Messages in this thread | | | From | Guenter Roeck <> | Date | Fri, 19 Aug 2016 05:47:24 -0700 | Subject | Re: Problem with atomic accesses in pstore on some ARM CPUs |
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On Fri, Aug 19, 2016 at 2:35 AM, Russell King - ARM Linux <linux@armlinux.org.uk> wrote: > On Tue, Aug 16, 2016 at 10:35:52AM -0700, Colin Cross wrote: >> persistent_ram uses atomic ops in uncached memory to store the start >> and end positions in the ringbuffer so that the state of the >> ringbuffer will be valid if the kernel crashes at any time. This was >> inherited from Android's ram_console implementation, and worked >> through armv7. > > That statement is actually inaccurate. It may have worked on _some_ > ARMv7 implementations, but it's not architecturally compliant. > > The exclusive access instructions are not portable to anything but > "normal memory": > > It is IMPLEMENTATION DEFINED whether LDREX and STREX operations can > be performed to a memory region with the Device or Strongly-ordered > memory attribute. Unless the implementation documentation explicitly > states that LDREX and STREX operations to a memory region with the > Device or Strongly-ordered attribute are permitted, the effect of > such operations is UNPREDICTABLE. > > pgprot_noncached() gives strongly-ordered memory, and so is unsuitable > to place semaphores in for all ARMv7 implementations. > > Also: > > + if (memtype) > + va = ioremap(start, size); > > this returns *device* memory, which is also unsuitable for all ARMv7 > implementations. > > It seems that pstore is playing in areas of the architecture which are > implementation defined, so it's no surprise that folk are seeing > different behaviours with different implementations. > > The code isn't architecturally wrong, it just isn't portable to all ARMv7 > implementations. > > So, saying that it works on some ARMv7 implementations is irrelevent. > > Note that LDREX and STREX are used for all operations that require > atomicity - iow, atomics and locks. > > pgprot_writecombine() and ioremap_wc() will return memory which is > suitable for these exclusive accesses - it maps to the architectures' > "normal memory, uncacheable". >
Unfortunately, pgprot_writecombine() doesn't work in my case (for rk3288 and rk3399). It was also reported not to work on Tegra Logan by Nvidia. As mentioned earlier, PAGE_KERNEL works, at least for rk3288 and rk3399.
Guenter
> So, I suspect the OP should not be using mem_type=1 or using the > "unbuffered" DT attribute, but should leave it was the default > (mem_type=0). > > > -- > RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ > FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up > according to speedtest.net.
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