From mboxrd@z Thu Jan 1 00:00:00 1970 From: marc_gonzalez@sigmadesigns.com (Marc Gonzalez) Date: Thu, 16 Nov 2017 17:42:36 +0100 Subject: [RFC] Improving udelay/ndelay on platforms where that is possible In-Reply-To: <20171116163254.GK31757@n2100.armlinux.org.uk> References: <4b707ce0-6067-ab36-e167-1acf348d26bf@free.fr> <11393e07-b042-180c-3bcd-484bf51eada6@sigmadesigns.com> <20171115131351.GE31757@n2100.armlinux.org.uk> <1fa81694-7bd2-564b-e5b9-ae53b9ea6620@sigmadesigns.com> <20171116153625.GJ31757@n2100.armlinux.org.uk> <9a4cfa9d-3940-b7f2-5a4d-59e89af85bb7@sigmadesigns.com> <48c38055-20f7-e565-aa56-74f360e6e3d9@sigmadesigns.com> <20171116163254.GK31757@n2100.armlinux.org.uk> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 16/11/2017 17:32, Russell King - ARM Linux wrote: > On Thu, Nov 16, 2017 at 05:26:32PM +0100, Marc Gonzalez wrote: >> On 16/11/2017 17:08, Nicolas Pitre wrote: >> >>> On Thu, 16 Nov 2017, Marc Gonzalez wrote: >>> >>>> On 16/11/2017 16:36, Russell King - ARM Linux wrote: >>>>> On Thu, Nov 16, 2017 at 04:26:51PM +0100, Marc Gonzalez wrote: >>>>>> On 15/11/2017 14:13, Russell King - ARM Linux wrote: >>>>>> >>>>>>> udelay() needs to offer a consistent interface so that drivers know >>>>>>> what to expect no matter what the implementation is. Making one >>>>>>> implementation conform to your ideas while leaving the other >>>>>>> implementations with other expectations is a recipe for bugs. >>>>>>> >>>>>>> If you really want to do this, fix the loops_per_jiffy implementation >>>>>>> as well so that the consistency is maintained. >>>>>> >>>>>> Hello Russell, >>>>>> >>>>>> It seems to me that, when using DFS, there's a serious issue with loop-based >>>>>> delays. (IIRC, it was you who pointed this out a few years ago.) >>>>>> >>>>>> If I'm reading arch/arm/kernel/smp.c correctly, loops_per_jiffy is scaled >>>>>> when the frequency changes. >>>>>> >>>>>> But arch/arm/lib/delay-loop.S starts by loading the current value of >>>>>> loops_per_jiffy, computes the number of times to loop, and then loops. >>>>>> If the frequency increases when the core is in __loop_delay, the >>>>>> delay will be much shorter than requested. >>>>>> >>>>>> Is this a correct assessment of the situation? >>>>> >>>>> Absolutely correct, and it's something that people are aware of, and >>>>> have already catered for while writing their drivers. >>>> >>>> In their cpufreq driver? >>>> In "real" device drivers that happen to use delays? >>>> >>>> On my system, the CPU frequency may ramp up from 120 MHz to 1.2 GHz. >>>> If the frequency increases at the beginning of __loop_delay, udelay(100) >>>> would spin only 10 microseconds. This is likely to cause issues in >>>> any driver using udelay. >>>> >>>> How does one cater for that? >>> >>> You make sure your delays are based on a stable hardware timer. >>> Most platforms nowadays should have a suitable timer source. >> >> So you propose fixing loop-based delays by using clock-based delays, >> is that correct? (That is indeed what I did on my platform.) >> >> Russell stated that there are platforms using loop-based delays with >> cpufreq enabled. I'm asking how they manage the brokenness. > > Quite simply, they don't have such a wide range of frequencies that can > be selected. They're on the order of 4x. For example, the original > platform that cpufreq was developed on, a StrongARM-1110 board, can > practically range from 221MHz down to 59MHz. Requesting 100 ?s and spinning only 25 ?s is still a problem, don't you agree? > BTW, your example above is incorrect. A 10x increase in frequency causes a request of 100 ?s to spin only 10 ?s, as written above. The problem is not when the frequency drops -- this makes the delay longer. The problem is when the frequency increases, which makes the delay shorter. Regards.