Update Linux to v5.4.2
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+=================================
+Kernel Memory Layout on ARM Linux
+=================================
+
+ Russell King <rmk@arm.linux.org.uk>
+
+ November 17, 2005 (2.6.15)
+
+This document describes the virtual memory layout which the Linux
+kernel uses for ARM processors. It indicates which regions are
+free for platforms to use, and which are used by generic code.
+
+The ARM CPU is capable of addressing a maximum of 4GB virtual memory
+space, and this must be shared between user space processes, the
+kernel, and hardware devices.
+
+As the ARM architecture matures, it becomes necessary to reserve
+certain regions of VM space for use for new facilities; therefore
+this document may reserve more VM space over time.
+
+=============== =============== ===============================================
+Start End Use
+=============== =============== ===============================================
+ffff8000 ffffffff copy_user_page / clear_user_page use.
+ For SA11xx and Xscale, this is used to
+ setup a minicache mapping.
+
+ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
+
+ffff1000 ffff7fff Reserved.
+ Platforms must not use this address range.
+
+ffff0000 ffff0fff CPU vector page.
+ The CPU vectors are mapped here if the
+ CPU supports vector relocation (control
+ register V bit.)
+
+fffe0000 fffeffff XScale cache flush area. This is used
+ in proc-xscale.S to flush the whole data
+ cache. (XScale does not have TCM.)
+
+fffe8000 fffeffff DTCM mapping area for platforms with
+ DTCM mounted inside the CPU.
+
+fffe0000 fffe7fff ITCM mapping area for platforms with
+ ITCM mounted inside the CPU.
+
+ffc00000 ffefffff Fixmap mapping region. Addresses provided
+ by fix_to_virt() will be located here.
+
+fee00000 feffffff Mapping of PCI I/O space. This is a static
+ mapping within the vmalloc space.
+
+VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
+ Memory returned by vmalloc/ioremap will
+ be dynamically placed in this region.
+ Machine specific static mappings are also
+ located here through iotable_init().
+ VMALLOC_START is based upon the value
+ of the high_memory variable, and VMALLOC_END
+ is equal to 0xff800000.
+
+PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
+ This maps the platforms RAM, and typically
+ maps all platform RAM in a 1:1 relationship.
+
+PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
+ One way of mapping HIGHMEM pages into kernel
+ space.
+
+MODULES_VADDR MODULES_END-1 Kernel module space
+ Kernel modules inserted via insmod are
+ placed here using dynamic mappings.
+
+00001000 TASK_SIZE-1 User space mappings
+ Per-thread mappings are placed here via
+ the mmap() system call.
+
+00000000 00000fff CPU vector page / null pointer trap
+ CPUs which do not support vector remapping
+ place their vector page here. NULL pointer
+ dereferences by both the kernel and user
+ space are also caught via this mapping.
+=============== =============== ===============================================
+
+Please note that mappings which collide with the above areas may result
+in a non-bootable kernel, or may cause the kernel to (eventually) panic
+at run time.
+
+Since future CPUs may impact the kernel mapping layout, user programs
+must not access any memory which is not mapped inside their 0x0001000
+to TASK_SIZE address range. If they wish to access these areas, they
+must set up their own mappings using open() and mmap().