From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kalle Valo Date: Wed, 24 Nov 2021 10:24:02 +0200 Subject: [PATCH 01/17] bitfield: Add non-constant field_{prep, get}() helpers In-Reply-To: (Geert Uytterhoeven's message of "Tue, 23 Nov 2021 09:30:14 +0100") References: <3a54a6703879d10f08cf0275a2a69297ebd2b1d4.1637592133.git.geert+renesas@glider.be> <01b44b38c087c151171f8d45a2090474c2559306.camel@sipsolutions.net> Message-ID: <87sfvm55ct.fsf@codeaurora.org> List-Id: To: linux-aspeed@lists.ozlabs.org MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Geert Uytterhoeven writes: > Hi Johannes, > > On Mon, Nov 22, 2021 at 5:33 PM Johannes Berg wrote: >> On Mon, 2021-11-22 at 16:53 +0100, Geert Uytterhoeven wrote: >> > The existing FIELD_{GET,PREP}() macros are limited to compile-time >> > constants. However, it is very common to prepare or extract bitfield >> > elements where the bitfield mask is not a compile-time constant. >> > >> >> I'm not sure it's really a good idea to add a third API here? >> >> We have the upper-case (constant) versions, and already >> {u32,...}_get_bits()/etc. > > These don't work for non-const masks. > >> Also, you're using __ffs(), which doesn't work for 64-bit on 32-bit >> architectures (afaict), so that seems a bit awkward. > > That's a valid comment. Can be fixed by using a wrapper macro > that checks if typeof(mask) == u64, and uses an __ffs64() version when > needed. > >> Maybe we can make {u32,...}_get_bits() be doing compile-time only checks >> if it is indeed a constant? The __field_overflow() usage is already only >> done if __builtin_constant_p(v), so I guess we can do the same with >> __bad_mask()? > > Are all compilers smart enough to replace the division by > field_multiplier(field) by a shift? It looks like the answer is no as few weeks back I received a comment internally that a team is seeing a slow down with u32_get_bits(): "Time taken for executing both the macros/inline function (in terms of microseconds) (out of 3 Trails) FIELD_GET : 32, 31, 32 u32_get_bits : 6379, 6664, 6558" Sadly I didn't realise to ask what compiler they were using. But I still prefer {u32,...}_get_bits() over FIELD_GET(), they are just so much cleaner to use. -- https://patchwork.kernel.org/project/linux-wireless/list/ https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches