linux-kernel - Re: [PATCH/RFC] mtd: spi-nor: honour max_message

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Message-ID: <87mux7afkv.fsf@notabene.neil.brown.name>
Date:   Thu, 10 May 2018 21:57:20 +1000
From:   NeilBrown <neil@...wn.name>
To:     Marek Vasut <marek.vasut@...il.com>,
        Boris Brezillon <boris.brezillon@...tlin.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Richard Weinberger <richard@....at>,
        linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC] mtd: spi-nor: honour max_message_size for spi-nor writes.

On Thu, May 10 2018, Marek Vasut wrote:

> On 05/10/2018 12:28 AM, NeilBrown wrote:
>> On Wed, May 09 2018, Boris Brezillon wrote:
>> 
>>> On Fri, 27 Apr 2018 16:18:05 +1000
>>> NeilBrown <neil@...wn.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.
>
> Nice, which hardware is that ?

Mediatek MT7621 SOC (particularly in the gnubee.org NAS platform).

Thanks,
NeilBrown

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