linux-kernel - Re: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO

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Date:   Wed, 25 Oct 2023 17:05:13 +0200
From:   Vasily Gorbik <gor@...ux.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Sebastian Reichel <sebastian.reichel@...labora.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        Niklas Schnelle <schnelle@...ux.ibm.com>, kernel@...labora.com
Subject: Re: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO_OVERFLOW

On Tue, Oct 24, 2023 at 12:53:03PM -1000, Linus Torvalds wrote:
>+/*
>+ * The other cases need to be split by type.
>+ *
>+ * Signed cases seem badly defined, but do exist. We should
>+ * consider removing them from this _Generic(), and fixing any
>+ * result 'not compatible with any association' cases.
>+ */
>+#define __div_round_up(n,d) _Generic((n)+(d),		\
>+	unsigned long long: __div_round_up_ull(n,d),	\
>+	long long: __div_round_up_ll(n,d),		\
>+	unsigned long: __div_round_up_ul(n,d),		\
>+	long: __div_round_up_l(n,d),			\
>+	unsigned int: __div_round_up_u(n,d),		\
>+	int: __div_round_up_i(n,d))

You probably want

 #define __div_round_up(n,d) _Generic((n)+(d),		\
	unsigned long long: __div_round_up_ull,		\
	long long: __div_round_up_ll,			\
	unsigned long: __div_round_up_ul,		\
	long: __div_round_up_l,				\
	unsigned int: __div_round_up_u,			\
	int: __div_round_up_i)(n,d)

to avoid early type-checking for expressions that will be discarded
and prevent errors like:

drivers/net/ethernet/microchip/sparx5/sparx5_tc_flower.c: In function 'sparx5_tc_flower_parse_act_police':
drivers/net/ethernet/microchip/sparx5/sparx5_main.h:435:34: error: conversion from 'long long unsigned int' to 'long unsigned int' changes value from '25000000000' to '3525163520' [-Werror=overflow]
  435 | #define SPX5_SDLB_GROUP_RATE_MAX 25000000000ULL
      |                                  ^~~~~~~~~~~~~~
./include/linux/div_round_up.h:68:42: note: in definition of macro '__div_round_up'
   68 |         unsigned long: __div_round_up_ul(n,d),          \
      |                                          ^
./include/linux/div_round_up.h:50:9: note: in expansion of macro '__keep_const'
   50 |         __keep_const(n, div_round_up(n,d))
      |         ^~~~~~~~~~~~
./include/linux/math.h:37:22: note: in expansion of macro '__KERNEL_DIV_ROUND_UP'
   37 | #define DIV_ROUND_UP __KERNEL_DIV_ROUND_UP
      |                      ^~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/microchip/sparx5/sparx5_tc_flower.c:732:38: note: in expansion of macro 'SPX5_SDLB_GROUP_RATE_MAX'
  732 |         if (pol->rate > DIV_ROUND_UP(SPX5_SDLB_GROUP_RATE_MAX, 1000)) {
      |                                      ^~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/microchip/sparx5/sparx5_main.h:435:34: error: overflow in conversion from 'long long unsigned int' to 'long int' changes value from '25000000000' to '-769803776' [-Werror=overflow]
...
drivers/net/ethernet/microchip/sparx5/sparx5_main.h:435:34: error: conversion from 'long long unsigned int' to 'unsigned int' changes value from '25000000000' to '3525163520' [-Werror=overflow]
...
drivers/net/ethernet/microchip/sparx5/sparx5_main.h:435:34: error: overflow in conversion from 'long long unsigned int' to 'int' changes value from '25000000000' to '-769803776' [-Werror=overflow]

Plus typos fixes below passes allyesconfig for s390, 32-bit x86 and arm.

 static inline unsigned long long __div_round_up_ull(unsigned long long n, unsigned long d)
 {
 #ifdef CONFIG_32BIT
 	if (!n)
-		return 0
-	do_div(n-1, d);
-	return n+1;
+		return 0;
+	n--;
+	do_div(n, d);
+	return n + 1;
 #else
 	return __const_div_round_up(n,d);
 #endif