On Tue, Oct 11, 2022, at 11:38 PM, Michal Suchánek wrote:

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

quoted

On Tue, Oct 11, 2022, at 1:30 PM, Thomas Zimmermann wrote:

quoted

Am 11.10.22 um 09:46 schrieb Javier Martinez Canillas:

quoted

quoted

+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.

Are you referring to CPU hardware or framebuffer hardware here?

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.

Agreed, assuming big-endian here clearly makes sense.

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'm not aware of any users on Arm, most likely they all use
simplefb/simpledrm or a gpu specific binding. There might be
users on sparc, but they would obviously be big-endian
as well.

I do not see any bindings in dts.

Right, that is the real problem I see as well. I found the original
CHRP binding document at
https://www.devicetree.org/open-firmware/bindings/devices/html/lfb-1_0d.html

Unfortunately, this only specifies an 8-bit-per-pixel mode, and the
multi-byte pixel support that was added in linux-2.1.125 was
probably powermac specific without a public specification.

I think ideally we should add a binding document that describes what
the driver actually expects, but in this case I would just drop the
#ifdef check and always assume the framebuffer is big-endian unless
the "little-endian" property is set, in order to have a sensible
definition that does not depend on what OS (i.e. Linux
CONFIG_CPU_BIG_ENDIAN) you are running.

       Arnd