linux-kernel - Re: [PATCHv2 04/10] fbdev: ssd1307fb: Unify init code and obtain hw specific bits from DT

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Message-ID: <20150305214958.GB4522@lukather>
Date:	Thu, 5 Mar 2015 22:49:58 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Thomas Niederprüm <niederp@...sik.uni-kl.de>
Cc:	plagnioj@...osoft.com, tomi.valkeinen@...com,
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 04/10] fbdev: ssd1307fb: Unify init code and obtain hw
 specific bits from DT

On Sun, Mar 01, 2015 at 11:27:57PM +0100, Thomas Niederprüm wrote:
> The SSD130X controllers are very similar from the configuration point of view.
> The configuration registers for the SSD1305/6/7 are bit identical (except the
> the VHCOM register and the the default values for clock setup register). This
> patch unifies the init code of the controller and adds hardware specific
> properties to DT that are needed to correctly initialize the device.
> 
> The SSD130X can be wired to the OLED panel in various ways. Even for the
> same controller this wiring can differ from one display module to another
> and can not be probed by software. The added DT properties reflect these
> hardware decisions of the display module manufacturer.
> The 'com-sequential', 'com-lrremap' and 'com-invdir' values define different
> possibilities for the COM signals pin configuration and readout direction
> of the video memory. The 'segment-remap' allows the inversion of the memory-
> to-pin mapping ultimately inverting the order of the controllers output pins.
> The 'prechargepX' values need to be adapted according the capacitance of the
> OLEDs pixel cells.
> 
> So far these hardware specific bits are hard coded in the init code, making
> the driver usable only for one certain wiring of the controller. This patch
> makes the driver usable with all possible hardware setups, given a valid hw
> description in DT. If the values are not set in DT the default values
> according to the controllers datasheet are assumed.

Unfortunately, this is not a reasonable thing to do, even if you fix
the existing user, there's still the case where you have an older DT
with a newer kernel.

Keeping (and documenting) the previous defaults is the only easy way
to support this.

Maxime

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

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