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