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

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Message-ID: <9bc530d1fdaf4490a00fee150f963ac7@AcuMS.aculab.com>
Date:   Wed, 16 Mar 2022 13:55:17 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Vignesh Raghavendra' <vigneshr@...com>,
        Michael Walle <michael@...le.cc>
CC:     Tudor Ambarus <tudor.ambarus@...rochip.com>,
        "p.yadav@...com" <p.yadav@...com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "miquel.raynal@...tlin.com" <miquel.raynal@...tlin.com>,
        "richard@....at" <richard@....at>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "nicolas.ferre@...rochip.com" <nicolas.ferre@...rochip.com>
Subject: RE: [PATCH v2 0/6] spi-mem: Allow specifying the byte order in DTR
 mode

Thought...

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.

	David

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