September 2012 - kernelnewbies@kernelnewbies.org

LDT - Linux Driver Template
by Constantine Shulyupin Sept. 26, 2012

Sept. 26, 2012

Hi I develop template of Linux driver. It can be used as sample for Linux driver development beginners and starting point for development of new drivers. The driver uses following Linux facilities: module, platform driver, file operations (read/write, mmap, ioctl, blocking and non-blocking mode, polling), kfifo, completion, interrupt, tasklet, work, kthread, timer, misc device, proc fs, UART 0x3f8, HW loopback, SW loopback Readme and sources: https://github.com/makelinux/ldt/blob/master/README.md To run driver with test script just run: git clone git://github.com/makelinux/ldt.git && cd ldt && ./ldt-test You feedback, suggestions, discussions, recommendations patched will be greatly appreciated! Thanks -- Constantine Shulyupin http://www.MakeLinux.com/ Embedded Linux Systems, Device Drivers, TI DaVinci

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Why a locked page must be copied in COW?
by Parmenides Sept. 26, 2012

Sept. 26, 2012

Hi, The do_wp_page() implements the Copy-On-write approach. I have no idea about its doing concerning locked pages. static int do_wp_page(struct mm_struct *mm, struct vm_area_struct * vma, unsigned long address, pte_t *page_table, pmd_t *pmd, pte_t pte) { old_page = pfn_to_page(pfn); <-- old_page is in quesiton. ... ... ... if (!TestSetPageLocked(old_page)) { <-- We test the 'PG_locked' flag of old_page. If it is not set, we can further check whether the page should be copied. int reuse = can_share_swap_page(old_page); unlock_page(old_page); if (reuse) { <-- If only one process owns old_page, we should copy it. flush_cache_page(vma, address); entry = maybe_mkwrite(pte_mkyoung(pte_mkdirty(pte)), vma); ptep_set_access_flags(vma, address, page_table, entry, 1); update_mmu_cache(vma, address, entry); pte_unmap(page_table); spin_unlock(&mm->page_table_lock); return VM_FAULT_MINOR; } } pte_unmap(page_table); /* * Ok, we need to copy. Oh, well.. */ <-- At this point, we should copy old_page. ... ... ... } In conclusion, if old_page is locked or it is not locked and two or more processes own old_page, we should copy it. I don't understand why a locked page must be copied regardless of how many processes own it. Why do we give a locked page a special consideration?

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FYI, open-sourced kernel, device driver and android courseware
by Robert P. J. Day Sept. 26, 2012

Sept. 26, 2012

for those interested, i'm now doing some work with a chap whose name might be familiar to some of you -- karim yaghmour -- who has recently open-sourced his linux and android courseware here: http://www.opersys.com/training as you can see, for all four of those classes, you're welcome to download the full courseware. at the moment, i'm working on the two linux courses and updating the material to reflect the 3.x kernel, but you're welcome to download and peruse whatever you want. i'm hoping to have fully-updated courses there at some point in the near future. in my copious free time. :-) rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================

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what's with the zeroing out of memory when kexec is configured?
by Robert P. J. Day Sept. 25, 2012

Sept. 25, 2012

(undoubtedly another in an upcoming series of nitpicky questions as i poke around in kernel code i haven't visited in a while and i learn that there are lots of things i don't know.) consider the following free_initrd() routine in init/initramfs.c: ===== start ===== static void __init free_initrd(void) { #ifdef CONFIG_KEXEC unsigned long crashk_start = (unsigned long)__va(crashk_res.start); unsigned long crashk_end = (unsigned long)__va(crashk_res.end); #endif if (do_retain_initrd) goto skip; #ifdef CONFIG_KEXEC /* * If the initrd region is overlapped with crashkernel reserved region, * free only memory that is not part of crashkernel region. */ if (initrd_start < crashk_end && initrd_end > crashk_start) { /* * Initialize initrd memory region since the kexec boot does * not do. */ memset((void *)initrd_start, 0, initrd_end - initrd_start); if (initrd_start < crashk_start) free_initrd_mem(initrd_start, crashk_start); if (initrd_end > crashk_end) free_initrd_mem(crashk_end, initrd_end); } else #endif free_initrd_mem(initrd_start, initrd_end); skip: initrd_start = 0; initrd_end = 0; } ===== end ===== so, one step at a time. simplest case is if i boot with "retain_initrd" kernel parameter, at which point it doesn't matter if kexec is configured or not, control just jumps to "skip:" label, and those variables are set to zero and no memory is freed. easy enough. next scenario is if kexec *isn't* configured, and i *don't* use retain_initrd, then what's run is: free_initrd_mem(initrd_start, initrd_end); skip: initrd_start = 0; initrd_end = 0; so free_initrd_mem() is call to free the *entire* range of initrd content. also, there is no zeroing/clearing of that space first, it's just freed. again, not too difficult to grasp. here's the third possibility, and the one i'm curious about: kexec is configured and i don't retain the initrd. in that case, here's the code that's executed: ===== start ===== * If the initrd region is overlapped with crashkernel reserved region, * free only memory that is not part of crashkernel region. */ if (initrd_start < crashk_end && initrd_end > crashk_start) { /* * Initialize initrd memory region since the kexec boot does * not do. */ memset((void *)initrd_start, 0, initrd_end - initrd_start); if (initrd_start < crashk_start) free_initrd_mem(initrd_start, crashk_start); if (initrd_end > crashk_end) free_initrd_mem(crashk_end, initrd_end); } else #endif free_initrd_mem(initrd_start, initrd_end); ===== end ===== now, if kexec is configured but there is no overlap between the initrd space and the kexec space, then free_initrd_mem() is called normally. but if there *is* overlap, there are two possible calls to free_initrd_mem() to free the non-kexec space, but it's that call to memset() that i don't understand -- what's the value of setting the entire range of the initrd space to zero? i don't know enough about kexec to know what that might mean but, in the normal case, memory about to be freed is not cleared, so what's the reasoning for that single line: memset((void *)initrd_start, 0, initrd_end - initrd_start); note that that will clear the *entire* initrd range -- both the part that overlaps the kexec range, and that part that doesn't. so what's the reasoning here? is that zeroing of space actually necessary? rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================

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Books for protocol development
by V.Ravikumar Sept. 25, 2012

Sept. 25, 2012

Hello all , Could some body please suggest any books/links for Linux networking protocol development. Protocol development at L2/L3 layers. Thanks in advance. Ravi

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combining "make O=" and "oldconfig" when configuring a kernel?
by Robert P. J. Day Sept. 25, 2012

Sept. 25, 2012

embarrassed to admit i'm not sure of the proper answer to this, but what is the proper way to combine the features of "make O=" and configuring with "oldconfig" and an existing .config file? it *appears* that what i would do with a pristine kernel source tree is something like: $ mkdir ../build [create a build directory] $ cp <some config file> ../build/.config $ make O=../build oldconfig and take it from there. does that sound right? rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================

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Problem during /init execve
by stl Sept. 25, 2012

Sept. 25, 2012

Hello all, What is the purpose of the inode->i_op->getxattr method? During the boot of linux-2.6.37 on a new architecture, it crashes in the get_vfs_caps_from_disk() function because of the following security check, (in my case, this field is ot initialized). if (!inode || !inode->i_op->getxattr) return -ENODATA; Has anyone seen anything similar before ? Where is this method initialized? Thanks in advance

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Assembly code fragment explanation
by K Arun Kumar Sept. 23, 2012

Sept. 23, 2012

Hi, I am trying to understand the x86 boot code where I encountered this assembly code fragment - asm volatile("lcallw *%0" : : "m" (boot_params.hdr.realmode_swtch) : "eax", "ebx", "ecx", "edx"); "lcallw *%0" - what does this mean ?? Regards, Arun

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Real mode switch hooks ??
by K Arun Kumar Sept. 23, 2012

Sept. 23, 2012

Hi I was looking at the x86 boot code - I found this - "boot_params.hdr.realmode"_swtch. why are these real mode switch hooks required ? Is this any boot param passed to the kernel cmd line ? Any pointers to read more about this ?? Regards, Arun

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ANSI C rules for variable declaration
by Lokesh Walase Sept. 21, 2012

Sept. 21, 2012

Hi all, 0.I read that ANSI C rules for variable declaration says that one must declare the variables at the beginning of any scope block. C99 onwards we are allowed to declare variables as & how required. 1.NOW, my doubt is what happens in the actual "practical/real-world" codes that are written in industry. *Do everyone *strictly* follow the ANSI C rules & declare the variables at the beginning of the scope ? * 2.Also , why are we still following that old rule ? *Is it so that old codes of 70s can be used with new codes ?* 3.There must be some standard that all of us need to follow , so what are they ? This point is very crucial for free & open-source projects , where people all around world write codes. *Can any one redirect me to some links/tutorials for the same ?* Regards, free4freedom

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