[PATCH/RFC] mtd: spi-nor: honour max_message_size for spi-nor writes.

[NeilBrown]

Hi,
 I've labeled this an RFC because I'm really not sure about removing the
 error path from spi_nor_write() -- maybe that really matters.  But on
 my hardware, performing multiple small spi writes to the flash seems
 to work.

 The spi driver is drivers/staging/mt7621-spi.  Possibly this needs to
 use DMA instead of a FIFO (assuming the hardware can) - or maybe
 drivers/spi/spi-mt65xx.c can be made to work on this hardware, though
 that is for an ARM SOC and mt7621 is a MIPS SOC.

 I note that openwrt has similar patches:
  target/linux/generic/pending-4.14/450-mtd-spi-nor-allow-NOR-driver-to-write-fewer-bytes-th.patch

 They also change the spi driver to do a short write, rather
 than change m25p80 to request a short write.

 Is there something horribly wrong with this?

Thanks,
NeilBrown

-----------------------8<------------------------

m25p80 honours max_message_size and max_transfer_size
for reads, but not for writes.
I have a driver that has a max message size of 36 bytes
(command, address, 32 bytes data, all places in a FIFO
in the controller).
This requires m25p80_write() to honour the size limits.
For that to work, spi-nor needs to quietly accept partial
writes.

With this, I can successfully re-flash my device.

Signed-off-by: NeilBrown <neil at brown.name>
---
 drivers/mtd/devices/m25p80.c  | 3 ++-
 drivers/mtd/spi-nor/spi-nor.c | 7 -------
 2 files changed, 2 insertions(+), 8 deletions(-)

diff --git a/drivers/mtd/devices/m25p80.c b/drivers/mtd/devices/m25p80.c
index a4e18f6aaa33..7ded13507604 100644
--- a/drivers/mtd/devices/m25p80.c
+++ b/drivers/mtd/devices/m25p80.c
@@ -117,7 +117,8 @@ static ssize_t m25p80_write(struct spi_nor *nor, loff_t to, size_t len,
 
 	t[data_idx].tx_buf = buf;
 	t[data_idx].tx_nbits = data_nbits;
-	t[data_idx].len = len;
+	t[data_idx].len = min3(len, spi_max_transfer_size(spi),
+			       spi_max_message_size(spi) - cmd_sz);
 	spi_message_add_tail(&t[data_idx], &m);
 
 	ret = spi_sync(spi, &m);
diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
index 42ae9a1529bb..cfa15f2801ad 100644
--- a/drivers/mtd/spi-nor/spi-nor.c
+++ b/drivers/mtd/spi-nor/spi-nor.c
@@ -1445,13 +1445,6 @@ static int spi_nor_write(struct mtd_info *mtd, loff_t to, size_t len,
 			goto write_err;
 		*retlen += written;
 		i += written;
-		if (written != page_remain) {
-			dev_err(nor->dev,
-				"While writing %zu bytes written %zd bytes\n",
-				page_remain, written);
-			ret = -EIO;
-			goto write_err;
-		}
 	}
 
 write_err:
-- 
2.14.0.rc0.dirty

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