linux-kernel - What is the best way to compare an unsigned and a constant?

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Message-ID: <20191227123938.5271-1-sjpark@amazon.com>
Date:   Fri, 27 Dec 2019 13:39:38 +0100
From:   SeongJae Park <sjpark@...zon.com>
To:     <brendanhiggins@...gle.com>
CC:     <linux-kselftest@...r.kernel.org>, <kunit-dev@...glegroups.com>,
        <linux-kernel@...r.kernel.org>, SeongJae Park <sj38.park@...il.com>
Subject: What is the best way to compare an unsigned and a constant?

Hello,


I have a function returning 'unsigned long', and would like to write a kunit
test for the function, as below.

    unsigned long foo(void)
    {
    	return 42;
    }

    static void foo_test(struct kunit *test)
    {
        KUNIT_EXPECT_EQ(test, 42, foo());
    }

However, this kunit gives me below warning for the above code:

    /.../linux/include/linux/kernel.h:842:29: warning: comparison of distinct pointer types lacks a cast
       (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
                                 ^
    /.../linux/include/kunit/test.h:493:9: note: in expansion of macro ‘__typecheck’
      ((void)__typecheck(__left, __right));           \
             ^~~~~~~~~~~
    /.../linux/include/kunit/test.h:517:2: note: in expansion of macro ‘KUNIT_BASE_BINARY_ASSERTION’
      KUNIT_BASE_BINARY_ASSERTION(test,           \
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~
    /.../linux/include/kunit/test.h:606:2: note: in expansion of macro ‘KUNIT_BASE_EQ_MSG_ASSERTION’
      KUNIT_BASE_EQ_MSG_ASSERTION(test,           \
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~
    /.../linux/include/kunit/test.h:616:2: note: in expansion of macro ‘KUNIT_BINARY_EQ_MSG_ASSERTION’
      KUNIT_BINARY_EQ_MSG_ASSERTION(test,           \
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /.../linux/include/kunit/test.h:979:2: note: in expansion of macro ‘KUNIT_BINARY_EQ_ASSERTION’
      KUNIT_BINARY_EQ_ASSERTION(test, KUNIT_EXPECTATION, left, right)
      ^~~~~~~~~~~~~~~~~~~~~~~~~
    /.../linux/mm/foo-test.h:565:2: note: in expansion of macro ‘KUNIT_EXPECT_EQ’
      KUNIT_EXPECT_EQ(test, 42, foo());
      ^~~~~~~~~~~~~~~

I could remove the warning by explicitly type casting the constant as below:

        KUNIT_EXPECT_EQ(test, (unsigned long)42, foo());

However, now 'checkpatch.pl' complains about the type casting as below.

    WARNING: Unnecessary typecast of c90 int constant
    #565: FILE: mm/foo-test.h:565:
    +       KUNIT_EXPECT_EQ(test, (unsigned long)42, foo());

Of course, there could be several work-arounds for these warnings, such as
using 'EXPECT_TRUE(test, 42 == foo())' or casting the function's return value.
Nonetheless, I'm not sure what is the right way.  Could you please let me know
what is the recommended way for this case?


Thanks,
SeongJae Park