linux-kernel - Re: [PATCH 1/3] eeprom: at25: Add DT support for EEPROMs with odd address bits

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Message-ID: <20171204211705.543xjzvttbrt65pm@rob-hp-laptop>
Date:   Mon, 4 Dec 2017 15:17:05 -0600
From:   Rob Herring <robh@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Ivo Sieben <meltedpianoman@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Chris Wright <chrisw@...s-sol.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] eeprom: at25: Add DT support for EEPROMs with odd
 address bits

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@...der.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@...der.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.

> AT25M01 uses 3-byte addressing (it needs 17 bits).

Do you need to know it is 17-bit vs. 24-bits? I'm guessing not as the 
unused bits are probably don't care.

Rob