On Mon, Nov 5, 2012 at 2:40 PM, Grant Likely <grant.likely@xxxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
