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. > 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. 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.
