Re: [PATCH v4 5/5] drm/ofdrm: Support big-endian scanout buffers

["Arnd Bergmann" <arnd@arndb.de>]

On Tue, Oct 11, 2022, at 1:30 PM, Thomas Zimmermann wrote:
> Am 11.10.22 um 09:46 schrieb Javier Martinez Canillas:
>>> +static bool display_get_big_endian_of(struct drm_device *dev, struct device_node *of_node)
>>> +{
>>> +	bool big_endian;
>>> +
>>> +#ifdef __BIG_ENDIAN
>>> +	big_endian = true;
>>> +	if (of_get_property(of_node, "little-endian", NULL))
>>> +		big_endian = false;
>>> +#else
>>> +	big_endian = false;
>>> +	if (of_get_property(of_node, "big-endian", NULL))
>>> +		big_endian = true;
>>> +#endif
>>> +
>>> +	return big_endian;
>>> +}
>>> +
>> 
>> Ah, I see. The heuristic then is whether the build is BE or LE or if the Device
>> Tree has an explicit node defining the endianess. The patch looks good to me:
>
> Yes. I took this test from offb.

Has the driver been tested with little-endian kernels though? While
ppc32 kernels are always BE, you can build kernels as either big-endian
or little-endian for most (modern) powerpc64 and arm/arm64 hardware,
and I don't see why that should change the defaults of the driver
when describing the same framebuffer hardware.

I could understand having a default to e.g. big-endian on all powerpc and
a default for little-endian on all arm, but having it tied to the
way the kernel is built seems wrong, and doesn't make sense in a
DT binding either.

      Arnd