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. Robert has been talking about the actions required to clean-up Debian Jessie support in another thread (https://groups.google.com/d/msg/beagleboard/2b8rArtfABY/A8d1JzmJa4IJ) and I suggested we should add a bit of a detour by cleaning up the cape support for the mainline kernel and switching away our primary process of supporting capes from using overlays to using a single devicetree file provided at boot. I promised a pull-request and hadn't gotten around to sending it until now (below). No architecture changes have been made in my pull-request, just bringing in the kernel devicetree source history. This suggestion is based on several assumptions, any number of which might be wrong. The assumptions (for which I'm looking for feedback/corrections): * The overlays pretty much all need to be compiled into the kernel if they are going to be loaded using kernel command-line arguments or /etc/capemgr for the majority of distros. While many cape devicetree use cases are perfectly happy with loading at run-time rather than boot-time, it seems there should be a mechanism for pushing cape support into the category of being available at boot-time across distributions. * The devicetree sources, including the primary boot .dts files, will eventually be removed from the kernel source tree. I'm not too sure if and when it'll really happen, but starting up a project to maintain the definitive beagleboard.org board devicetree files outside the kernel seems to make sense. Given the interdependency of the boot .dtb and the overlay .dtbo files, combining them into a single repository where every distribution can pick them up seems like a natural and obvious choice. There are of course some dependencies on kernel versions, but I believe most of those have settled out by now and we should be OK moving forward. * There seems to be little or no interest in my previous proposal to use cape EEPROMs to store the overlay fragments. Given some churn in the devicetree node definitions, it is likely this would have failed in bad ways anyway. It seems mostly reasonable to expect users to update their kernel and firmware to gain support for new add-on hardware and we mostly try to avoid regressions (with the seemingly ever-living exception of 3D graphics support), so I think I'm better aligned with the community to drop my older suggestion. Some people, including CircuitCo, are building capes without configuration EEPROMs now, so a different recommended mechanism seems like a requirement. * With the patches in our vendor 3.8 kernels and with all recent mainline kernels, performing userspace muxing operations has become easy again. It seems to be possible to turn on drivers not currently in use without an unacceptable level of bloat or conflict. This has been partially proven out using Charles' universal cape (https://github.com/cdsteinkuehler/beaglebone-universal-io), though I still have some concerns about conflicts. The result might be that there is still some number of overlays, but the approach of minimizing the overlays and instead relying on the existing loading/unloading mechanisms of the mainline drivers as much as possible feels right to me. * It will still be some time before devicetree overlay support is adopted in the mainline kernel. While I still see a strong need to have devicetree overlay support and CapeMgr in the mainline, the desire here is to optimize the user experience in the shortest term possible. Users get really confused by the errors that get generated by loading incorrect devicetree overlays and it is always nice when you can avoid confusing users. My suggestion is: * Maintain the source for .dtb and .dtbo files for all BeagleBoard.org boards at http://github.com/beagleboard/devicetree-source, overriding the sources in the mainline kernel when performing kernel builds. We can host pre-built .dtb and .dtbo files at a normailzed location outside the source repository for those that don't want to perform the builds, since I don't believe the binary format is changing any time soon other than the overlay support we utilize. Any patches accepted into mainline as long as these files still exist there as well should be merged to try to keep these in-line as long as possible, anticipating eventual removal from mainline. * Have cape makers submit pull requests to have their cape enabled in the single boot-time devicetree overlay for each board they support. The base drivers should be enabled at the earliest point where they don't cause a conflict, minimizing what gets put into their overlays. The final mux configuration and other points of conflict should be kept in a board-specific overlay to make that configuration available at boot. If the cape is disabled on the kernel command-line, the kernel doesn't include CapeMgr support, or the board doesn't include a configuration EEPROM, users would still have the ability to perform run-time userspace mux configuration without CapeMgr unless conflicting drivers need to be loaded. Alternatively, if we don't think this is where things belong, perhaps we could try to move all of this into the u-boot sources, though people using other bootloaders might not find this the most useful approach. The pull request for http://github.com/beagleboard/devicetree-source: The following changes since commit a524bf5d5e173795ce4cb214280f304588b1d78d: capes: add BB-VIEW-LCD4-01-00A0 & BB-VIEW-LCD7-01-00A0 (2014-05-29 15:59:31 -0500) are available in the git repository at: g...@github.com:jadonk/cape-firmware master for you to fetch changes up to 9de20db80056b72d6e1f61b68447ba8f2d12cf9c: Beagle DTS: remove deleted files from Makefile (2014-06-02 12:35:28 -0400) ---------------------------------------------------------------- [...] Too many changes to list bringing in kernel history. -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
