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testevdev: Explain why the test data is encoded the way it is 

Signed-off-by: Simon McVittie <smcv@collabora.com>
This commit is contained in:
Simon McVittie 2022-10-12 12:41:30 +01:00 committed by Ozkan Sezer
parent 81dee31949
commit 7d230af51d
1 changed files with 14 additions and 1 deletions
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15
test/testevdev.c
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@ -1,6 +1,6 @@
/*
Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
Copyright (C) 2020 Collabora Ltd.
Copyright (C) 2020-2022 Collabora Ltd.
This software is provided 'as-is', without any express or implied
warranty. In no event will the authors be held liable for any damages
@ -935,6 +935,19 @@ static const GuessTest guess_tests[] =
}
};
/* The Linux kernel provides capability info in EVIOCGBIT and in /sys
* as an array of unsigned long in native byte order, rather than an array
* of bytes, an array of native-endian 32-bit words or an array of
* native-endian 64-bit words like you might have reasonably expected.
* The order of words in the array is always lowest-valued first: for
* instance, the first unsigned long in abs[] contains the bit representing
* absolute axis 0 (ABS_X).
*
* The constant arrays above provide test data in little-endian, because
* that's the easiest representation for hard-coding into a test like this.
* On a big-endian platform we need to byteswap it, one unsigned long at a
* time, to match what the kernel would produce. This requires us to choose
* an appropriate byteswapping function for the architecture's word size. */
SDL_COMPILE_TIME_ASSERT(sizeof_long, sizeof(unsigned long) == 4 || sizeof(unsigned long) == 8);
#define SwapLongLE(X) \
((sizeof(unsigned long) == 4) ? SDL_SwapLE32(X) : SDL_SwapLE64(X))