On 02/05/2018 17:06, 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 fcounting 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.
A third approach, simpler still, is to allow an alternative encoding of a boolean property,
only to be used when deleting a boolean using an overlay. That encoding could be a
zero-valued cell or byte, or the string "false" - it doesn't really matter, but it ought
to be efficient to process so a zero cell seems easiest. The point is that the only code
that has to change is the interpretation of a boolean property, and the behaviour in the
presence of old code is almost the same as you got before the addition of this facility
- that a property remains true when you would like it to be false. The "almost" comes
from the fact that, to old code, any attempt to modify a boolean with an overlay would
have the same effect as setting it, but since an overlay hasn't previously been able to
set a boolean to false I don't see this as a significant difference.
Phil
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