Subject: ARM topic: Is DT on ARM the solution, or is there something better?
Date: Sunday 20th October 2013 21:26:54 UTC (over 3 years ago)
IIRC (and perhaps I don't; it was really slightly before my active involvement in kernel development) Linus triggered the whole ARM DT conversion in response to disliking the volume of changes, and conflicts, in board files. The idea of DT conversion being that all the board-specific details could be moved out of the kernel and into DT files, thus causing him not to have to see it. Note: As part of implementing DT on ARM, we've also cleaned up and modularized a lot of code, and created new subsystems and APIs. I think this is a separate issue, and much of that could have happened completely independently from doard->DT conversion. I wonder if DT is solving the problem at the right level of abstraction? The kernel still needs to be aware of all the nitty-gritty details of how each board is hooked up different, and have explicit code to deal the union of all the different board designs. For example, if some boards have a SW-controlled regulator for a device but others don't, the kernel still needs to have driver code to actively control that regulator, /plus/ the regulator subsystem needs to be able to substitute a dummy regulator if it's optional or simply missing from the DT. Another example: MMC drivers need to support some boards detecting SD card presence or write-protect via arbitrary GPIOs, and others via dedicated logic in the MMC controller. In general, the kernel still needs a complete driver to every last device on every strange board, and needs to support every strange way some random board hooks all the devices together. The only thing we've really moved out of the kernel is the exact IDs of which GPIOS, interrupts, I2C/SPI ports the devices are connected to; the simple stuff not the hard stuff. The code hasn't really been simplified by DT - if anything, it's more complicated since we now have to parse those values from DT rather than putting them into simple data-structures. I wonder if some other solution with a higher level of abstraction wouldn't be a better idea? Would it make more sense to define some kind of firmware interface that the kernel deals with, so that all HW details are hidden behind that firmware interface, and the kernel just deals with the firmware interface, which hopefully has less variation than the actual HW (or even zero variation). * Would UEFI/ACPI/similar fulfill this role? * Perhaps a standard virtualization interface could fulfil this role? IIUC, there are already standard mechanisms of exposing e.g. disks, USB devices, PCI devices, etc. into VMs, and recent ARM HW supports virtualization well now. A sticking point might be graphics, but it sounds like there's work to transport GL or Gallium command streams over the virtualization divide. Downsides might be: - Overhead, due to invoking the para-virtualized VM host for IO, and extra resources to run the host. - The host SW still has to address the HW differences. Would it be more acceptable to run a vendor kernel as the VM host if it meant that the VMs could be a more standardized environment, with a more single-purpose upstream kernel? Would it be easier to create a simple VM host than a full Linux kernel with a full arbitrary Linux distro, thus allowing the HW differences to be addressed in a simple way? These techniques would allow distros to target a single HW environment, e.g. para-virtualized KVM, rather than many many different SoCs and boards each with different bootloaders, bootloader configurations, IO peripherals, DT storage locations, etc. Perhaps a solution like this would allow distros to easily support a similar environment across a range of HW in a way that "just works" for many users, while not preventing people with more specific needs crafting more HW-specific environments? Note: This is all just slightly random thinking that came to me while I couldn't sleep last night, so apologies if it isn't fully coherent. It's certainly not a proposal, just perhaps something to mull over.  All /recent/ consumer-grade ARM laptop or desktop HW that I'm aware of that's shipped has Cortex A15 cores that support virtualization. -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html