From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Borkmann Subject: Re: [RFCv2 01/16] add basic register-field manipulation macros Date: Mon, 29 Aug 2016 16:34:25 +0200 Message-ID: <57C447F1.60807@iogearbox.net> References: <1472234775-29453-1-git-send-email-jakub.kicinski@netronome.com> <1472234775-29453-2-git-send-email-jakub.kicinski@netronome.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: ast@kernel.org, dinan.gunawardena@netronome.com, jiri@resnulli.us, john.fastabend@gmail.com, kubakici@wp.pl To: Jakub Kicinski , netdev@vger.kernel.org Return-path: Received: from www62.your-server.de ([213.133.104.62]:40769 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756135AbcH2Oe3 (ORCPT ); Mon, 29 Aug 2016 10:34:29 -0400 In-Reply-To: <1472234775-29453-2-git-send-email-jakub.kicinski@netronome.com> Sender: netdev-owner@vger.kernel.org List-ID: On 08/26/2016 08:06 PM, Jakub Kicinski wrote: > 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 [...] > + * Bitfield access macros > + * > + * FIELD_{GET,PREP} macros take as first parameter shifted mask > + * from which they extract the base mask and shift amount. > + * Mask must be a compilation time constant. > + * > + * Example: > + * > + * #define REG_FIELD_A GENMASK(6, 0) > + * #define REG_FIELD_B BIT(7) > + * #define REG_FIELD_C GENMASK(15, 8) > + * #define REG_FIELD_D GENMASK(31, 16) > + * > + * Get: > + * a = FIELD_GET(REG_FIELD_A, reg); > + * b = FIELD_GET(REG_FIELD_B, reg); > + * > + * Set: > + * reg = FIELD_PREP(REG_FIELD_A, 1) | > + * FIELD_PREP(REG_FIELD_B, 0) | > + * FIELD_PREP(REG_FIELD_C, c) | > + * FIELD_PREP(REG_FIELD_D, 0x40); > + * > + * Modify: > + * reg &= ~REG_FIELD_C; > + * reg |= FIELD_PREP(REG_FIELD_C, c); > + */ > + > +#define _bf_shf(x) (__builtin_ffsll(x) - 1) > + > +#define _BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \ Nit: if possible, please always use "__" instead of "_" as prefix, which is more common coding style in the kernel. > + ({ \ > + BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask), \ > + _pfx "mask is not constant"); \ > + BUILD_BUG_ON_MSG(!(_mask), _pfx "mask is zero"); \ > + BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \ > + ~((_mask) >> _bf_shf(_mask)) & (_val) : 0, \ > + _pfx "value too large for the field"); \ > + BUILD_BUG_ON_MSG((_mask) > (typeof(_reg))~0ull, \ > + _pfx "type of reg too small for mask"); \ > + __BUILD_BUG_ON_NOT_POWER_OF_2((_mask) + \ > + (1ULL << _bf_shf(_mask))); \ > + }) > + > +/** > + * FIELD_PREP() - prepare a bitfield element > + * @_mask: shifted mask defining the field's length and position > + * @_val: value to put in the field > + * > + * FIELD_PREP() masks and shifts up the value. The result should > + * be combined with other fields of the bitfield using logical OR. > + */ > +#define FIELD_PREP(_mask, _val) \ > + ({ \ > + _BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \ > + ((typeof(_mask))(_val) << _bf_shf(_mask)) & (_mask); \ > + }) > + > +/** > + * FIELD_GET() - extract a bitfield element > + * @_mask: shifted mask defining the field's length and position > + * @_reg: 32bit value of entire bitfield > + * > + * FIELD_GET() extracts the field specified by @_mask from the > + * bitfield passed in as @_reg by masking and shifting it down. > + */ > +#define FIELD_GET(_mask, _reg) \ > + ({ \ > + _BF_FIELD_CHECK(_mask, _reg, 0U, "FIELD_GET: "); \ > + (typeof(_mask))(((_reg) & (_mask)) >> _bf_shf(_mask)); \ > + }) No strong opinion, but FIELD_PREP() sounds a bit weird. Maybe rather a FIELD_GEN() (aka "generate") and FIELD_GET() pair? > +#endif > diff --git a/include/linux/bug.h b/include/linux/bug.h > index e51b0709e78d..292d6a10b0c2 100644 > --- a/include/linux/bug.h > +++ b/include/linux/bug.h > @@ -13,6 +13,7 @@ enum bug_trap_type { > struct pt_regs; > > #ifdef __CHECKER__ > +#define __BUILD_BUG_ON_NOT_POWER_OF_2(n) (0) > #define BUILD_BUG_ON_NOT_POWER_OF_2(n) (0) > #define BUILD_BUG_ON_ZERO(e) (0) > #define BUILD_BUG_ON_NULL(e) ((void*)0) > @@ -24,6 +25,8 @@ struct pt_regs; > #else /* __CHECKER__ */ > > /* Force a compilation error if a constant expression is not a power of 2 */ > +#define __BUILD_BUG_ON_NOT_POWER_OF_2(n) \ > + BUILD_BUG_ON(((n) & ((n) - 1)) != 0) Is there a reason BUILD_BUG_ON_NOT_POWER_OF_2(n) cannot be reused? Because the (n) == 0 check would trigger (although it shouldn't ...)? > #define BUILD_BUG_ON_NOT_POWER_OF_2(n) \ > BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0)) > >