On Mon, Jan 08, 2018 at 12:03:34PM +0100, Lucas Stach wrote: > Am Freitag, den 05.01.2018, 15:43 +0000 schrieb Lorenzo Pieralisi: > > On Fri, Jan 05, 2018 at 02:26:34PM +0000, Bharat Kumar Gogada wrote: > > > On Fri, Dec 22, 2017 at 01:02:28PM +0000, Bharat Kumar Gogada > > > wrote: > > > > Bjorn wrote: > > > > > In the PCI config access path, the *_pcie_valid_device() > > > > > functions in > > > > > the dwc, altera, rockchip, and xilinx drivers all check whether > > > > > the > > > > > link is up. > > > > > > > > > > I think this is racy because the link may go down after we > > > > > check but > > > > > before we perform the config access. > > > > > > > > > > What would blow up if we removed the *_pcie_link_up() checks? > > > > > > > > > > I'd like to either remove the checks or add comments about why > > > > > the > > > > > race is acceptable. If we've covered this before, I apologize. > > > > > Adding a comment will keep me from pestering you about this > > > > > again in > > > > > the future. > > > > In both Xilinx driver cases when link is down, hardware responds > > > > by > > > > AXI DECERR/SLVERR status which causes an exception, synchronous > > > > external abort to CPU. This causes system to hang, so we need > > > > this > > > > check for both of our drivers. We will add comments. > > > > > > This is a problem, and checking whether the link is up is a > > > workaround but not a real solution. That means your system may > > > hang if the link happens to go down at the wrong time. > > > > > > A real solution would be to handle the synchronous external abort > > > so it doesn't cause a system hang. > > > > > > Yes, I agree that this is workaround. For pcie-xilinx.c for arm32, > > > we can have fault handling similar to "imx6q_pcie_abort_handler" in > > > drivers/pci/dwc/pci-imx6.c. > > > Since this driver is same for Microblaze architecture also, it > > > requires separate handling. > > > > > > For pcie-xilinx-nwl.c ARM64 as per link [1], linux kernel will hang > > > for the above AXI responses. > > > As of now arm64 RAS is still work in progress [2]. > > > > > > [1] https://www.spinics.net/lists/arm-kernel/msg624203.html > > > > > > [2] https://patchwork.kernel.org/patch/9973967/ > > > > > > The check can be removed, if above issues were addressed. > > > > I do not see why the above "issues" should be addressed in order to > > remove that check - as it was pointed out in this thread it just does > > not solve anything, so what's the reason for keeping it ? > > I solves the issue that you hang the system on PCIe enumeration in 100% > of the cases when the link is down and you don't have the abort handler > in place. There is a mechanism to detect if the link is up before starting enumeration or am I wrong ? Probably what we should be discussing here is what are the causes for the link to go down *unexpectedly* - in other words - what makes that racy check "likely" to work - which was the initial question, by the way. > It doesn't solve the race issue, but that is a lot less likely to be > hit in the real world. I guess it's not a good idea to remove something > that covers 98% of the problem just because it doesn't cover the > remaining 2%, right? See above - I want to understand what your 98% and 2% actually represent. Thanks, Lorenzo
