[RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2)

[Tabi Timur-B04825]

On Mon, Nov 5, 2012 at 2:40 PM, Grant Likely <grant.likely at secretlab.ca> wrote:

> Jane is building custom BeagleBone expansion boards called 'capes'. She
> can boot the system with a stock BeagleBoard device tree, but additional
> data is needed before a cape can be used. She could replace the FDT file
> used by U-Boot with one that contains the extra data, but she uses the
> same Linux system image regardless of the cape, and it is inconvenient
> to have to select a different device tree at boot time depending on the
> cape.

What's wrong with having the boot loader detect the presence of the
Cape and update the device tree accordingly?  We do this all the time
in U-Boot.  Doing stuff like reading EEPROMs and testing for the
presence of hardware is easier in U-Boot than in Linux.

For configurations that can be determined by the boot loader, I'm not
sure overlays are practical.

> Nigel is building a real-time video processing system around a MIPS SoC
> and a Virtex FPGA. Video data is streamed through the FPGA for post
> processing operations like motion tracking or compression. The FPGA is
> configured via the SPI bus, and is also connected to GPIO lines and the
> memory mapped peripheral bus. Nigel has designed several FPGA
> configurations for different video processing tasks. The user will
> choose which configuration to load which can even be reprogrammed at
> runtime to switch tasks.

Now this, on the other hand, makes more sense.  If the hardware
configuration is literally user-configurable, then okay.  However, I'm
not sure I see the need to update the device tree.  The device tree is
generally for hardware that cannot be discovered/probed by the device
driver.  If we're loading a configuration from user space, doesn't the
driver already know what the hardware's capabilities are, since it's
the one doing the uploading of a new FPGA code?  Why not skip the
device tree update and just tell the driver what the new capabilities
are?

-- 
Timur Tabi
Linux kernel developer at Freescale