minmax.h: update some comments

- Change three to several.
- Remove the comment about retaining constant expressions, no longer true.
- Realign to nearer 80 columns and break on major punctiation.
- Add a leading comment to the block before __signed_type() and __is_nonneg()
  Otherwise the block explaining the cast is a bit 'floating'.
  Reword the rest of that comment to improve readability.

Link: https://lkml.kernel.org/r/85b050c81c1d4076aeb91a6cded45fee@AcuMS.aculab.com
Signed-off-by: David Laight <david.laight@aculab.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: david-laight

include/linux/minmax.h

@@ -8,13 +8,10 @@
 #include <linux/types.h>
 
 /*
- * min()/max()/clamp() macros must accomplish three things:
+ * min()/max()/clamp() macros must accomplish several things:
  *
  * - Avoid multiple evaluations of the arguments (so side-effects like
  *   "x++" happen only once) when non-constant.
- * - Retain result as a constant expressions when called with only
- *   constant expressions (to avoid tripping VLA warnings in stack
- *   allocation usage).
  * - Perform signed v unsigned type-checking (to generate compile
  *   errors instead of nasty runtime surprises).
  * - Unsigned char/short are always promoted to signed int and can be
@@ -31,45 +28,43 @@
  *   bit #0 set if ok for unsigned comparisons
  *   bit #1 set if ok for signed comparisons
  *
- * In particular, statically non-negative signed integer
- * expressions are ok for both.
+ * In particular, statically non-negative signed integer expressions
+ * are ok for both.
  *
- * NOTE! Unsigned types smaller than 'int' are implicitly
- * converted to 'int' in expressions, and are accepted for
- * signed conversions for now. This is debatable.
+ * NOTE! Unsigned types smaller than 'int' are implicitly converted to 'int'
+ * in expressions, and are accepted for signed conversions for now.
+ * This is debatable.
  *
- * Note that 'x' is the original expression, and 'ux' is
- * the unique variable that contains the value.
+ * Note that 'x' is the original expression, and 'ux' is the unique variable
+ * that contains the value.
  *
- * We use 'ux' for pure type checking, and 'x' for when
- * we need to look at the value (but without evaluating
- * it for side effects! Careful to only ever evaluate it
- * with sizeof() or __builtin_constant_p() etc).
+ * We use 'ux' for pure type checking, and 'x' for when we need to look at the
+ * value (but without evaluating it for side effects!
+ * Careful to only ever evaluate it with sizeof() or __builtin_constant_p() etc).
  *
- * Pointers end up being checked by the normal C type
- * rules at the actual comparison, and these expressions
- * only need to be careful to not cause warnings for
- * pointer use.
+ * Pointers end up being checked by the normal C type rules at the actual
+ * comparison, and these expressions only need to be careful to not cause
+ * warnings for pointer use.
  */
 #define __signed_type_use(x, ux) (2 + __is_nonneg(x, ux))
 #define __unsigned_type_use(x, ux) (1 + 2 * (sizeof(ux) < 4))
 #define __sign_use(x, ux) (is_signed_type(typeof(ux)) ? \
 	__signed_type_use(x, ux) : __unsigned_type_use(x, ux))
 
 /*
- * To avoid warnings about casting pointers to integers
- * of different sizes, we need that special sign type.
+ * Check whether a signed value is always non-negative.
  *
- * On 64-bit we can just always use 'long', since any
- * integer or pointer type can just be cast to that.
+ * A cast is needed to avoid any warnings from values that aren't signed
+ * integer types (in which case the result doesn't matter).
  *
- * This does not work for 128-bit signed integers since
- * the cast would truncate them, but we do not use s128
- * types in the kernel (we do use 'u128', but they will
- * be handled by the !is_signed_type() case).
+ * On 64-bit any integer or pointer type can safely be cast to 'long'.
+ * But on 32-bit we need to avoid warnings about casting pointers to integers
+ * of different sizes without truncating 64-bit values so 'long' or 'long long'
+ * must be used depending on the size of the value.
  *
- * NOTE! The cast is there only to avoid any warnings
- * from when values that aren't signed integer types.
+ * This does not work for 128-bit signed integers since the cast would truncate
+ * them, but we do not use s128 types in the kernel (we do use 'u128',
+ * but they are handled by the !is_signed_type() case).
  */
 #ifdef CONFIG_64BIT
   #define __signed_type(ux) long