On Wed, May 25, 2016 at 08:32:51AM -0700, Frank Rowand wrote: > On 5/25/2016 2:20 AM, Mark Rutland wrote: > > Linux for legacy reasons, documenting it as a binding is not necessarily > > in anyone's best interest. If we want to document it, we may want to > > mark it as deprecated, with a pointer to better alternatives. > Lack of documentation and bad documentation are a MAJOR problem for > devicetree. > Refusing to accept documentation of existing behavior makes no > sense to me. Sometimes the best thing to do is remove the behaviour, some of these things are just bugs. That's not quite the case here but it's in the spectrum of things that happen so clearly just blindly documenting everything people find happens to work is not great - if something isn't being used because it wasn't discoverable sometimes we should be thankful for that. Adding documentation for every last implementation that happens to work in a given situation through layers of indirection isn't going to help with the usability issues DT has, one of the things that the documentation does is tell both users and other OS vendors (and now I guess also people writing ACPI machine descriptions) what they should be doing when they implement or use the bindings. This is especially true if the documentation doesn't even cover the intended effects of the implementation detail, that's just checkbox documentation. One of the big problems we have with getting people to write high quality bindings is getting them to understand that they're supposed to be describing the hardware, not just dumping the current Linux implementation into an external data structure. If that's all we're doing then device tree isn't buying us a huge amount, we're just putting the same things in another format with worse tooling. This is like the issues we've got with all the historic bindings that just dumped raw numeric constants in the DT - people see those and just write a binding which dumps whatever internal constants Linux has into DTs. Consider this case, the proposal is: | +Normally SPI buses are assigned dynamic bus numbers starting at 32766 | +and counting downwards. It is possible to assign the bus number | +statically using devicetee aliases. For example, on the MPC5200 the This has no practical meaning as a spec since it doesn't say what a bus number either is or means so an implementation can happily ignore it with no effect. The details on how Linux currently does dynamic IDs are unhelpful and possibly misleading if the bus gets reinstantiated but that's somewhat secondary. The actual effect Christer is intending to generate is that his systems end up with stable names for spidev devices which are a very obvious implementation detail of Linux on current systems - using raw spidev directly in a binding rather than a compatible string for the attached device is something that generates loud warnings since that's not something that meets the DT goal of describing the hardware. With the compatible string it's fine since we have a description of the hardware and the OS can bind whatever the most suitable support is to the device, without we have literally no idea what we're supposed to be controlling. Just documenting bus numbers is not going to say anything about how Linux supports the particular devices, how spidev works and how userspace names devices, nor is it going to help anyone who wants stable naming over a class of boards with multiple sockets (eg, board A has two SPI sockets on one SPI controller, board B has one controller per socket) - the whole using one ramdisk over multiple boards use case that Christer mentioned. What would seem to be a lot more sensible here would be to define a binding for whatever device is being described with some support for providing a descriptive name which we can then bring out to userspace for it to match on (and perhaps use for the device name so you get spidev-socket1, spidev-gpschip or something which would be a lot more useful for this type of application since it's easier to map onto the physical system). That directly addresses the need is a more robust and general fashion. I do wonder if such naming support should be at a more general level, possibly even DT wide, since it seems like something that will apply elsewhere. If it's just some raw signals on an expansion connector then this seems to be something that should be handled as part of the support for things like BeagleBone capes, if no overlay is loaded perhaps that should default to raw userspace access to devices.
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