On Fri, Apr 27, 2018 at 4:39 PM, Alexander Graf agraf@suse.de wrote:
Hi Rob,
On 27.04.18 18:40, Rob Herring wrote:
On Fri, Apr 27, 2018 at 2:47 AM, Alexander Graf agraf@suse.de wrote:
On 27.04.18 08:24, Udit Kumar wrote:
Hi There is bit of discussion on linux-efi too , to handle DT update
I guess some members of this forum are active there too.
https://www.spinics.net/lists/linux-efi/msg13700.html
To summaries 1/ Ownership of DTB IMO should be firmware and we should retain this ownership in EBBR as well, Any objections/thoughts ?
I fully agree. On top of that we need to make clear that backward and forward compatibility are not optional.
For that I think we may need to actually give people workable solutions to create device trees that are compatible with multiple levels of kernel support. The main areas I'm aware of that keep breaking are:
- fine grained interrupt controller support
Do you have an example of that? The only thing I can think of is people switching interrupts from the GIC to an always-on, low-power mode custom interrupt controller.
The last time I've seen that breakage was:
https://github.com/raspberrypi/linux/blob/rpi-4.14.y/arch/arm/boot/dts/bcm27...
which indeed does switch interrupts from the GIC to an interrupt muxer behind the GIC.
The problem is that once support for that lands upstream, you will have very little option but to break backwards compatibility today.
This one is unfortunate. It could have been handled better. An interrupt-map property in the aux ctrlr could have mapped the interrupts to GIC without any aux driver. Then when the aux driver lands, it just needs to remove the interrupt-map (on boot). Alternatively, I skimmed thru some discussions of the issue, but I'm not clear why the devices behind the aux controller can't all just treat their interrupts as shared. But that would be a simple change to the drivers' irq handlers, so I'm probably missing something. If that worked, then the DT would never need to change.
I guess whether this could have been handled better depends if folks knowingly ignored the issue or this was found after upstreaming support. The latter case may be unavoidable, but maybe we can make it rare enough we only need overlays in some exceptions. Whether we try to be stricter and do better up front or have some overlay based solution, either one is going to require folks be aware of the issues and effort to avoid them.
- clock support
Are there cases other than going from fixed, fake clocks to a real clock controller node. I'm inclined to stop allowing people to do that. A better way this could be done is just provide a clock controller driver with a bunch of fixed clocks. Then the switch from the dumb driver to the real driver is just a kernel change.
Going from fixed to in-kernel clock driver is one case I'm aware of, yes.
The other one I can think of right now is going from one clock interface to another. For example if you configure clocks using a firmware interface and that firmware interface moves to a newer, more shiny interface (say proprietary to SCMI). That switch may also regress users of the device tree.
If not only the clocks but the whole firmware interface changes, then I don't see how a stable DT matters at that point.
I think Andre had other examples of where DT compatibility broke, so I'll let him chime in as well.
- power domain support
Example?
https://github.com/Xilinx/linux-xlnx/blob/master/arch/arm64/boot/dts/xilinx/...
Booting with the power-domains properties included in the device tree fails for me, because the driver framework considers them critical to load a device and just errors out with -EDEFER.
- pinctrl support
This would be the firmware initially does all the pin setup, then you move it to DT and drop the setup from the firmware? Otherwise I don't understand the problem in this case. We'd start with no pinctrl and then add it to the DT. Why wouldn't the kernel just ignore it?
Because it doesn't. The linux driver framework as it works today looks at the pinctrl parent for a specific device and if it can't find it loaded, throws -EDEFER for the driver getting loaded. I suppose the rationale behind that is that pinctrl drivers could be modules that get loaded after the driver you want to load.
Every time a device tree changes in any of the above, that usually ends up in backwards incompatibility.
TBC, you're talking about new dtb with old kernels. We've mainly cared
Yes. Both really.
about old dtbs and new kernels. So first we should agree the former is important too. I do, because simply you wouldn't want a BIOS update to make your PC stop booting your already installed OS.
Yes.
I'd like to solve this with policy and good practice before we try to apply technical solutions on top of a mess.
I'm not holding my hopes up, but we can try ;). Another full dimension of messiness are downstream, unapproved bindings that some times can't live in the same device tree as their upstream counterparts once upstream chose a different route.
For pinctrl and power domain, I suppose we could have a compatible whitelist in the kernel and instead of -EDEFER just consider the targets immutable if we hit none of the whitelisted strings.
Yes, I had similar thought as well, but was thinking a blacklist rather. We could modify the DT on boot to disable the listed nodes. Or various subsystems could have lists of compatibles to not return -EPROBE_DEFER. The one complexity with a blacklist is we'd need patches for stable that may not be in mainline unless we add the compatible to the blacklist and then immediately remove it when adding the real driver.
Also, this is not just a old vs. new compatibility issue. It can also be an on-going kernel config issue. If you fail to enable all the right drivers that are dependencies because you've added them to the DT, then you won't boot any more. A whitelist based on the kernel config would solve that problem. The challenge is how to create the list. The information is already present, just not easily accessed.
Anyway, now I have a clear picture of this problem and need to think some more about it. This particular issue is bit outside of EBBR and this audience. We should discuss with more kernel folks.
Rob