This conversation seems to have pretty much entirely ended up happening
on ksummit-discuss, which doesn't seem that useful. So let's have
another go with a wider audience (and where I don't have to sign up for
yet another mailing list...)
ACPI and DT overlap in that they both provide a mechanism for
enumerating non-enumerable devices, and both also provide a mechanism
for attaching additional configuration data to devices (which may or may
not be otherwise enumerable). There's a sufficient overlap in
functionality that at least one platform that's traditionally been
Device Tree (arm) is also adding support for ACPI - there's even
ACPI-based arm hardware on the market already.
Right now that's a problem for us. The same hardware may end up shipped
with either ACPI or DT-based firmware, and at the moment we'd need to
either write two drivers or one driver with two glue layers. This is
somewhat suboptimal.
The biggest difference between DT and ACPI is that DT is effectively a
somewhat structured mechanism for passing arbitrary data, while ACPI
tends (with exceptions) to provide standardised data. For instance, in
DT we might have:
sdhci@c8000000 {
status = "okay";
power-gpios = <&gpio TEGRA_GPIO(K, 6) GPIO_ACTIVE_HIGH>;
bus-width = <4>;
keep-power-in-suspend;
};
In ACPI the normal way to handle this would be to have a GPIO operation
region that listed the GPIOs:
OperationRegion(GPO1, GeneralPurposeIO, 0, 1)
Field(GP01, ByteAcc, NoLock, Preserve)
{
Connection (GpioIo(Exclusive, PullUp,,,,,)
TG06, 1
}
And then an ACPI device:
Device (\_SB.SDHC)
{
Method (_PS0, 0, NotSerialized)
{
Store (1, TG06)
}
Method (_PS3, 0, NotSerialized)
{
Store (0, TG06)
}
}
with the ACPI core then executing these methods whenever a device is
powered up or down.
How can we unify these two different representations? The only terribly
plausible way of doing so seems to be to push that out to a helper
function and then have it handled as part of device runtime power
management, and just have the driver make pm_runtime_get() and _put()
calls.
So we can theoretically handle cases like power lines without /too/ much
trouble, while representing the same thing in either ACPI or DT. But we
have plenty of other data - in the case of SDHCI alone, there's the bus
width and potentially card detect and write protect gpio lines. These
can easily be represented in ACPI, but not in a terribly generic way. We
could easily add new functions to retrieve this information, but doing
this through the standards body is likely to prove tedious, especially
when new hardware types appear and we want to be able to ship support
pretty much immediately.
The other choice is to ignore most of the existing ACPI functionality
and just use it as a mechanism for providing additional metadata. Apple
already do this using the _DSM methods:
Method (_DSM, 4, NotSerialized)
{
Store (Package (0x07)
{
"refnum",
0x00,
"address",
0x39,
"device-id",
0x0cc8,
Buffer (0x01)
{
0x00
}
}, Local0)
DTGP (Arg0, Arg1, Arg2, Arg3, RefOf (Local0))
Return (Local0)
}
where the DTGP() method verifies that the caller passed in an
appropriate UUID and then hands back this buffer.
Putting all our existing DT metadata in there would be straightforward,
but we would then effectively just be using ACPI to repackage DT -
drivers would continue to make DT-specific calls in order to obtain the
information they need.
This seems like the least-effort approach, but it doesn't really solve
much. Any other OS on the same hardware is unlikely to use the DT data,
and if someone wants to run Linux on hardware that was intended for
another OS they're unlikely to have any DT data to use.
So while unifying this seems hugely desirable, right now it's not
incredibly obvious how we can actually achieve that. In some cases it's
easy to take DT information and rewrite it the ACPI way, but in others
we just don't seem to have the primitives we need for that. One approach
would be to work through the existing DT bindings documentation and see
what's missing from ACPI and work on adding it, but we'll still then
need helper functions that are able to obtain the same information from
either source.
In any case, it seems like this is something that should be discussed. A
bunch of people in the earlier discussion mentioned that they were going
to be in New Orleans, so I'd suggest that we arrange a time for
in-person discussion there. That should give us a solid basis for
further mailing list discussion, and then there'll be another
opportunity for discussion in Edinburgh.
--
Matthew Garrett | mjg59@xxxxxxxxxxxxx
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