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Date: Wed, 1 Nov 2017 20:28:18 +0100 From: Marc Gonzalez <marc_gonzalez@...madesigns.com> To: Russell King - ARM Linux <linux@...linux.org.uk> Cc: Doug Anderson <dianders@...omium.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Mark Rutland <mark.rutland@....com>, Jonathan Austin <jonathan.austin@....com>, Arnd Bergmann <arnd@...db.de>, Peter Zijlstra <peterz@...radead.org>, Will Deacon <will.deacon@....com>, Michael Turquette <mturquette@...libre.com>, Nicolas Pitre <nico@...aro.org>, Stephen Boyd <sboyd@...eaurora.org>, Steven Rostedt <rostedt@...dmis.org>, LKML <linux-kernel@...r.kernel.org>, Kevin Hilman <khilman@...nel.org>, John Stultz <john.stultz@...aro.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, Mason <slash.tmp@...e.fr> Subject: Re: [RFC] Improving udelay/ndelay on platforms where that is possible On 01/11/2017 10:26, Russell King - ARM Linux wrote: > On Tue, Oct 31, 2017 at 05:23:19PM -0700, Doug Anderson wrote: > >> On Tue, Oct 31, 2017 at 10:45 AM, Linus Torvalds wrote: >> >>> So I'm very much open to udelay improvements, and if somebody sends >>> patches for particular platforms to do particularly well on that >>> platform, I think we should merge them. But ... >> >> If I'm reading this all correctly, this sounds like you'd be willing >> to merge <https://patchwork.kernel.org/patch/9429841/>. This makes >> udelay() guaranteed not to underrun on arm32 platforms. > > That's a mis-representation again. It stops a timer-based udelay() > possibly underrunning by one tick if we are close to the start of > a count increment. However, it does nothing for the loops_per_jiffy > udelay(), which can still underrun. It is correct that improving the clock-based implementation does strictly nothing for the loop-based implementation. Is it possible to derive a higher bound on the amount of under-run when using the loop-based delay on arm32? > My argument against merging that patch is that with it merged, we get > (as you say) a udelay() that doesn't underrun _when using a timer_ > but when we end up using the loops_per_jiffy udelay(), we're back to > the old problem. > > My opinion is that's bad, because it encourages people to write drivers > that rely on udelay() having "good" behaviour, which it is not guaranteed > to have. So, they'll specify a delay period of exactly what they want, > and their drivers will then fail when running on systems that aren't > using a timer-based udelay(). > > If we want udelay() to have this behaviour, it needs to _always_ have > this behaviour irrespective of the implementation. So that means > the loops_per_jiffy version also needs to be fixed in the same way, > which IMHO is impossible. Let's say some piece of HW absolutely, positively, unequivocally, uncompromisingly, requires a strict minimum of 10 microseconds elapsing between operations A and B. You say a driver writer must not write udelay(10); They have to take into account the possibility of under-delay. How much additional delay should they add? 10%? 20%? 50%? A percentage + a fixed quantity? If there is an actual rule, then it could be incorporated in the loop-based implementation? If it is impossible to say (as Linus hinted for some platforms) then this means there is no way to guarantee a minimal delay? Regards.
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