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

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Message-ID: <e41bb2d9ad144736b858b186acf2a47b@AcuMS.aculab.com>
Date:   Sun, 4 Oct 2020 21:36:20 +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: 04 October 2020 22:12
> 
> On 10/2/20 9:50 AM, David Laight wrote:
> > From: Bert Vermeulen
> >> 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?
> 
> I'm working with Realtek RTL838x/RTL839x SoCs. Reading it out is a
> pretty convoluted procedure involving different I/O registers depending
> on the SoC model.

How do they manage to let you do read/write without 'read control'.
Actually I can imagine...

The problem we had was getting the IO pins to link up to user
logic on an Altera Cyclone-V fpga.
Then it was just a 'SMOP' to get reads and write converted to
nibble 'bit-bang' with the chipselect and output enable (IIRC)
controlled by the register address.
I doubt any 'standard' interface is as efficient.

I think I found a hardware bug in the chips we are using.
There seemed to be a timing window in which the 'read status'
command (after a write/erase) was completely ignored by
the device.
Everything looked write on a scope - but the data line
wasn't driven.

	David

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