[PATCH v2] mtd: cfi_cmdset_0002: cap the write-buffer chunk at 256 bytes on an x8 device

[Orgad Shaneh]

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
regardless of the buffer size it advertises.

A Micron M29EW (an x16 part wired in x8 mode) advertises a 512-byte
write buffer. cfi_amdstd_write_buffers() used the full 512, CMD(511)
truncated the count to 0xff on the way out, and the chip aborted the
program on the 257th data byte. Every full-size chunk failed while the
partial ones at the start of a write went through:

  MTD do_write_buffer_wait(): software timeout, address:0x03278bff.
  jffs2: Write of 4164 bytes at 0x02c789b4 failed. returned -5, retlen 68
  jffs2: No space for garbage collection. Aborting GC thread

Clamp MaxBufWriteSize in the write-buffer fixup for x8 devices, and keep
mtd->writebufsize (computed before the fixups run) consistent with it.

The bug predates the git history, so there is no Fixes: tag.

Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Claude:claude-opus-5
Signed-off-by: Orgad Shaneh <orgads@xxxxxxxxx>
---

v2: test the device width (cfi->device_type) instead of the aggregate bus
    width. map_bankwidth_is_1() is false for x8 devices interleaved on a
    wider bus, which have the same 256-byte ceiling: do_write_buffer()
    sends CMD(words - 1), and CMD() replicates that count into each
    device's lane, where it is truncated to 8 bits - so two x8 chips with
    a 512-byte buffer are told 255 words and are still sent 512 bytes
    each. device_type is also immune to map_bankwidth_is_1() compiling to
    a constant 0 when CONFIG_MTD_MAP_BANK_WIDTH_1 is off. Caught by the
    Sashiko review bot.

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
@@ -283,6 +283,21 @@ static void fixup_use_write_buffers(struct mtd_info *mtd)
 		pr_debug("Using buffer write method\n");
 		mtd->_write = cfi_amdstd_write_buffers;
 	}
+
+	/*
+	 * 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.
+	 */
+	if (cfi->device_type == CFI_DEVICETYPE_X8 &&
+	    cfi->cfiq->MaxBufWriteSize > 8) {
+		cfi->cfiq->MaxBufWriteSize = 8;
+		mtd->writebufsize = cfi_interleave(cfi) <<
+				    cfi->cfiq->MaxBufWriteSize;
+	}
 }
 #endif /* !FORCE_WORD_WRITE */
 
-- 
2.47.0