From mboxrd@z Thu Jan 1 00:00:00 1970 From: shawn.guo@freescale.com (Shawn Guo) Date: Fri, 9 Sep 2011 15:32:48 +0800 Subject: [PATCH 6/6] arm/imx6q: add suspend/resume support In-Reply-To: <20110908162452.GA20292@n2100.arm.linux.org.uk> References: <1315303120-24203-1-git-send-email-shawn.guo@linaro.org> <1315303120-24203-7-git-send-email-shawn.guo@linaro.org> <20110908062301.GK8148@S2100-06.ap.freescale.net> <20110908074718.GA14953@n2100.arm.linux.org.uk> <20110908152222.GA31581@S2100-06.ap.freescale.net> <20110908162452.GA20292@n2100.arm.linux.org.uk> Message-ID: <20110909073247.GD31581@S2100-06.ap.freescale.net> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Russell, Really appreciate the explanation. On Thu, Sep 08, 2011 at 05:24:52PM +0100, Russell King - ARM Linux wrote: > On Thu, Sep 08, 2011 at 11:22:23PM +0800, Shawn Guo wrote: > > On Thu, Sep 08, 2011 at 08:47:18AM +0100, Russell King - ARM Linux wrote: > > > On Thu, Sep 08, 2011 at 02:23:02PM +0800, Shawn Guo wrote: > > > > Yes, if you lose L2 power in suspend, it's the easiest way for you to > > > > resume L2. > > > > > > > > For my case which retains L2, I actually do not want to call this > > > > function which will invalidate L2. But since we still have problem > > > > to use rmk's generic suspend/resume updates (ARM: pm: add L2 cache > > > > cleaning for suspend), we have to flush the entire L2 on imx6q for now. > > > > > > OMAP44xx has the same problem but they re-initialize L2 on resume in > > > their pre-cpu_resume() assembly code. > > > > > Are you suggesting that this might be the reason why imx6q has problem > > with your patch? > > Consider the mechanics of what is happening. > > On suspend, when we enter cpu_suspend(), we assume the L2 cache is > still enabled. > > We _always_ assume that L1 cache state is lost, so we always flush the > entire L1 cache. > > As SoCs can (and do) preserve the L2 contents over suspend/resume cycles, > we leave it to the platform's finisher to decide whether it needs to > flush the entire L2 cache to RAM or not. > > If L2 is preserved, then we want to ensure that as much data as possible > is retained in the L2 cache (if the hardware preserves its contents, we > don't want waste time flushing data out of the cache needlessly - > especially if we're using these paths for cpuidle.) However, we need > access to a certain amount of data to bring the system back up, and as > the L2 cache typically will not be searched before the MMU is enabled, > we have to flush out a certain minimal amount of data (the location > of the stacked restore information and the stacked restore information > itself.) > > However, we don't flush out anything else from the L2 cache. > > Now, upon resume, the resume code will be able to read the data it needs > to restore the system as we ensured that was flushed out to memory - as > I mentioned above, the L2 cache won't be searched for this irrespective > of whether the control registers have enabled it or not. > > We will continue to the point where we hit the first bit of information > stored in L2, which will probably be the stacked SVC register set. If > at this point we're not able to search the L2 cache, then we'll read > stale data from the backing memory instead, and the system will crash. > > Consider what generic code could do about this - if we flushed out that > register set to memory, then we could get past that point - but we then > would need to call a function to enable the L2 cache. What if that > function needs data which is sitting in the L2 cache to function (eg, > it may need values from the device tree). We would need to ensure > that that data were also flushed out of the L2 cache. What about > spinlocks? Maybe some other CPU has dragged the spinlock data into > the L2 cache. That gets _much_ harder to solve. > > Now to the physical act of enabling the L2 cache. The L2 cache control > registers are subject to security restrictions when running in non-secure > mode, needing platform specific SMC calls to reprogram the cache. Generic > code is unable to do this. > I could be very possibly wrong here. But isn't the core in secure mode upon reset? Do we really have to reprogram L2 through SMC calls there? > Hence why it's left up to the platform to figure out how to enable the > L2 cache before calling cpu_resume(). The platform is best placed to > work out what it needs to do to setup the L2 cache so that the L2 cache > is available by the time the system control register is written, enabling > the MMU and caches. > > > Are we supposed to re-initialize L2 before calling > > into generic cpu_resume()? > > So, the above is the long way of saying "yes" to this question. I hope > it gives the full picture about why this is so. > Yes, it does. Thanks, again. -- Regards, Shawn