Documentation/features: Explain kernel feature descriptions and add visualization script

The previous patches added arch support matrices for more than 40 generic kernel features
that need per architecture support.

The structure of the feature descriptions is the following:

Each feature has its own directory under Documentation/features/subsystem_name/feature_name/,
and the arch-support.txt file shows its current arch porting status.

For example, lockdep support is shown the following way:

    triton:~/tip> cat Documentation/features/locking/lockdep/arch-support.txt
    #
    # Feature name:          lockdep
    #         Kconfig:       LOCKDEP_SUPPORT
    #         description:   arch supports the runtime locking correctness debug facility
    #
        -----------------------
        |         arch |status|
        -----------------------
        |       alpha: | TODO |
        |         arc: |  ok  |
        |         arm: |  ok  |
        |       arm64: |  ok  |
        |       avr32: |  ok  |
        |    blackfin: |  ok  |
        |         c6x: | TODO |
        |        cris: | TODO |
        |         frv: | TODO |
        |       h8300: | TODO |
        |     hexagon: |  ok  |
        |        ia64: | TODO |
        |        m32r: | TODO |
        |        m68k: | TODO |
        |       metag: |  ok  |
        |  microblaze: |  ok  |
        |        mips: |  ok  |
        |     mn10300: | TODO |
        |       nios2: | TODO |
        |    openrisc: | TODO |
        |      parisc: | TODO |
        |     powerpc: |  ok  |
        |        s390: |  ok  |
        |       score: |  ok  |
        |          sh: |  ok  |
        |       sparc: |  ok  |
        |        tile: |  ok  |
        |          um: |  ok  |
        |   unicore32: |  ok  |
        |         x86: |  ok  |
        |      xtensa: |  ok  |
        -----------------------

For generic kernel features that need architecture support, the
arch-support.txt file in each feature directory shows the arch
support matrix, for all upstream Linux architectures.

The meaning of entries in the tables is:

    | ok |  # feature supported by the architecture
    |TODO|  # feature not yet supported by the architecture
    | .. |  # feature cannot be supported by the hardware

This directory structure can be used in the future to add other
files - such as porting guides, testing description, etc.

The Documentation/features/ hierarchy may also include generic
kernel features that works on every architecture, in that case
the arch-support.txt file will list every architecture as
supported.

To list an architecture's unsupported features, just do something
like:

  triton:~/tip>  git grep -lE 'x86.*TODO' Documentation/features/*/*/arch-support.txt
  Documentation/features/lib/strncasecmp/arch-support.txt
  Documentation/features/time/arch-tick-broadcast/arch-support.txt

which will print the list of not yet supported features.

The Documentation/features/list-arch.sh script will print the current
support matrix of one architecture:

  triton:~/tip> Documentation/features/list-arch.sh
  #
  # Kernel feature support matrix of the 'x86' architecture:
  #
      core/ BPF-JIT              :  ok  |                        HAVE_BPF_JIT #  arch supports BPF JIT optimizations
      core/ generic-idle-thread  :  ok  |             GENERIC_SMP_IDLE_THREAD #  arch makes use of the generic SMP idle thread facility
      core/ jump-labels          :  ok  |                HAVE_ARCH_JUMP_LABEL #  arch supports live patched, high efficiency branches
      core/ tracehook            :  ok  |                 HAVE_ARCH_TRACEHOOK #  arch supports tracehook (ptrace) register handling APIs
     debug/ gcov-profile-all     :  ok  |           ARCH_HAS_GCOV_PROFILE_ALL #  arch supports whole-kernel GCOV code coverage profiling
     debug/ KASAN                :  ok  |                     HAVE_ARCH_KASAN #  arch supports the KASAN runtime memory checker
     debug/ kgdb                 :  ok  |                      HAVE_ARCH_KGDB #  arch supports the kGDB kernel debugger
     debug/ kprobes              :  ok  |                        HAVE_KPROBES #  arch supports live patched kernel probe
     debug/ kprobes-on-ftrace    :  ok  |              HAVE_KPROBES_ON_FTRACE #  arch supports combined kprobes and ftrace live patching
     debug/ kretprobes           :  ok  |                     HAVE_KRETPROBES #  arch supports kernel function-return probes
     debug/ optprobes            :  ok  |                      HAVE_OPTPROBES #  arch supports live patched optprobes
     debug/ stackprotector       :  ok  |              HAVE_CC_STACKPROTECTOR #  arch supports compiler driven stack overflow protection
     debug/ uprobes              :  ok  |               ARCH_SUPPORTS_UPROBES #  arch supports live patched user probes
     debug/ user-ret-profiler    :  ok  |           HAVE_USER_RETURN_NOTIFIER #  arch supports user-space return from system call profiler
        io/ dma-api-debug        :  ok  |                  HAVE_DMA_API_DEBUG #  arch supports DMA debug facilities
        io/ dma-contiguous       :  ok  |                 HAVE_DMA_CONTIGUOUS #  arch supports the DMA CMA (continuous memory allocator)
        io/ dma_map_attrs        :  ok  |                      HAVE_DMA_ATTRS #  arch provides dma_*map*_attrs() APIs
        io/ sg-chain             :  ok  |                   ARCH_HAS_SG_CHAIN #  arch supports chained scatter-gather lists
       lib/ strncasecmp          : TODO |             __HAVE_ARCH_STRNCASECMP #  arch provides an optimized strncasecmp() function
   locking/ cmpxchg-local        :  ok  |                  HAVE_CMPXCHG_LOCAL #  arch supports the this_cpu_cmpxchg() API
   locking/ lockdep              :  ok  |                     LOCKDEP_SUPPORT #  arch supports the runtime locking correctness debug facility
   locking/ queued-rwlocks       :  ok  |             ARCH_USE_QUEUED_RWLOCKS #  arch supports queued rwlocks
   locking/ queued-spinlocks     :  ok  |           ARCH_USE_QUEUED_SPINLOCKS #  arch supports queued spinlocks
   locking/ rwsem-optimized      :  ok  |               Optimized asm/rwsem.h #  arch provides optimized rwsem APIs
      perf/ kprobes-event        :  ok  |      HAVE_REGS_AND_STACK_ACCESS_API #  arch supports kprobes with perf events
      perf/ perf-regs            :  ok  |                      HAVE_PERF_REGS #  arch supports perf events register access
      perf/ perf-stackdump       :  ok  |           HAVE_PERF_USER_STACK_DUMP #  arch supports perf events stack dumps
     sched/ numa-balancing       :  ok  |        ARCH_SUPPORTS_NUMA_BALANCING #  arch supports NUMA balancing
   seccomp/ seccomp-filter       :  ok  |            HAVE_ARCH_SECCOMP_FILTER #  arch supports seccomp filters
      time/ arch-tick-broadcast  : TODO |             ARCH_HAS_TICK_BROADCAST #  arch provides tick_broadcast()
      time/ clockevents          :  ok  |                 GENERIC_CLOCKEVENTS #  arch support generic clock events
      time/ context-tracking     :  ok  |               HAVE_CONTEXT_TRACKING #  arch supports context tracking for NO_HZ_FULL
      time/ irq-time-acct        :  ok  |            HAVE_IRQ_TIME_ACCOUNTING #  arch supports precise IRQ time accounting
      time/ modern-timekeeping   :  ok  |            !ARCH_USES_GETTIMEOFFSET #  arch does not use arch_gettimeoffset() anymore
      time/ virt-cpuacct         :  ok  |            HAVE_VIRT_CPU_ACCOUNTING #  arch supports precise virtual CPU time accounting
        vm/ ELF-ASLR             :  ok  |              ARCH_HAS_ELF_RANDOMIZE #  arch randomizes the stack, heap and binary images of ELF binaries
        vm/ huge-vmap            :  ok  |                 HAVE_ARCH_HUGE_VMAP #  arch supports the ioremap_pud_enabled() and ioremap_pmd_enabled() VM APIs
        vm/ ioremap_prot         :  ok  |                   HAVE_IOREMAP_PROT #  arch has ioremap_prot()
        vm/ numa-memblock        :  ok  |              HAVE_MEMBLOCK_NODE_MAP #  arch supports NUMA aware memblocks
        vm/ PG_uncached          :  ok  |               ARCH_USES_PG_UNCACHED #  arch supports the PG_uncached page flag
        vm/ pmdp_splitting_flush :  ok  |    __HAVE_ARCH_PMDP_SPLITTING_FLUSH #  arch supports the pmdp_splitting_flush() VM API
        vm/ pte_special          :  ok  |             __HAVE_ARCH_PTE_SPECIAL #  arch supports the pte_special()/pte_mkspecial() VM APIs
        vm/ THP                  :  ok  |      HAVE_ARCH_TRANSPARENT_HUGEPAGE #  arch supports transparent hugepages

Cc: <linux-api@vger.kernel.org>
Cc: <linux-arch@vger.kernel.org>
Cc: <linux-kernel@vger.kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/features/arch-support.txt

+
+For generic kernel features that need architecture support, the
+arch-support.txt file in each feature directory shows the arch
+support matrix, for all upstream Linux architectures.
+
+The meaning of entries in the tables is:
+
+    | ok |  # feature supported by the architecture
+    |TODO|  # feature not yet supported by the architecture
+    | .. |  # feature cannot be supported by the hardware
+

Documentation/features/list-arch.sh

+#
+# Small script that visualizes the kernel feature support status
+# of an architecture.
+#
+# (If no arguments are given then it will print the host architecture's status.)
+#
+
+ARCH=${1:-$(arch | sed 's/x86_64/x86/' | sed 's/i386/x86/')}
+
+cd $(dirname $0)
+echo "#"
+echo "# Kernel feature support matrix of the '$ARCH' architecture:"
+echo "#"
+
+for F in */*/arch-support.txt; do
+  SUBSYS=$(echo $F | cut -d/ -f1)
+  N=$(grep -h "^# Feature name:"        $F | cut -c25-)
+  C=$(grep -h "^#         Kconfig:"     $F | cut -c25-)
+  D=$(grep -h "^#         description:" $F | cut -c25-)
+  S=$(grep -hw $ARCH $F | cut -d\| -f3)
+
+  printf "%10s/%-22s:%s| %35s # %s\n" "$SUBSYS" "$N" "$S" "$C" "$D"
+done
+