Re: device tree for arm soc ?

[Matt Thomas <matt%3am-software.com@localhost>]

On Sep 21, 2013, at 12:05 PM, Manuel Bouyer <bouyer%antioche.eu.org@localhost> 
wrote:

> Hello,
> with arm SoCs (and this probably applies to other SoCs as well), we have to
> setup I/O pins. The difficulty being that a I/O pin can be multiplexed
> to several different peripherals. Additionnally a peripheral function
> can be routed to different pins. For example, on a AM3359,
> pin E18 can be configured to be either eCAP0_in_PWM0_out, uart3_txd,
> spi1_cs1 pr1_ecap0_ecap_capin_apwm_0, spi1_sclk, mmc0_sdwp,
> xdma_event_intr2 or gpio0_7.
> On the other hand, uart3_txd can be configured to be routed to E18,
> N16 or G18. So enabling a peripheral is not enough to get it working,
> we have to choose to which I/O pins its signal will be routed. And this is
> at minimum board-dependant (but in the general case it's application
> dependant: where you want uart3_txd depends on how you wired the cape
> you plug on top of your beaglebone :)
> 
> Right now we don't have a good way to deal with this; sitara_cm.c and
> am335x_cm_padconf.c is a start for a driver to request a pin when it needs 
> one,
> but the pins are hardcoded in the driver at this time (look at ti_iic.c).
> I can't think of a good way to specify this in a kernel config file;
> in addition we probably want something more dynamic. Providing the
> information from userland after boot is probably not good enough, because you
> want to configure some peripheral and associated I/O pins before mounting root
> (for the mass storage or network controller, or the console).
> 
> Linux solved this by providing a device tree at boot time, which describe
> the hardware including the device to I/O ping associations.
> The device tree file is loaded at the same time as the kernel by the
> boot loader. FreeBSD has also gone this route. Of course they're not
> using the same DT ...
> 
> Before I start looking deeper at this, does anyone has comments,
> objections, etc ... ?

fdt is a pain and isn't always available.

Look at the way I did this for cubie/allwinner.