfi_interleave(cfi) == 1 &&
 	    cfi->cfiq->MaxBufWriteSize > 8) {
 		cfi->cfiq->MaxBufWriteSize = 8;
 		mtd->writebufsize = cfi_interleave(cfi) <<
-- 
2.47.0