On Wed, May 27, 2020 at 12:50:01PM +0000, Schrempf Frieder wrote: > On 27.05.20 13:53, Russell King - ARM Linux admin wrote: > > On Wed, May 27, 2020 at 10:39:12AM +0000, Schrempf Frieder wrote: > >> Hi, > >> > >> on our i.MX6UL/ULL boards running mainline kernels, we see an issue with > >> RS485 collisions on the bus. These are caused by the resetting of the > >> RTS signal being delayed after each transmission. The TXDC interrupt > >> takes several milliseconds to trigger and the slave on the bus already > >> starts to send a reply in the meantime. > >> > >> We found out that these delays only happen when the CPU is in "low power > >> idle" mode (ARM power off). When we disable cpuidle state 2 or put some > >> background load on the CPU everything works fine and the delays are gone. > >> > >> echo 1 > /sys/devices/system/cpu/cpu0/cpuidle/state2/disable > >> > >> It seems like also other interfaces (I2C, etc.) might be affected by > >> these increased latencies, we haven't investigated this more closely, > >> though. > >> > >> We currently apply a patch to our kernel, that disables low power idle > >> mode by default, but I'm wondering if there's a way to fix this > >> properly? Any ideas? > > > > Let's examine a basic fact about power management: > > > > The deeper PM modes that the system enters, the higher the latency to > > resume operation. > > > > So, I'm not surprised that you have higher latency when you allow the > > system to enter lower power modes. Does that mean that the kernel > > should not permit entering lower power modes - no, it's policy and > > application dependent. > > > > If the hardware is designed to use software to manage the RTS signal > > to control the RS485 receiver, then I'm afraid that your report really > > does not surprise me - throwing that at software to manage is a really > > stupid idea, but it seems lots of people do this. I've held this view > > since I worked on a safety critical system that used RS485 back in the > > 1990s (London Underground Jubilee Line Extension public address system.) > > > > So, what we have here is several things that come together to create a > > problem: > > > > 1) higher power savings produce higher latency to resume from > > 2) lack of hardware support for RS485 half duplex communication needing > > software support > > 3) an application that makes use of RS485 half duplex communication > > without disabling the higher latency power saving modes > > > > The question is, who should disable those higher latency power saving > > modes - the kernel, or userspace? > > > > The kernel knows whether it needs to provide software control of the > > RTS signal or not, but the kernel does not know the maximum permissible > > latency (which is application specific.) So, the kernel doesn't have > > all the information it needs. However, there is a QoS subsystem which > > may help you. > > > > There's also tweaks available via > > /sys/devices/system/cpu/cpu*/power/pm_qos_resume_latency_us > > > > which can be poked to configure the latency that is required, and will > > prevent the deeper PM states being entered. > > Thanks for the detailed explanation. This all makes perfect sense to me. > I will keep in mind that we need to consider this aspect of power saving > vs. latency when designing systems and also that we need to provide the > information for the kernel to decide which of the two is more important. > > Also thanks for pointing out the QoS subsystem. I'm not quite sure if it > would work for us to use pm_qos_resume_latency_us in our specific case. > The actual latency we observe is something like 2 to 3 milliseconds > longer with low power idle than without, but the exit_latency for low > power idle specified in the cpuidle driver is only 300 us.
I wonder whether the exit latencies are correct in that case. >From the comments, it seems 80us is allowed for the software overhead of entering/leaving the idle state vs 220us for the hardware. It may be a good idea for someone to add some tracing points in there to try and measure the minimum software latencies. > So as far as I can see with this difference even if we would set > pm_qos_resume_latency_us to 1000 us (which should be fast enough for the > RS485 to work properly), the low power idle wouldn't be disabled. > > It's rather this discrepancy between the latency set in the driver and > what we see in reality which makes me wonder if there's something I'm > missing. It's possible that there's something missing from the kernel's estimation of the latency required for entering / exiting those states. There is an amount of cache flushing that is required when entering those lower states, and I wonder if that has been accounted for. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC for 0.8m (est. 1762m) line in suburbia: sync at 13.1Mbps down 424kbps up