MIPS: memset.S: Add comments to fault fixup handlers

It is not immediately obvious what the expected inputs to these fault
handlers is and how they calculate the number of unset bytes. Having
stared deeply at this in order to fix some corner cases, add some
comments to assist those who follow.

Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/19339/
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: <linux-mips@linux-mips.org>
Cc: <linux-kernel@vger.kernel.org>

arch/mips/lib/memset.S

@@ -232,16 +232,25 @@
 
 #ifdef CONFIG_CPU_MIPSR6
 .Lbyte_fixup\@:
+	/*
+	 * unset_bytes = (#bytes - (#unaligned bytes)) - (-#unaligned bytes remaining + 1) + 1
+	 *      a2     =             a2                -              t0                   + 1
+	 */
 	PTR_SUBU	a2, t0
 	jr		ra
 	 PTR_ADDIU	a2, 1
 #endif /* CONFIG_CPU_MIPSR6 */
 
 .Lfirst_fixup\@:
+	/* unset_bytes already in a2 */
 	jr	ra
 	 nop
 
 .Lfwd_fixup\@:
+	/*
+	 * unset_bytes = partial_start_addr +  #bytes   -     fault_addr
+	 *      a2     =         t1         + (a2 & 3f) - $28->task->BUADDR
+	 */
 	PTR_L		t0, TI_TASK($28)
 	andi		a2, 0x3f
 	LONG_L		t0, THREAD_BUADDR(t0)
@@ -250,6 +259,10 @@
 	 LONG_SUBU	a2, t0
 
 .Lpartial_fixup\@:
+	/*
+	 * unset_bytes = partial_end_addr +      #bytes     -     fault_addr
+	 *      a2     =       a0         + (a2 & STORMASK) - $28->task->BUADDR
+	 */
 	PTR_L		t0, TI_TASK($28)
 	andi		a2, STORMASK
 	LONG_L		t0, THREAD_BUADDR(t0)
@@ -258,10 +271,15 @@
 	 LONG_SUBU	a2, t0
 
 .Llast_fixup\@:
+	/* unset_bytes already in a2 */
 	jr		ra
 	 nop
 
 .Lsmall_fixup\@:
+	/*
+	 * unset_bytes = end_addr - current_addr + 1
+	 *      a2     =    t1    -      a0      + 1
+	 */
 	PTR_SUBU	a2, t1, a0
 	jr		ra
 	 PTR_ADDIU	a2, 1