linuxkpi: Document why <linux/pci.h> can't include <linux/ioport.h>

Reviewed by:	bz, emaste
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55738

sys/compat/linuxkpi/common/include/linux/pci.h

@@ -60,6 +60,17 @@
 #include <linux/pci_ids.h>
 #include <linux/pm.h>
 
+/*
+ * <linux/ioport.h> should be included here, like Linux, but we can't have that
+ * because Linux `struct resource` definition would conflict with FreeBSD
+ * native definition.
+ *
+ * At least the amdgpu DRM driver (amdgpu_isp.c at the time of this writing)
+ * relies on this indirect include to get the definition of Linux `struct
+ * resource`. As a workaround, we include <linux/ioport.h> from
+ * <linux/mfd/core.h>.
+ */
+
 #include <linux/kernel.h>	/* pr_debug */
 
 struct pci_device_id {