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

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Message-ID: <cb612906-3d52-66cd-fbbc-7872bd2d9a92@gmail.com>
Date:   Thu, 10 May 2018 12:21:52 +0200
From:   Marek Vasut <marek.vasut@...il.com>
To:     NeilBrown <neil@...wn.name>,
        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 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 ?

-- 
Best regards,
Marek Vasut