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

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Message-ID: <CAL_Jsq+-w=bbSN1LvRy7G9Dtc=7-C+CiyQnrtZkMLK23S98P9Q@mail.gmail.com>
Date:   Tue, 5 Dec 2017 07:56:29 -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 Tue, Dec 5, 2017 at 3:09 AM, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> Hi Rob,
>
> On Tue, Dec 5, 2017 at 9:57 AM, Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>> On Mon, Dec 4, 2017 at 10:17 PM, Rob Herring <robh@...nel.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@...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.
>>
>> 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".

I wasn't suggesting throwing out all these. Just add a compatible for
the one oddball 9-bit part.

But I'm fine adding address-width=9 too.

Rob