From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jakub Kicinski Subject: [RFC 01/12] add basic register-field manipulation macros Date: Wed, 1 Jun 2016 17:50:03 +0100 Message-ID: <1464799814-4453-2-git-send-email-jakub.kicinski@netronome.com> References: <1464799814-4453-1-git-send-email-jakub.kicinski@netronome.com> Cc: ast@kernel.org, daniel@iogearbox.net, dinan.gunawardena@netronome.com, Jakub Kicinski , Ivo van Doorn , Felix Fietkau To: netdev@vger.kernel.org Return-path: Received: from mail-wm0-f49.google.com ([74.125.82.49]:37699 "EHLO mail-wm0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753052AbcFAQvo (ORCPT ); Wed, 1 Jun 2016 12:51:44 -0400 Received: by mail-wm0-f49.google.com with SMTP id z87so37551007wmh.0 for ; Wed, 01 Jun 2016 09:51:43 -0700 (PDT) In-Reply-To: <1464799814-4453-1-git-send-email-jakub.kicinski@netronome.com> Sender: netdev-owner@vger.kernel.org List-ID: C bitfields are problematic and best avoided. Developers interacting with hardware registers find themselves searching for easy-to-use alternatives. Common approach is to define structures or sets of macros containing mask and shift pair. Operation 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: field = FIELD_GET(REG_FIELD, reg); reg &= ~REG_FIELD; reg |= FIELD_PUT(REG_FIELD, field); FIELD_{GET,PUT} 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. Compared to Felix Fietkau's version I: - edited the comments a bit; - gave auxiliary macros _bf_ prefix; - dropped the UL specifier from 1 in _bf_low_bits, - renamed the FIELD_SET to FIELD_PUT; - added 64bit versions. CC: Ivo van Doorn CC: Felix Fietkau Signed-off-by: Jakub Kicinski Reviewed-by: Dinan Gunawardena Reviewed-by: Simon Horman --- include/linux/bitfield.h | 89 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 89 insertions(+) create mode 100644 include/linux/bitfield.h diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h new file mode 100644 index 000000000000..3815c81f5b10 --- /dev/null +++ b/include/linux/bitfield.h @@ -0,0 +1,89 @@ +/* + * Copyright (C) 2014 Felix Fietkau + * Copyright (C) 2004 - 2009 Ivo van Doorn + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License version 2 + * as published by the Free Software Foundation + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + */ + +#ifndef _LINUX_BITFIELD_H +#define _LINUX_BITFIELD_H + +#include + +/* + * Macros to find first set bit in a variable. + * These macros behave the same as the __ffs() functions but the most important + * difference that this is done during compile-time rather than run-time. + */ +#define compile_ffs2(__x) \ + __builtin_choose_expr(((__x) & 0x1), 0, 1) + +#define compile_ffs4(__x) \ + __builtin_choose_expr(((__x) & 0x3), \ + (compile_ffs2((__x))), \ + (compile_ffs2((__x) >> 2) + 2)) + +#define compile_ffs8(__x) \ + __builtin_choose_expr(((__x) & 0xf), \ + (compile_ffs4((__x))), \ + (compile_ffs4((__x) >> 4) + 4)) + +#define compile_ffs16(__x) \ + __builtin_choose_expr(((__x) & 0xff), \ + (compile_ffs8((__x))), \ + (compile_ffs8((__x) >> 8) + 8)) + +#define compile_ffs32(__x) \ + __builtin_choose_expr(((__x) & 0xffff), \ + (compile_ffs16((__x))), \ + (compile_ffs16((__x) >> 16) + 16)) + +#define compile_ffs64(__x) \ + __builtin_choose_expr(((__x) & 0xffffffff), \ + (compile_ffs32((__x))), \ + (compile_ffs32(((__x) >> 16) >> 16) + 32)) + +/* + * Macros to validate that the mask given contains a contiguous set of bits. + * Note that we cannot use the is_power_of_2() function since this check + * must be done at compile-time. + */ +#define _bf_is_power_of_two(x) ( !((x) & ((x) - 1)) ) +#define _bf_low_bits(x) ( ((x) - 1) & ~(x) ) +#define _bf_is_valid_mask(x) _bf_is_power_of_two(1U + (x) + _bf_low_bits(x)) + +#define _BF_FIELD_CHECK(_mask) \ + BUILD_BUG_ON(!(_mask) || !_bf_is_valid_mask(_mask)) + +#define FIELD_PUT(_mask, _val) \ + ({ \ + _BF_FIELD_CHECK(_mask); \ + (((u32)(_val)) << compile_ffs32(_mask)) & _mask; \ + }) + +#define FIELD_GET(_mask, _val) \ + ({ \ + _BF_FIELD_CHECK(_mask); \ + (u32)(((_val) & _mask) >> compile_ffs32(_mask)); \ + }) + +#define FIELD_PUT64(_mask, _val) \ + ({ \ + _BF_FIELD_CHECK(_mask); \ + (((u64)(_val)) << compile_ffs64(_mask)) & _mask; \ + }) + +#define FIELD_GET64(_mask, _val) \ + ({ \ + _BF_FIELD_CHECK(_mask); \ + (u64)(((_val) & _mask) >> compile_ffs64(_mask)); \ + }) + +#endif -- 1.9.1