linux-kernel - Re: [PATCH v2 2/4] i2c: sunxi: Add Reduced Serial Bus (RSB) DT bindings documentation

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Date:	Wed, 4 Mar 2015 17:39:35 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Chen-Yu Tsai <wens@...e.org>
Cc:	Wolfram Sang <wsa@...-dreams.de>, Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, linux-i2c@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-sunxi@...glegroups.com,
	Hans de Goede <hdegoede@...hat.com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2 2/4] i2c: sunxi: Add Reduced Serial Bus (RSB) DT
 bindings documentation

On Mon, Mar 02, 2015 at 04:24:44PM +0800, Chen-Yu Tsai wrote:
> Reduced Serial Bus (RSB) is an SMBus like bus used to communicate
> with some PMICs (like the AXP223) or other peripherals.
> 
> The RSB DT bindings are pretty much the same as the one defined for
> the marvell's mv64xxx controller, with the additional RSB specific
> "allwinner,rsb-hw-addr" property for slave device nodes.
> 
> There are 2 types of addresses for RSB devices, a hardware address
> and a runtime (software) configurable address. The former is only
> used when configuring the latter. All read/write accesses use the
> runtime address.
> 
> It would seem straightforward to use the hardware address in the
> DT bindings as the slave's address. However this will not work as
> the hardware address is 12 bits wide, and at least 1 device, the
> AC100 audio codec, has the highest bit set. This address would be
> incompatible with I2C (7 or 10 bit addresses) and likely rejected.
> 
> Hence this binding uses statically allocated (by the author of the
> DT) runtime addresses for the slave's "reg" property. The hardware
> address is put in a separete named property. When writing a new DT,
                      ^ separate
> the author must take care to not have multiple slave devices use
> the same address. It is recommended to follow whatever conventions
> the hardware vendor uses.

While very complete, the three last paragraphs should rather be, or at
least duplicated, in the file itself.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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