On 06/17/2014 05:09 AM, Russell King - ARM Linux wrote: > On Mon, Jun 16, 2014 at 09:22:50AM -0400, Jason Kridner wrote: >> Adding devicetree and linux-arm-kernel lists based on feedback on IRC... >> >> On Tue, Jun 10, 2014 at 12:46 PM, Jason Kridner <jkridner@xxxxxxxxx> wrote: >>> I'd like to discuss moving our current library of cape devicetree >>> overlay sources into a single tree, including the boot .dtb files for >>> BeagleBoard.org boards and moving towards enabling as much of the cape >>> support into a single boot-time .dtb file with an approach similar to >>> the cape-universal overlay >>> (https://github.com/cdsteinkuehler/beaglebone-universal-io), but not >>> in an overlay. >>> >>> First of all, I want to note this doesn't change my view on the >>> importance of mainline support for devicetree overlays. They are still >>> absolutely critical and highly useful, solving problems that cannot be >>> solved through boot-time devicetrees. I'm simply looking for an >>> approach that will complement the availability of overlays and provide >>> the best user experience. > > Here's the most obvious question in the world on this topic. Are capes > hot-pluggable? > > Looking at the posts on google+ from David Anders, they're using pin > headers for connectivity, with no additional protection against hot- > plugging, and no sequencing of pin connection. In other words, they are > not hot-pluggable. > > So, why do we need to add a load of infrastructure to the kernel to allow > the device tree to be modified at run time? At present, the way the > entire DT infrastructure works is that it assumes the DT remains static > and never changes - this applies not only to the core DT code, but also > to all the drivers which have been converted. > > So, you're asking for a feature which is impossible to really make use > of on the hardware which you want to use it. > > Why should kernel developers go to the extent of adding support for DT > modification at runtime when the platform you want this for doesn't even > support hotplugging of these capes? > > The logical way to deal with this is to have the boot loader merge DT > fragments together before it calls the kernel, so the kernel sees a single > DT blob which describes the whole hardware. Frankly, if we're going to push device tree merging to be someone elses problem, I'd like to push it out to userspace. > A good way that this could have been done is to put an I2C EEPROM on > each cape, and have that store the DT fragment. The boot loader could > have then read that from each cape, and used that information to build > up the final DT. Why this hasn't been thought of, considering that the > kernel has been moving towards DT for years, is quite unbelievable. I had actually talked about this a long while back (face to face) with people, but the problem was (and still kind of is) the bindings changing, etc. -- Tom -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
