From mboxrd@z Thu Jan 1 00:00:00 1970 From: shinya.kuribayashi.px@renesas.com (Shinya Kuribayashi) Date: Tue, 17 Jul 2012 12:10:06 +0900 Subject: [PATCH v2 2/2] ARM: delay: allow timer-based delay implementation to be selected In-Reply-To: <20120713111337.GH18079@mudshark.cambridge.arm.com> References: <1340991231-17682-1-git-send-email-will.deacon@arm.com> <1340991231-17682-3-git-send-email-will.deacon@arm.com> <4FFE7DB2.4040702@renesas.com> <20120712084432.GA2816@mudshark.cambridge.arm.com> <4FFE9A69.3060301@renesas.com> <4FFEFDE3.5000403@codeaurora.org> <4FFF8509.2050302@renesas.com> <20120713085746.GA18079@mudshark.cambridge.arm.com> <20120713111337.GH18079@mudshark.cambridge.arm.com> Message-ID: <5004D78E.4050606@renesas.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Will, Stephen and Santosh, On 7/13/2012 8:13 PM, Will Deacon wrote: > I was anticipating that the platform would set the initial loops_per_jiffy > value if it requires udelays before loop calibration and the default of 4k > is wildly off. I overlooked two different lpj setups were involved at hand. First one was, the initial loops_per_jiffy value of 4k was too small for almost all processors running Linux today, so I set up loops_per_jiffy _early_, calculated from the CPU clock speed. I didn't mentioned this before, sorry for confusion. So my initial loops_per_jiffy is not 4k at this point. It's optimized for loop-based delay with the CPU running at 1.2GHz (much bigger than default 4k). And later, init_current_timer_delay() got processed. Actual udelay() behavior switched from loop-based delay to timer-based one immediately, while my loops_per_jiffy was not changed/updated to appropriate value. This is why my udelay()s, used after init_current_timer_delay(), were taking considerable long time to expire. Note that my initial tests for Will's patchset was done using a loadable module dedicated for udelay tests, that was prepared for 2.6.35/3.0 kernels beforehand. And this time, I confirmed that updating loops_per_jiffy at the same time as lpj_fine, works perfectly as expected for me. > If people need loops_per_jiffy to be updated at the same time as lpj_fine, > I can post that as a separate patch (below) as Russell has merged v2 of these > patches into his delay branch. That said, I'd certainly like to know if this > is actually a real problem (and can't be solved by choosing a compromise value > as the initial loops_per_jiffy). I think Shinya was doing some tests so > I'll wait to see how those went. >>From my observations: (1) loops_per_jiffy can easily be calculated from the CPU clock speed. If your platform is capable of detecting CPU frequency at run-time, settingi up loops_per_jiffy _early_ can allow you early use of udelay()s. Or even if you don't need udelay() early, setting up lpj_fine (or having calibrate_delay_is_known()) allows you to skip calibrate_delay() later. This is useful and can be applied to both UP and SMP systems. (2) For SMP platforms, if you need ealy use of udelay(), you have to update loops_per_jiffy at the same time as init_current_timer_delay(). It could be done in init_current_timer_delay(), or platforms can take care of that, that need udelay() available early. Either one should be fine with me. -- Shinya Kuribayashi Renesas Electronics