On 03/02/2015 11:28 AM, Kevin Cernekee wrote: > On Mon, Mar 2, 2015 at 8:08 AM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: >> On 03/02/2015 09:56 AM, Kevin Cernekee wrote: >>> On Mon, Mar 2, 2015 at 5:14 AM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: >>>> On 11/24/2014 06:36 PM, Kevin Cernekee wrote: >>>>> These apply to newly converted drivers, like serial8250/libahci/... >>>>> The examples were adapted from the regmap bindings document. >>>>> >>>>> Signed-off-by: Kevin Cernekee <cernekee@xxxxxxxxx> >>>>> --- >>>>> .../devicetree/bindings/common-properties.txt | 60 ++++++++++++++++++++++ >>>>> 1 file changed, 60 insertions(+) >>>>> create mode 100644 Documentation/devicetree/bindings/common-properties.txt >>>>> >>>>> diff --git a/Documentation/devicetree/bindings/common-properties.txt b/Documentation/devicetree/bindings/common-properties.txt >>>>> new file mode 100644 >>>>> index 0000000..21044a4 >>>>> --- /dev/null >>>>> +++ b/Documentation/devicetree/bindings/common-properties.txt >>>>> @@ -0,0 +1,60 @@ >>>>> +Common properties >>>>> + >>>>> +The ePAPR specification does not define any properties related to hardware >>>>> +byteswapping, but endianness issues show up frequently in porting Linux to >>>>> +different machine types. This document attempts to provide a consistent >>>>> +way of handling byteswapping across drivers. >>>>> + >>>>> +Optional properties: >>>>> + - big-endian: Boolean; force big endian register accesses >>>>> + unconditionally (e.g. ioread32be/iowrite32be). Use this if you >>>>> + know the peripheral always needs to be accessed in BE mode. >>>>> + - little-endian: Boolean; force little endian register accesses >>>>> + unconditionally (e.g. readl/writel). Use this if you know the >>>>> + peripheral always needs to be accessed in LE mode. This is the >>>>> + default. >>>> >>>> There is a fundamental problem with specifying the default in DT bindings. >>>> How can drivers which are currently native-endian support big-endian? >>>> >>>> If the driver is converted to support big-endian, every previous >>>> devicetree will be invalid with the new kernel (because those devicetrees >>>> don't specify 'native-endian'). >>>> >>>> IOW, consider if the default were 'native-endian'. How would the 8250 >>>> driver support existing devicetrees? >>> >>> Correct. This scheme is intended for drivers like 8250 and libahci >>> which currently default to little-endian by virtue of using >>> readl/writel for MMIO accesses. Drivers that default to native-endian >>> should specify that in their bindings documents, similar to >>> Documentation/devicetree/bindings/regmap/regmap.txt. >> >> Which effectively means if a user can't upgrade their devicetree, they >> can't upgrade their kernel. I don't think that flies. > > This doesn't change the behavior of pre-existing drivers that > implement the *-endian properties in a different way. There are not > many of these drivers and they can be documented as special cases. Yeah, ok, as long as there's no expectation that existing drivers meet this criteria when they add big-endian support. >> It's exactly this kind of stuff that prompted Jonathan Corbet's article, >> "Device trees as ABI" http://lwn.net/Articles/561462 >> >> Why not leave the default unspecified? > > The document aims to provide a consistent way of handling DT > endianness properties across (compliant) drivers. It is confusing if > one new driver defaults to little-endian, and another new driver > defaults to native-endian. Ok. How many 4.0 driver + DT submissions that are native-endian are declaring this binding? > And since most of the commonly used drivers already implement > little-endian MMIO accesses, that is the default. My personal > preference would have been native-endian since that seems more common > on the hardware side, but defaulting to little-endian prevents > breaking the device tree "ABI" on existing systems. That was basically my point; there's no way to meet these goals for existing, native-endian drivers without breakage (just as there would have been no way if native-endian had been the default). Regards, Peter Hurley
