Re: [PATCH v2 0/6] spi-mem: Allow specifying the byte order in DTR mode

From: Michael Walle <michael@walle.cc>
To: David Laight <David.Laight@aculab.com>
Cc: 'Vignesh Raghavendra' <vigneshr@ti.com>,
	Tudor Ambarus <tudor.ambarus@microchip.com>,
	p.yadav@ti.com, broonie@kernel.org, miquel.raynal@bootlin.com,
	richard@nod.at, linux-mtd@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-spi@vger.kernel.org,
	nicolas.ferre@microchip.com
Subject: Re: [PATCH v2 0/6] spi-mem: Allow specifying the byte order in DTR mode
Date: Thu, 17 Mar 2022 10:40:09 +0100	[thread overview]
Message-ID: <bc2083a8903fdabc65083f7e5232ca2d@walle.cc> (raw)
In-Reply-To: <9bc530d1fdaf4490a00fee150f963ac7@AcuMS.aculab.com>

Hi,

Am 2022-03-16 14:55, schrieb David Laight:
> Thought...

Thank you for your proposal.

> Can you read the device in STR mode until you get a suitable
> non-palindromic value, then read it in DTR mode and dynamically
> determine the byte order?
> 
> Clearly this won't work if the device is erased to all 0xff.
> But a check could be done on/after the first write.
> 
> I suspect write times are actually dominated by the time spent
> waiting for the write to complete?
> (Never mind the earlier block erase time.)
> So always writing in STR mode probably makes little difference?
> Writes really ought to be uncommon as well.
> 
> Speeding up reads is a different matter - and probably useful.
> 
> Of course, if you've got hardware reading the spi memory in DTR
> mode for config data you might need to byteswap it (compared
> to the STR writes) - but that is probably a 2nd order problem.
> 
> I've got some bespoke logic on an PCIe fpga for accessing spi memory.
> Uses address bits for the control signals and converts a 32bit
> read/write into 8 nibble transfers to the chip.
> (uses byte enables - don't an odd number of clocks.)
> mmapp()ed to userspace for updating the 6MB fpga image.

Our problem is not how to detect that we have to swap it, but
rather what we do when we have to do it.

If we have a controller which can swap the bytes for us on the
fly, we are lucky and can enable swapping if we need it. We are
also lucky when we don't have to swap the flash contents, obviously.

But what do we do when we need to swap it and the controller
doesn't support it. We could do it in software which will slow
things down. So depending on the use case this might or might not
work. We can degrade it to a speed which doesn't have this issue;
which might be 1-1-1 in the worst case. We could also do just
nothing special; but this will lead to inconsistencies between
reading in 1-1-1 and 8d-8d-8d.

-michael