quoted

+#ifndef __PPE_H__
+#define __PPE_H__
+
+#include <linux/compiler.h>
+#include <linux/interconnect.h>
+
+struct device;

#include <linux/device.h> ?

quoted

+struct regmap;

Same with previous one, include it's header file?

If the structure is opaque at this level, it is fine to not include
the header. There is nothing in the header actually needed. By not
including it, the build it faster. A large part of the kernel build
time is spent processing headers, so less headers are better.

     Andrew