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

[NeilBrown]

On Wed, May 09 2018, Boris Brezillon wrote:

> On Fri, 27 Apr 2018 16:18:05 +1000
> NeilBrown <neil at brown.name> wrote:
>
>> 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?
>
> Marek, any opinion on this patch?
>

Hi,
 thanks for following up.
 I have since found that I don't need this patch, though maybe others
 still do(??).
 My hardware can only send 36 bytes and receive 32 in a single
 transaction.  However I can run a sequence of transactions
 to process a whole message no matter how large that message is.  As
 long as I keep chip-select asserted, all the slave device sees is that
 the clock period isn't quite constant, and the slave shouldn't care
 much about that.
 When reading from flash, I found that handling large messages with
 multiple hardware transactions was 50% faster than breaking the
 read down into lots of 32 byte messages.

 So, I won't object if this patch is forgotten.  Thanks for
 your time anyway.

NeilBrown

>> 
>> 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:
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