December 2016 - 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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Driver to allow DMA from user space
by Ran Shalit Dec. 31, 2016

Dec. 31, 2016

Hello, I want to use DMA from userspace. I already use dma in kernel, and now I want can create a character device which will be responsible for this. The only problem is that I want to use the same memory which was allocated in kernel with dma_alloc_coherent. Is it correct to use mmap in order to use the phsyical memory which was allocated with dma_alloc_coherent ? If it's that simple it can be surely helpful, and the simple driver which wraps dma_alloc_coherent can do the job for dmaing from userspace. In kernel: dma_buffer_info->buf_info[i].virtAddr = (uint8_t *)dma_alloc_coherent( &ti667x_pci_dev[dev_id]->dev, dma_buffer_info->buffer_size, (dma_addr_t *)&dma_buffer_info->buf_info[i].pcieAddr, GFP_KERNEL); In userspace: buf_desc[i].virtAddr = mmap(0, size_of_buffer, PROT_READ | PROT_WRITE, MAP_SHARED, pcie_drv_inst[dsp_id].dev_desc, buf_desc[i].pcieAddr); Thanks, Ran

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Re: how to periodically monitor the qdisc size ?
by Shiyao Ma Dec. 29, 2016

Dec. 29, 2016

Hi Valdis, I am in the process of doing an emulation of a dumbbell topology, on my linux box. > > I am using the RED qdsic. > > You have our condolences. RED is probably not the best choice of > qdisc, because it requires very careful tuning of multiple parameters. > I am using RED for the part of emulating a switch. Since most switches are use AQM of RED/ECN. > > You might want to investigate using fq_codel, or for very new > kernels, use the fq qdisc and the bbr congestion control method. > Yes. For the sending side, I am already using fq+bbr. -- 吾輩は猫である。ホームーページはhttps://introo.me <http://introo.me>。

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how to periodically monitor the qdisc size ?
by Shiyao Ma Dec. 29, 2016

Dec. 29, 2016

Hi, I am using the RED qdsic. I want to see how many skbuffs are in the red qdisc, and the total bytes of the skbuffs, like in every 2 seconds. Regards

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[MMC_SPI] Getting CRC error on CMD18 for MMC card on SPI
by Amit Mahadik Dec. 29, 2016

Dec. 29, 2016

Hello, I am using ARM (CortexA9)development board which has SPI controller running at 6MHz. I want to interface MMC card on SPI bus. I have made changes to my board.c file to include the mmc_spi driver and mentioned the Chip select, mode (MODE 0) and frequency of operation (started with 40kHz). The initial card detection goes well. However, there are CRC errors when the block layer tries to read the partition table.I have attached log for reference. The card gets detected on Linux based system. The card is formatted with DOS partition (single partition). Please help me regarding this issue. Regards,Amit.

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In netlink, what is interface address and what is local address.
by Douglas Su Dec. 29, 2016

Dec. 29, 2016

I am totally confused with these two addresses, don't they are the same? Can a same IP be set on different interface?

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Fwd: Need some help regarding write_begin, write_end and writepage(s).
by Pranay Srivastava Dec. 29, 2016

Dec. 29, 2016

Hello, I'm trying to understand the write_begin, write_end and writepages / writepage call sequence by having a small file system coded myself. I'm not using block_write_begin / block_write_end also not using mpage_writepages as well. The issue however is that apart from write_begin and write_end, writepages never gets called. However if I do write_inode_now ( in write_end) then writepage do get called, but that's not how it's supposed to be. 1. in write_begin : I'm able to grab_cache_page_begin and returned it locked. 2. in write_end: I'm able to allocate disk space and do SetPageDirty and SetPageUptodate unlock_page and page_cache_release. 3. I also tried __mark_inode_dirty(inode, I_DIRTY_PAGES) 4. I also tried to set_page_writeback, but apparently that then makes the iput_final stuck since it waits for PG_Writeback to be cleared and that I've coded in writepage which never gets called. but writepages never gets called. Even iput_final doesn't make the write_inode_now call. Any help would be appreciated. Regards,

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disable input event device
by Ran Shalit Dec. 28, 2016

Dec. 28, 2016

Hello, Is there some way to disable input event device ? Maybe a way to disable its irq usage ? I actually rather do that from shell, not from kernel, because the kernel source is not available, and I thought that issue is also relevant in the forum. Thank you, Ranchu

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