February 2017 - kernelnewbies@kernelnewbies.org

where is __percpu_seg initialized?
by 落痕 March 28, 2018

March 28, 2018

Hi all: I knew the kernel implement the per-cpu variable via __percpu_seg. For example on x86 architecture, it's defined as: #ifdef CONFIG_X86_64 #define __percpu_seg gs #define __percpu_mov_op movq #else #define __percpu_seg fs #define __percpu_mov_op movl #endif But I'm curious where __percpu_seg is initialized?? I fail to find the relative code. Thanks.

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How to handle Hotplug with UIO userspace driver
by Mandeep Sandhu Sept. 28, 2017

Sept. 28, 2017

Hi All, We're working on a custom PCIe based hardware, which can be hotplugged into the system. We're using the UIO approach of having a minimal kernel driver which basically handles the interrupt and provides a mmap of the device memory to userspace and userspace driver which mmap's the provided memory and manages the device. We're using systemd and udev rules to spawn off a user process whenever the hardware is plugged into the system. Now my question is, how can we gracefully inform the userspace process if someone pulls out the hardware (sitting on a PCI slot). Even with full hotplug support, there can always be a case of "surprise removal", where someone yanks the board without going through the whole "attention button" routine. While the kernel driver has no problem with hot-removal, I see that if I "unregister" our UIO driver in response to the removal, then the mapping for the hardware's physical memory would become "invalid" (done by UIO on unregsiter). This could possibly cause the user-space process to crash due to illegal memory access. Is there a "standard" way to handle this scenario, i.e for hotpluggable hardware using UIO? Basically, we'd want to "unregister" the UIO device only after the userspace process is informed of the removal so that it could unmap/close the device and exit gracefully. We could try doing it using some systemd trick, where we put a dependency on one of the UIO created files (/dev/uioX), so that when the file is destroyed, systemd could kill/shutdown the process. Although, there's no guarantee that the userspace process won't try to access the mmaped space during that interval, so this isn't a reliable solution either. Any hints, thoughts? Thanks., -mandeep

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What are need to makes humidifier?
by 윤영석 April 1, 2017

April 1, 2017

Hi, I want to make own humidifier. What sensor need when making humidifier?? humidity indicator (detecting humidity), ultrasonic waves maker?? Please shares your wisdom to me. Thanks. Best regards.

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CMA question
by Johannes Thoma Feb. 28, 2017

Feb. 28, 2017

Dear Kernel hackers, As far as I understood CMA (contiguous memory allocation) memory is used for other purposes as long it isn't requested via cma_alloc() by a driver. cma_alloc then tries to free the memory by relocating it and returns the contiguous area. I have a case where cma_alloc() sometimes fails to relocate the memory which causes my driver (a GPU driver) to fail starting up (the driver is started when the system is running for a while). Is there a way to prevent the CM allocator to use the memory for any other purpose? Or should I better use another mechanism to allocate contiguous memory at boot time (is there a framework for doing so)? PS: I once had a patch which tries harder to relocate the pages (I patched cma_alloc()), but it didn't really work well, blocking the allocation for seconds. Thanks for your advice, - Johannes

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staging-testing branch
by Tobin C. Harding Feb. 28, 2017

Feb. 28, 2017

Greg Thanks for your unfathomable patience dealing with kernel newbies. I have a small suggestion, in a most humble manner. It seems that when patches don't apply the response is not automated, if it is possible within your system to automate checking if a patch actually applies would it be beneficial to point the patch submitter towards tracking remote branches (i.e are they basing their patches on on your staging-testing branch). I have wasted some of your time previously because it took me a while to work this out. Perhaps we can save other people from also unnecessarily using up your time. thanks again, Tobin.

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Is sg_set_page will not work with IOMMU?
by Harsh Jain Feb. 28, 2017

Feb. 28, 2017

Hi All, I am trying to create new sg list from old list. The new list will have length < 2048. When I populate the new sg list using sg_set_page(). IOMMU starts giving following error. [ 388.463656] DMAR: DRHD: handling fault status reg 2 [ 388.468539] DMAR: [DMA Write] Request device [03:00.0] fault addr ef66d000 [fault reason 05] PTE Write access is not set [ 389.464146] DMAR: DRHD: handling fault status reg 102 [ 389.469203] DMAR: [DMA Write] Request device [03:00.0] fault addr ef65d000 [fault reason 05] PTE Write access is not set Function to create new sg list: { struct scatterlist *newsg, *sg; int i, len, processed = 0; size_t size = 2048; newsg = kmalloc_array(nents, sizeof(struct scatterlist), GFP_KERNEL); if (!newsg) return ERR_PTR(-ENOMEM); sg = newsg; sg_init_table(sg, nents); for (i = 0; i < nents; i++) { len = min_t(u32, sgl->length - processed, 2048); sg_set_page(sg, sg_page(sgl), len, sgl->offset + processed); processed += len; if (processed == sgl->length) { processed = 0; sgl = sg_next(sgl); } sg = sg_next(sg); } return newsg; } Same driver works fine if I disable IOMMU.

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wifi not detected. Ubuntu 16.04.1 4.8.0.39-generic
by Vasu M Feb. 28, 2017

Feb. 28, 2017

Hi, Not sure if this is a correct forum to discuss but I am facing an issue on Ubuntu 16.04.1 64 bit kernel 4.8.0-39-generic using wifi adapter AR2413/AR241 which is running ATH5K version 4.8.0.39-generic. If this is not the correct alias, please excuse and suggest the right alias. After a cold reboot, the NM is not detecting the WIFI. Machine is not able to recover from this. I also restarted network manager and reloaded the driver but didn't fix the issue. Tried: sudo rmmod -f ath5k (got a msg -- disconnected - you are offline) sudo rfkill unblock all sudo modprobe ath5k (Nothing happened) When i check the /etc/modprobe.d/blacklist-ath_pci.conf, I see the following - #For some Atheros 5K RF MACs, the madwifi driver loads but fails to #correctly initialize the hardware, leaving it in a state from which ath5k #cannot recover. To prevent this condition, stop madwifi from loading by #default. Use Jockey to select one driver or the other. (Ubuntu #315056, #323830) blacklist ath_pci There are some bugs reported in the past but I don't come across anything that was reported in the latest kernel which is similar. Not sure if this is a bug still lurking around but I found someone reporting a similar problem here - https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/1569590. Any guidance is much appreciated. Thanks, Vasu

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Fwd: want to be a kernel developer
by Ozgur Karatas Feb. 24, 2017

Feb. 24, 2017

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want to be a kernel developer
by sourav mondal Feb. 24, 2017

Feb. 24, 2017

hi, I want to be a kernel developer.I don't really know much about linux kernel. can anyone suggests me some good books that can help to learn more about the kernel development? thanks, sourav

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Cannot activate multiple CPUs.
by chris warth Feb. 22, 2017

Feb. 22, 2017

Is there a trick to bringing up multiple CPUs on an SOC? My kernel is not bringing up secondary CPUs. Running a kernel compiled from v4.10 sources on an LS1046A reference board. Support for this platform only arrived in the mainline sources in v4.10, so it is pretty new, and there are clearly differences from the vendor supplied SDK sources. Configured with generic arm64 defconfig. Loading the kernel via `booti` in u-boot. Provided fsl-ls1046a-rdb.dtb as the device tree. Console log indicates only a single CPU is available. [ 0.083735] EFI services will not be available. [ 0.096301] smp: Bringing up secondary CPUs ... [ 1.141306] CPU1: failed to come online [ 1.145159] CPU1: failed in unknown state : 0x0 [ 2.202164] CPU2: failed to come online [ 2.206016] CPU2: failed in unknown state : 0x0 [ 3.263021] CPU3: failed to come online [ 3.266873] CPU3: failed in unknown state : 0x0 [ 3.271449] smp: Brought up 1 node, 1 CPU Is it possible that multiple cores are not supported in the mainline code? Or am I missing some piece of configuration that would bring up multiple cores? Thanks in advance, - Chris

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