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[72.189.64.225]) by smtp.gmail.com with ESMTPSA id b10sm863066qkl.19.2020.04.24.08.09.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Apr 2020 08:09:39 -0700 (PDT) Date: Fri, 24 Apr 2020 11:09:26 -0400 From: William Breathitt Gray To: Lukas Wunner Cc: Syed Nayyar Waris , akpm@linux-foundation.org, andriy.shevchenko@linux.intel.com, arnd@arndb.de, Linus Walleij , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/6] bitops: Introduce the the for_each_set_clump macro Message-ID: <20200424150828.GA5034@icarus> References: <20200424122521.GA5552@syed> <20200424141037.ersebbfe7xls37be@wunner.de> <20200424150058.xadjxaga3csh3br6@wunner.de> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="ylS2wUBXLOxYXZFQ" Content-Disposition: inline In-Reply-To: <20200424150058.xadjxaga3csh3br6@wunner.de> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --ylS2wUBXLOxYXZFQ Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Apr 24, 2020 at 05:00:58PM +0200, Lukas Wunner wrote: > On Fri, Apr 24, 2020 at 08:22:38PM +0530, Syed Nayyar Waris wrote: > > On Fri, Apr 24, 2020 at 7:40 PM Lukas Wunner wrote: > > > > > > On Fri, Apr 24, 2020 at 05:55:21PM +0530, Syed Nayyar Waris wrote: > > > > +static inline void bitmap_set_value(unsigned long *map, > > > > + unsigned long value, > > > > + unsigned long start, unsigned lon= g nbits) > > > > +{ > > > > + const size_t index =3D BIT_WORD(start); > > > > + const unsigned long offset =3D start % BITS_PER_LONG; > > > > + const unsigned long ceiling =3D roundup(start + 1, BITS_PER_L= ONG); > > > > + const unsigned long space =3D ceiling - start; > > > > + > > > > + value &=3D GENMASK(nbits - 1, 0); > > > > + > > > > + if (space >=3D nbits) { > > > > + map[index] &=3D ~(GENMASK(nbits + offset - 1, offset)= ); > > > > + map[index] |=3D value << offset; > > > > + } else { > > > > + map[index] &=3D ~BITMAP_FIRST_WORD_MASK(start); > > > > + map[index] |=3D value << offset; > > > > + map[index + 1] &=3D ~BITMAP_LAST_WORD_MASK(start + nb= its); > > > > + map[index + 1] |=3D (value >> space); > > > > + } > > > > +} > > > > > > Sorry but what's the advantage of using this complicated function > > > as a replacement for the much simpler bitmap_set_value8()? > > > > > > The drivers calling bitmap_set_value8() *know* that 8-bit accesses > > > are possible and take advantage of that knowledge by using a small, > > > speed-optimized function. Replacing that with a more complicated > > > (potentially less performant) function doesn't seem to be a step > > > forward. > >=20 > > Actually this generic function can work with n-bits of any size (less > > than equal to BITS_PER_LONG), while the earlier bitmap_set_value8 > > worked with n-bits having size of 8 bits only. > >=20 > > In the case when n-bits is 8-bits, this new bitmap_set_value() > > function would behave very similar to the earlier bitmap_set_value8() > > function. For example, in case of n-bits being 8-bits it will always > > execute the 'if' condition and not the 'else' condition, hence > > offering the same performance (because of encountering similar code > > statements) as earlier bitmap_set_value8() function, most probably. > >=20 > > There is an additional advantage (this can happen when n-bits is not 8 > > bits): during setting value of n-bit in bitmap, if a situation arise > > that the width of next n-bit is exceeding the word boundary, then it > > will divide itself such that some portion of it is stored in that > > word, while the remaining portion is stored in the next higher word. > >=20 > > So, this function preserves the behaviour of earlier > > bitmap_set_value8() function and also adds extra functionality to > > that. >=20 > Please leave drivers as is which use exclusively 8-bit accesses, > e.g. gpio-max3191x.c and gpio-74x164.c. I'm fearing a performance > regression if your new generic variant is used. They work perfectly > fine the way they are and I don't see any benefit this series may have > for them. >=20 > If there are other drivers which benefit from the flexibility of your > generic variant then I'm not opposed to changing those. >=20 > Thanks, >=20 > Lukas We can leave of course bitmap_set_value8 alone, but for 8-bit values the difference in latency I suspect is primarily due to the conditional test for the word boundaries. This latency is surely overshadowed by the I/O latency of the GPIO drivers, so I don't think there's much harm in changing those to use the generic function when the bottleneck will not be due to the bitmap_set_value/bitmap_get_value operations. 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