David, On 03/05/2018 03:22, David Gibson wrote: > On Wed, May 02, 2018 at 05:06:06PM +0100, Phil Elwell wrote: >> Rob, >> >> On 02/05/2018 16:49, Rob Herring wrote: >>> On Wed, May 2, 2018 at 4:19 AM, Phil Elwell <phil@xxxxxxxxxxxxxxx> wrote: >>>> David et al., >>>> >>>> I've mentioned before the problem posed for overlays by boolean properties, i.e. >>>> that a boolean property that is "true" in a base DTB cannot be made "false" by an >>>> overlay because doing so requires that the property be deleted. A solution for this >>>> problem would be to define a new FDT tag - FDT_DEL_PROP, say - that is used to encode >>>> any /delete-property/ found in a node during overlay compilation. When the overlay is >>>> applied, the named property would be deleted if present. >>>> >>>> A heuristic would be needed to decide whether this property should be encoded or just >>>> acted on immediately - the use of the '-@' command line parameter would seem to fit the >>>> bill. >>>> >>>> Although one might consider extending this mechanism to cover node deletion, in practice >>>> I think this would be too problematic in terms of broken phandle references etc., and in >>>> most cases 'status = "disabled"' achieves the same objective, so I'm not proposing this >>>> be added. >>>> >>>> Is such a change something you would consider supporting, or do you have an alternative >>>> preferred solution? >>> >>> Can you give some examples of cases where you need this feature. >> >> The case I've encountered is the "non-removable" flag on an SD interface, specifically >> the SD interface used for an SDIO link to a WiFi chip on the Raspberry Pi 3B, which is >> marked "non-removable" for operational reasons. Some users prefer to repurpose the SD >> interface to drive a second SD card on a different set of pins, which they could do using >> an overlay except for the "non-removable". The ugly workaround is to disable the original >> interface node and create a near-clone in the overlay without the unwanted property. >> >>> I'm concerned about how an OS is supposed to deal with properties >>> disappearing. Do we need to start refcounting properties too? That's >>> not really a reason to not support this in dtc, but rather perhaps a >>> policy decision in the OS to not delete properties once a node is in >>> use/active. Would such a policy break your use case? >> >> I agree that deletion of a property at run-time could prove awkward - if only booleans >> were implemented another way - but a non-run-time restriction wouldn't bother me since >> we can require users to only apply the overlay via the firmware. >> >> Since writing the original email I've thought some more about the implementation, and >> perhaps a new tag is too disruptive a change. An alternative implementation would be to >> create a new node, in the same vein as "__symbols__", listing properties to delete in >> some suitable encoding. > > That has the advantage(?) of not actively breaking things that don't > understand the new tag. The reason I question whether that's an > advantage is that things that didn't understand the new encoding still > obviously wouldn't be able to process the deletion. So the question > is whether silently ignoring the delete is better or worse than > blowing up entirely when given an fdt with the new deletion encoding. Since the overlay doesn't know the state of the base DTB it is being applied to, there are going to be cases where overlays attempt to delete non-existent properties, which is harmless with one encoding and fatal (at least for that overlay) in the other. I don't have a strong opinion on the matter - it's not a big issue on the Pi where we usually update kernel, firmware and DTB together - but confining the required changes to the overlay generation and application code seems preferable. Phil -- To unsubscribe from this list: send the line "unsubscribe devicetree-compiler" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
