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

[Michal Suchánek]

On Tue, Oct 11, 2022 at 10:06:59PM +0200, Arnd Bergmann wrote:
> 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.

The original code was added with
commit 7f29b87a7779 ("powerpc: offb: add support for foreign endianness")

The hardware is either big-endian or runtime-switchable-endian. It makes
sense to assume big-endian when runnig big-endian and the DT does not
specify endian which is likely on a historical system.

It also makes sense to assume that on system with
runtime-switchable-endian the DT specifies the framebuffer endian.

If systems that only do little-endian exist or emerge later then it also
makes sense to assume that the framebuffer matches the host if not
specified.

I don't really see a problem here.

BTW is this used on arm and on what platform?

I do not see any bindings in dts.

Thanks

Michal