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From: Geert Uytterhoeven <geert@linux-m68k.org>
Date: Tue, 5 Dec 2017 10:09:11 +0100
Message-ID: <CAMuHMdWFzUbZ-wcyZmLEXcnFQ+RJBnkrxW+B0ie9cR=1LPtsgw@mail.gmail.com>
Subject: Re: [PATCH 1/3] eeprom: at25: Add DT support for EEPROMs with odd
 address bits
To: Rob Herring <robh@kernel.org>
Cc: Ivo Sieben <meltedpianoman@gmail.com>, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Chris Wright <chrisw@sous-sol.org>, Wolfram Sang <wsa@the-dreams.de>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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Hi Rob,

On Tue, Dec 5, 2017 at 9:57 AM, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> On Mon, Dec 4, 2017 at 10:17 PM, Rob Herring <robh@kernel.org> wrote:
>> On Mon, Dec 04, 2017 at 10:17:47AM +0100, Geert Uytterhoeven wrote:
>>> On Thu, Nov 30, 2017 at 2:29 PM, Geert Uytterhoeven
>>> <geert+renesas@glider.be> wrote:
>>> > Certain EEPROMS have a size that is larger than the number of address
>>> > bytes would allow, and store the MSB of the address in bit 3 of the
>>> > instruction byte.
>>> >
>>> > This can be described in platform data using EE_INSTR_BIT3_IS_ADDR, or
>>> > in DT using the obsolete legacy "at25,addr-mode" property.
>>> > But currently there exists no non-deprecated way to describe this in DT.
>>> >
>>> > Hence extend the existing "address-width" DT property to allow
>>> > specifying 9, 17, or 25 address bits, and enable support for that in the
>>> > driver.
>>> >
>>> > Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
>>> > ---
>>> > EEPROMs using 9 address bits are common (e.g. M95040, 25AA040/25LC040).
>>> > Do EEPROMs using 17 or 25 address bits, as mentioned in
>>> > include/linux/spi/eeprom.h, really exist?
>>> > Or should we just limit it to a single odd value (9 bits)?
>>>
>>> At least for the real Atmel parts, only the AT25040 part uses odd (8 +
>>> 1 bit) addressing.
>>
>> Seems like we should have a specific compatible for it.
>
> Possibly. But currently all configuration is done through DT properties, not
> through matching on compatible values.

Adding compatible values for all known/used parts could quickly become a
large table.
E.g. Atmel/Microchip has 3 variants of 512-byte EEPROMs: AT25040B,
25LC040A, and 25AA040A. The former uses an 8-byte pagesize, while the
latter parts use 16-byte pagesizes.
Not to mention "compatible" parts from other manufacturers, and all other
supported size.

Currently all of this is configured through the "pagesize", "size", and
"address-width" DT properties, with matching on generic "atmel,at25".

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds