Linux Kernel https://git.stealer.net/cgit.cgi/user/sven/linux.git/atom?h=v4.9.272 2016-09-09T09:09:24Z 2016-09-09T09:09:24Z Jakub Kicinski jakub.kicinski@netronome.com 2016-08-31T11:46:44Z urn:sha1:3e9b3112ec74f192eaab976c3889e34255cae940 Common approach to accessing register fields is to define structures or sets of macros containing mask and shift pair. Operations on the register are then performed as follows: field = (reg >> shift) & mask; reg &= ~(mask << shift); reg |= (field & mask) << shift; Defining shift and mask separately is tedious. Ivo van Doorn came up with an idea of computing them at compilation time based on a single shifted mask (later refined by Felix) which can be used like this: #define REG_FIELD 0x000ff000 field = FIELD_GET(REG_FIELD, reg); reg &= ~REG_FIELD; reg |= FIELD_PREP(REG_FIELD, field); FIELD_{GET,PREP} macros take care of finding out what the appropriate shift is based on compilation time ffs operation. GENMASK can be used to define registers (which is usually less error-prone and easier to match with datasheets). This approach is the most convenient I've seen so to limit code multiplication let's move the macros to a global header file. Attempts to use static inlines instead of macros failed due to false positive triggering of BUILD_BUG_ON()s, especially with GCC < 6.0. Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com> Reviewed-by: Dinan Gunawardena <dinan.gunawardena@netronome.com> Signed-off-by: Kalle Valo <kvalo@codeaurora.org>