January 2014 - kernelnewbies@kernelnewbies.org

Self Introduction
by Ganessh Kumar R P Jan. 7, 2014

Jan. 7, 2014

Hello guys, I have a fair knowledge about kernel after a course on Operating System as part of my under-grad and hands on in building kernel from source code. I want to start contributing to small projects. Is anyone needs some assistance or small work lying around please point it to me. Any guidance or pointers will be helpful. I will be also active in IRC channel. Thanks, Ganessh

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user space vs kernel space address space
by Pritam Bankar Jan. 7, 2014

Jan. 7, 2014

Lets consider 32 bit Linux system with 512 physical RAM. Suppose I have standard 3:1 address space split. Now what I understand is (In general) 1. In the fourth gigabyte I have kernel space 2. Out of 1GB for kernel address space only 896MB is used as direct mapping and other 128 MB is used for Noncontiguous Memory Area Management, Fixed Mapping and Permanent Mapping. PCMIIW Following are my doubts : 1. Since my system has only 512MB RAM, will there be only direct mappings since 896 is enough to hold 512 RAM? 2. When user space program do malloc, we get some virtual address from userspace region (from first 3GB) of process. So will it be like, when I access some memory from that region, there is some physical frame associated with it AND same physical frame will also be mapped in kernel space ? So what I want to know, for every physical frame is there a mapping in userspace as well as kernel space ? (given that some address in user space of process map to same physical frame) 3. If we consider example of Linux system with 4GB, now if I do malloc from user space from which memory region kernel will give memory ? Direct mapped region or fixed map or permanent map ? Thanks and regards, Pritam Bankar

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Breaking up a bvec in a bio for reading more than 512
by neha naik Jan. 7, 2014

Jan. 7, 2014

Hi All, I am getting a request with bvec->bv_len > 512. Now, the information to be read is scattered across the entire disk in 512 chunks. So that, information on disk can be : sector 8, sector 100, sector 9. Now if i get a request to read with the bvec->bv_len > 512 i need to pull in the information from multiple places on disk since the data is not sequentially located. I tried to look at the linux source code because i think raid must be doing it all the time. (eg : on disk 1 we may be storing sector 6 and on disk 2 we may be storing sector 7 and so on). However, i have not really got any useful information from it. Also scouring through articles on google has not helped much. I am hoping somebody points me in the right direction. Thanks in advance, Neha

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simple memory malloc and free in ip_output.c
by Guibin(Bill) Tian Jan. 7, 2014

Jan. 7, 2014

Hi, I am doing a very simple operation at the method of ip_queue_xmit in ip_output.c. The code is very simple. char *p = kmalloc(48, GFP_KERNEL); kfree(p); This will be called every time this method is called. But the result is that the whole system is not responsive at all. This is a very simple, not sure what I am doing wrong. Can anybody help? Thanks

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DMA, CMA, coherence and performance
by Steven Bell Jan. 7, 2014

Jan. 7, 2014

Hi, I'm working on a device driver for a video device which continuously reads and writes image frames using DMA. The frames are fairly large, in the range of 2-8MB, and I would like the buffers for them to be contiguous because of my hardware. My understanding is that using the contiguous memory allocator is the current "right way" to get the buffers, and that CMA operates entirely behind the scenes when calls are made to dma_alloc_coherent(). However, it seems that for this system, a streaming DMA setup would be more appropriate. The buffer gets filled with data once, handed to the device, and then isn't touched again until it gets reused with new data. The resources I've read have hinted that streaming DMA has some performance benefits over coherent DMA, so this seems like the way to go. But I haven't seen any discussion of how to use CMA with streaming DMA (or whether such a thing is even necessary). Does the CMA also work behind get_free_pages, or other kernel memory allocation methods? Does it matter? The kernel newbies page on memory allocation (http://kernelnewbies.org/KernelMemoryAllocation) says that get_free_pages up to about 8MB are ok. Is that a generalization based on typical memory fragmentation, or a guarantee? Thanks, Steven

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Interrupt fires when module is unloaded
by Eric Fowler Jan. 7, 2014

Jan. 7, 2014

I am trying to figure out interrupts by writing a shadow of Rubini's 'short' program. Recall that Rubini tells us to enable parallel port interrupts by wiring pins 9&10 together, then writing binary data to the parallel port's address. I am doing that, but: - I don't see interrupts when I write to the port - I do see one interrupt when I unload the driver (in the fop's .release method) - This happens whether or not the pins are wired up. What is going on here?

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Help request: Blocking access to USB mass storage device across momentary power loss
by Rick Ramstetter Jan. 6, 2014

Jan. 6, 2014

I sent the following email to "Matthew Dharm" whose name appears in Linux 2.6.35's "scsiglue.c" as that file's primary maintainer. I realize in retrospect I should have messaged the kernelnewbies and/or linux-usb lists first. To summarize the below text, "what we're hoping to do ... is reset the SD cards ... via a drop of the appropriate power rail ... such that in progress userspace calls to read() and write() are blocked until the hardware completes its power on initialization sequence" ------------------------------------------- From: J. Rick Ramstetter Sent: Monday, January 06, 2014 11:53 AM To: mdharm-usb(a)one-eyed-alien.net Subject: Help request: Blocking access to USB mass storage device across momentary power loss Hello Matthew, I'm emailing you because your name and email address were mentioned in the source "scsiglue.c" of Linux kernel version 2.6.35. I'm about to ask for help with aspects of that source file, and I'm hoping you'd be kind enough to entertain my questions. I work for a company that makes in flight entertainment (IFE) hardware for various non-US airlines. These units are, at their core, flight certified movie players with a secondary storage methodology of SD cards. Each unit interfaces with its SD card storage via an SMSC 2660 or SMSC 2640 chip; in other words, the individual SD cards appear to the IFE units as USB mass storage devices using the most-common forms of USB mass storage (drivers/usb/storage/scsiglue.c, Bulk-Only, Transparent SCSI). We've recently root-caused a bug in our use of the SMSC 26x0 chips. The cause is in hardware: in our board design, we failed to use CRD_PWR lines provided by the SMSC chips to power the SD cards, but instead wired the SD cards directly to the same 3.3v rail that powers the SMSC chips. We've been advised by SMSC that this is improper, and our own empirical testing suggests that the SMSC chips expect to be able to "power cycle" the SD cards by cycling the CRD_PWR lines. Though we've fixed the problem for future board revisions, we have thousands of affected IFE units in the field and reworking them is impossible. We have observed the following behaviors: (1) The SMSC chip becomes, in USB terminology, "orphaned" (2) The IFE units react by issuing a USB port reset (the usb mass storage driver's "scsiglue.c" sets a port reset as the scsi host bus reset function pointer) (3) That reset causes the SMSC chip to reset, but the SD cards are not reset due to improper wiring (4) The SMSC chip and SD cards cannot communicate due to non-matched state. Accesses fall through to scsi_lib's scsi_io_completion()'s "case NOT_READY ... action = ACTION_FAIL" logic. What we're hoping to do, and forgive me if this seems extreme, is reset the SD cards at step (3) above via a drop of the appropriate power rail. We have software control of that rail, and there are few devices on it, all of which are storage devices. We've demonstrated the feasibility of "suspending" all of the platform (hardwired platform_device) storage devices on that rail during a blackout, such that in progress userspace calls to read() and write() are blocked until the hardware completes its power on initialization sequence. We have not, however, demonstrated this feasibility with our USB Mass Storage SD card devices, and thus I'm emailing you. With regard to "suspending" the SD card devices, we've tried walking the LDM device tree from childmost device to parentmost device, invoking the appropriate power management suspend() operations as we go (including those for usb's endpoint.c devices), but have had no luck during our resume operation. Upon powerup of the SD cards and SMSC chip, USB re-enumerates the devices, and re-invokes scsi_add_host() (ref usb_stor_probe2()). The net result is a new scsi host, a new UEVENT, and a new udev device node. Can you offer any advice on the goal of blocking userspace read() and write() calls during the device's blackout? I can see a few different approaches to this, including: * A FUSE layer * Pausing the Host in the SCSI layers * Preventing the calls to scsi_add_host() et all in the USB layer Some more specific questions: * Is any documentation regarding the design and requirements of power operations in the Linux SCSI code available on the internet? * Besides host self-blocking (shost->host_self_blocked), does SCSI have any facility for host "pausing" or temporary disconnection? * Is this a problem for the SCSI layer at all, or does the USB / USB Mass Storage layer need to handle it? Thank you for your time, --Rick R.

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Re: how to use mmap() on a disk file to get a kernel address?
by Yu Bi Jan. 6, 2014

Jan. 6, 2014

Thanks for your idea. Because I implement a kernel storage service based on a disk file through VFS layer. To simplify the implementation, I prefer to read/write the mapped kernel space address instead of the file data access api(such as page cache API / VFS API). In FreeBSD, I can use vm_map_find to map a file into kernel space address( vnode pager) But in linux, I don't know how to implement this(map disk file into kernel space)? On Thu, Dec 26, 2013 at 3:42 PM, Yu Bi <lazysmartegg(a)gmail.com> wrote: > Hi, > I try to mmap a file in a linux kernel module. I have tried to use the > function do_mmap_pgoff. But the address returned is memory virtual > address in current process' user space, i.e., below the kernel boundary. > Instead, I want to map the file in the kernel space and get the kernel > virtual address of the mapped region. Is there any kernel API in Linux > support this operation? Or I have to reimplement a kernel mmap version ? > But reimplement a kernel version mmap() is not easy I guess. Do you > have any idea? > > > > > >

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how to build monolithic kernel
by Laxmi Meena Jan. 5, 2014

Jan. 5, 2014

Hi, has anybody created rpm of monolithic kernel. Thanks in advance.

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Cache coherency handling in user space
by jagadeesh kumar Jan. 4, 2014

Jan. 4, 2014

Hi, I am doing RDMA (InfiniBand) programming. When network device and CPU are using kernel buffer, then DMA synchronisation APIs are to be used for maintaining cache coherency. But in case of User space buffer there is no such provision provided for user and data is always synchronised with cache copy. Please help me to understand how cache coherency is maintained in user space. Thanks, Jagadeesh.

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