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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-429c1125ad0sm3503532f8f.13.2025.10.31.05.45.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 31 Oct 2025 05:45:31 -0700 (PDT) Date: Fri, 31 Oct 2025 12:45:30 +0000 From: David Laight To: Andy Shevchenko Cc: Jonathan Cameron , linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org, Jyoti Bhayana , Jonathan Cameron , David Lechner , Nuno =?UTF-8?B?U8Oh?= , Andy Shevchenko Subject: Re: [PATCH v1 1/1] iio: common: scmi_sensors: Replace const_ilog2() with ilog2() Message-ID: <20251031124530.3db7805b@pumpkin> In-Reply-To: References: <20251031074500.3958667-1-andriy.shevchenko@linux.intel.com> <20251031094336.6f352b4f@pumpkin> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Fri, 31 Oct 2025 11:54:30 +0200 Andy Shevchenko wrote: > On Fri, Oct 31, 2025 at 09:43:36AM +0000, David Laight wrote: > > On Fri, 31 Oct 2025 08:45:00 +0100 > > Andy Shevchenko wrote: =20 >=20 > ... >=20 > > > tstamp_scale =3D sensor->sensor_info->tstamp_scale + > > > - const_ilog2(NSEC_PER_SEC) / const_ilog2(10); > > > + ilog2(NSEC_PER_SEC) / ilog2(10); =20 > >=20 > > Is that just a strange way of writing 9 ? =20 >=20 > Why? It's correct way of writing log=C2=B9=E2=81=B0(NSEC_PER_SEC), the pr= oblem here is that > "i" people do not think about :-) Even without the "i" the division could easily give 8.999999. So you'd be relying on rounding to get the required integral value. > But we have intlog10(), I completely forgot about it. And it isn't the function the code is looking for. (The result is shifted left 24 and it doesn't have an optimisation for constants.) >=20 > > Mathematically log2(x)/log2(10) is log10(x) - which would be 9. > > The code does seem to be 'in luck' though. > > NSEC_PER_SEC is 10^9 or 0x3b9aca00, so ilog2(NSEC_PER_SEC) is 29. > > ilog2(10) is 3, and 29/3 is 9. > >=20 > > Do the same for 10^10 and you get 11. =20 >=20 > That code looks like working by luck entirely, TBH. I just took the scope= of > the patch to start dropping const_ilog2() usages. Something always crawls out of the woodwork... David