linux-kernel - RE: [PATCH] mtd: spi-nor: Fix 3-or-4 address byte mode logic

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Date:   Fri, 2 Oct 2020 07:50:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Bert Vermeulen' <bert@...t.com>, Pratyush Yadav <p.yadav@...com>
CC:     "tudor.ambarus@...rochip.com" <tudor.ambarus@...rochip.com>,
        "miquel.raynal@...tlin.com" <miquel.raynal@...tlin.com>,
        "richard@....at" <richard@....at>,
        "vigneshr@...com" <vigneshr@...com>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] mtd: spi-nor: Fix 3-or-4 address byte mode logic

From: Bert Vermeulen
> Sent: 01 October 2020 23:23
> 
> On 10/1/20 8:34 AM, Pratyush Yadav wrote:
> > So using an address width of 4 here is not necessarily the right thing
> > to do. This change would break SMPT parsing for all flashes that use
> > 3-byte addressing by default because SMPT parsing can involve register
> > reads/writes. One such device is the Cypress S28HS flash. In fact, this
> > was what prompted me to write the patch [0].
> >
> > Before that patch, how did MX25L25635F decide to use 4-byte addressing?
> 
> The SoCs I'm dealing with have an SPI_ADDR_SEL pin, indicating whether it
> should be in 3 or 4-byte mode. The vendor's hacked-up U-Boot sets the mode
> accordingly, as does their BSP. It seems to me like a misfeature, and I want
> to just ignore it and do reasonable JEDEC things, but I have the problem
> that the flash chip can be in 4-byte mode by the time it gets to my spi-nor
> driver.

If these are the devices I think they are, can't you read the
non-volatile config word (bit 0) to find out whether the device
expects a 3 or 4 byte address and how many 'idle' clocks there
are before the read data?

A device that requires 3 bytes of address can be set to a read
delay of 12 cycles (rather than the usual 10) so that 'hardware'
reads (typically from address 0) can transparently support
devices that require 3 or 4 bytes addresses.

	David

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