September 2012 - kernelnewbies@kernelnewbies.org

Why __force sparse annotation needed in copy_from_user() to vmalloc'ed area
by Thomas Petazzoni Sept. 5, 2012

Sept. 5, 2012

Hello, In the course of reviewing a new framebuffer driver [1], I discovered the following bit of code: + dst = (void __force *) (info->screen_base + p); + + if (copy_from_user(dst, buf, count)) + err = -EFAULT; Where info->screen_base is a vmalloc'ed memory area. Many other framebuffer drivers have used this __force sparse annotation: see [2] or [3] for example. I am confused as to why sparse requires the __force annotation for what seems to be a regular user to kernel copy. The __force annotation is typically not needed for user to kernel copy when the destination kernel buffer has been kmalloc'ed. Why does sparse handles vmalloc'ed buffer differently, and gives a warning in this case? Thanks, Thomas [1] http://marc.info/?l=linux-fbdev&m=134683775709128&w=2 [2] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdi… [3] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdi… -- Thomas Petazzoni, Free Electrons Kernel, drivers, real-time and embedded Linux development, consulting, training and support. http://free-electrons.com

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Re: Why exported const value modified by another driver not updated in original driver
by Manavendra Nath Manav Sept. 5, 2012

Sept. 5, 2012

On Tue, Sep 4, 2012 at 5:55 PM, Dan Carpenter <dan.carpenter(a)oracle.com> wrote: > On Tue, Sep 04, 2012 at 03:58:20PM +0530, Manavendra Nath Manav wrote: >> Is the above a genuine kernel bug, or i am missing something out here. Pls help. >> > > When you declare something as const then the compiler assumes it > really is const and uses a literal instead of reading from memory. > I'm surprised the compiler doesn't print a warning message. > > It has to do with compilers, nothing to do with kernels. > > regards, > dan carpenter Thanks All, I understood the problem and current gcc behaviour after looking at output of objdump of driver.ko file when the variable is declared as "const" and in second case as "const volatile". The compiler optimises by directly passing the value in first case and the address of variable in second case. Thanks for all the help and clarification. push $0x7b // 123 in decimal push $0x0 -- Manavendra Nath Manav

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Writing I2C chip driver in combination with other structure
by aurum spark Sept. 4, 2012

Sept. 4, 2012

Hi All, I have written and seen lot of code of i2c drivers for input subsystem devices and much familiar with that. Now I have started writing driver for one chip that is not exactly type of input device. So, little bit confused about how should be the driver architecture so that it will be a clean code yet simple. Following text and diagram explains short functional description of chip. Chip Interface :- ========== ====== ======= | | | I2C | | | ----- I2C --- | Device | | ARM | ----- Bus -- | With rw | ------------ Input Connect | | | regs | | | ---------------| | ====== INTR ======= So above picture depicts how chip is interfaced to ARM CPU. This chip has some registers which are rw and can be used to read certain information from chip or to program it. This chip is capable of giving intterupt to CPU. This chip has one input which it like some cable connection. It detects when this cable is connected and raises the interrupt. Functionality Expected in Driver:- =================== 1. Driver should communicate with chip over I2C. 2. Driver should report event to user space upon receiving interrupt from I2C device. 3. Driver should be capable of programming I2C chip @ the time of initialization as well should be capable of changing registers as and when Linux User Space application wants to. What I have thought of:- ============== #1 definitely makes this as i2c client driver(chip driver) #2 Here I have few options a) Shall I use input subsystem mechanism to report interrupt as an event to user space ? In this case it's combination of I2C + input subsystem. Then application can read something over i2c and decide next flow, OR b) Shall I use some sysfs based interface on which application can do select sys call and can receive event ? for this nothing extra required. #3 Since approach in #2 is not fixed here also there are multiple options a) This can be written as I2C client + char driver so that it can have ioctls to send commands to i2c chip based on applications demand. b) We can provide sysfs interface to user space to send commands to i2c device as and when required at runtime. As per my thoughts I am not sure shall I make this as input device just to report one event upon getting interrupt from device. Also will it be ok to have this as char driver in combination with i2c chip driver ? Please guide me and take me to correct source or documentation or to source code of some driver for this kind of device. Does i2c support ops like fbops ? Regards, ~/Aurum

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Why exported const value modified by another driver not updated in original driver
by Manavendra Nath Manav Sept. 4, 2012

Sept. 4, 2012

Hi, I have declared a static const int variable in one driver and exported that variable symbol. In another driver i am modifying that variable. The other driver prints the modified value but the original driver retains the original value. When both virtual and physical addresses of the variable as seen by both drivers are same, how is this even possible. Is it a kernel bug? [root@localhost bug]# uname -a Linux localhost.localdomain 3.5.3 #3 SMP Mon Sep 3 21:52:12 IST 2012 i686 i686 i386 GNU/Linux ================================ [root@localhost bug]# cat driver.c #include <linux/module.h> static const int value = 123; int init_module() { printk("Base driver (init): value = %d\n", value); printk("Base driver (init): virtual address of value = %p\n", (void *)&value); printk("Base driver (init): physical address of value = %p\n", (void *)__pa(&value)); return 0; } void cleanup_module() { printk("Base driver (exit): value = %d\n", value); printk("Base driver (exit): virtual address of value = %p\n", (void *)&value); printk("Base driver (exit): physical address of value = %p\n", (void *)__pa(&value)); } EXPORT_SYMBOL(value); ============================================== [root@localhost bug]# cat hacker.c #include <linux/module.h> extern int value; int init_module() { value = 987; printk("Hacker driver (init): value = %d\n", value); printk("Hacker Base driver (init): virtual address of value = %p\n", (void *)&value); printk("Hacker Base driver (init): physical address of value = %p\n", (void *)__pa(&value)); return 0; } void cleanup_module() { printk("Hacker driver (exit): value = %d\n", value); printk("Hacker driver (exit): virtual address of value = %p\n", (void *)&value); printk("Hacker driver (exit): physical address of value = %p\n", (void *)__pa(&value)); return; } ========================================= [root@localhost bug]# insmod driver.ko Base driver (init): value = 123 Base driver (init): virtual address of value = f89d61c8 Base driver (init): physical address of value = 389d61c8 [root@localhost bug]# insmod hacker.ko Hacker driver (init): value = 987 Hacker Base driver (init): virtual address of value = f89d61c8 Hacker Base driver (init): physical address of value = 389d61c8 [root@localhost bug]# rmmod hacker.ko Hacker driver (exit): value = 987 Hacker driver (exit): virtual address of value = f89d61c8 Hacker driver (exit): physical address of value = 389d61c8 [root@localhost bug]# rmmod driver.ko Base driver (exit): value = 123 Base driver (exit): virtual address of value = f89d61c8 Base driver (exit): physical address of value = 389d61c8 [root@localhost bug]# cat /proc/kallsyms | grep value f89f91c8 R value [driver] f89f9088 r __ksymtab_value [driver] f89f91cc r __kstrtab_value [driver] Interestingly, when i change the declaration of variable to "volatile" in driver.c as "static const volatile int" then the driver.ko prints modified value "987" during rmmod and the original value "123" is lost. What is the difference between this and previous declaration? Please forgive if you find this question silly? -- Manavendra Nath Manav

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Re: Why exported const value modified by another driver not updated in original driver
by Manavendra Nath Manav Sept. 4, 2012

Sept. 4, 2012

On Tue, Sep 4, 2012 at 4:09 PM, Julian Andres Klode <jak(a)jak-linux.org> wrote: > On Tue, Sep 04, 2012 at 03:58:20PM +0530, Manavendra Nath Manav wrote: >> On Tue, Sep 4, 2012 at 3:00 PM, Manavendra Nath Manav >> <mnm.kernel(a)gmail.com> wrote: >> > Hi, >> > >> > I have declared a static const int variable in one driver and exported >> > that variable symbol. In another driver i am modifying that variable. > > No, you did not export it. It's static (and static is kind of the opposite > of extern). And then you try to assign a value to a non-static variable with > the same name, which might not even exist (as the other is static, this should > not even work in my opinion). > >> > The other driver prints the modified value but the original driver >> > retains the original value. When both virtual and physical addresses >> > of the variable as seen by both drivers are same, how is this even >> > possible. Is it a kernel bug? > > It only works because the compiler optimized the value = 123 away, > and replaced value with 123 in the first module everywhere, as it > is declared const and thus cannot be changed, so there's no reason > to read it from memory. > > -- > Julian Andres Klode - Debian Developer, Ubuntu Member > > See http://wiki.debian.org/JulianAndresKlode and http://jak-linux.org/. Still I am not able to understand !! Why the output behaviour changes when the declaration is made as "const volatile". I get the same results as above when i remove "static" and retain "const". -- Manavendra Nath Manav

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Is it possible to use a local drive to cache an NFS share
by Graeme Russ Sept. 4, 2012

Sept. 4, 2012

Hi All, I am mounting /home over NFS which is great, but it really kills compile times. So I have a local HDD which I have copied all my source code over from which I do my coding and compiling. What I would love to do is use the local drive to cache /home. Specifically one user directory under /home - only one machine will ever be modifying the contents of this directory. Or should I just use rsync? On a side note - I moved my source code from a 1TB HDD on a 3Gb/s SATA port to an Intel 510 series 250GB SSD on a 6Gb/s SATA port. But I didn't see an appreciable increase in compile speed. Regards, Graeme

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itanium variable sized TLB entries support
by Shentino Sept. 3, 2012

Sept. 3, 2012

Is there a way for the kernel to take advantage of a flexible TLB entry coverage size, as in itanium, or is the pgd/pud/pmd/pte abstraction pretty much hard coded?

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Query regarding some blk_queue_XXX functions. (For kernel 2.6.32 SLES 11)
by Pranay Kumar Srivastava Sept. 3, 2012

Sept. 3, 2012

Hi All, I'm using SLES11 (kernel 2.6.32) I recently did a dummy code for block device driver where the whole disk is made of pages in memory. I have some query regarding usage of some of the functions, I've absolutely no idea what the first 2 do. What I was trying to do was that I wanted the block layer to give my driver requests which are aligned according to my device's minimum sector size. Since I wanted to play with the driver I chose 1024 bytes instead of 512, however I've tried with 2048 bytes device sector size as well and all seems well :D Now the problem is kernel wants to give everything in 512 byte but I want to receive everything aligned properly according to my device's sector size. So I looked in kernel code and I thought I could use the functions below... 1. blk_limits_io_min: No clue about this one, in-fact the device didn't got added when I used this. 2. blk_queue_physical_block_size: This one checks the logical block size before it sets physical block size itself. But not really sure about it's purpose 3. blk_queue_logical_block_size: I think this is the one responsible that I get requests properly aligned to device's sector size? Not really sure... 4. set_capacity: There's also a callback in block_device_operations by this name? However this function just sets the size of part0 of the gendisk. I've not seen the callback being called upon, I used some print statements and I never see them. So what's the purpose of the callback? Lastly: There's a field gd->start in gendisk which apparently sets the start of data sector? But why would block layer be bothered with that? Is this used by filesystems since application programs certainly can write to sector 0 as well right? And a couple more :P , When I try to use fdisk on the disk, it seems to create partition table (I think), however when it tries to read the partition table again I get invalid argument to ioctl. So I think I'm missing an ioctl call in my driver correct? I've none yet though. However this issue aside, if I'm able to create a partition then my driver should be able to handle additional disks correct? And it would follow the same operations when I added my module and called add_disk the first time correct? Thanks a lot for reading! Regards, Pranay Kr. Srivastava pranay.shrivastava(a)hcl.com Software Engineer ERS,HCL Technologies A-5, Sector 24, Noida 201301, U.P. (India) ::DISCLAIMER:: ---------------------------------------------------------------------------------------------------------------------------------------------------- The contents of this e-mail and any attachment(s) are confidential and intended for the named recipient(s) only. E-mail transmission is not guaranteed to be secure or error-free as information could be intercepted, corrupted, lost, destroyed, arrive late or incomplete, or may contain viruses in transmission. The e mail and its contents (with or without referred errors) shall therefore not attach any liability on the originator or HCL or its affiliates. Views or opinions, if any, presented in this email are solely those of the author and may not necessarily reflect the views or opinions of HCL or its affiliates. Any form of reproduction, dissemination, copying, disclosure, modification, distribution and / or publication of this message without the prior written consent of authorized representative of HCL is strictly prohibited. If you have received this email in error please delete it and notify the sender immediately. Before opening any email and/or attachments, please check them for viruses and other defects. ----------------------------------------------------------------------------------------------------------------------------------------------------

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Difference between kernel 3.0.15 and kernel 3.4
by Ritesh Harjani Sept. 3, 2012

Sept. 3, 2012

Where i can find all the differences between the kernel version 3.0.15 and 3.4. Ritesh

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Re: Kernelnewbies Digest, Vol 22, Issue 1
by kishore kumar Sept. 3, 2012

Sept. 3, 2012

hello everybody, I am new to kernel programming. Can anyone tell me how to start it. kishore kumar On Sun, Sep 2, 2012 at 9:30 PM, <kernelnewbies-request(a)kernelnewbies.org>wrote: > Send Kernelnewbies mailing list submissions to > kernelnewbies(a)kernelnewbies.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > or, via email, send a message with subject or body 'help' to > kernelnewbies-request(a)kernelnewbies.org > > You can reach the person managing the list at > kernelnewbies-owner(a)kernelnewbies.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Kernelnewbies digest..." > > > Today's Topics: > > 1. Re: map bss to addr zero (Ritesh Harjani) > 2. Re: map bss to addr zero (Mulyadi Santosa) > 3. Re: Kernel booting problem. (Jim Cromie) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Sun, 2 Sep 2012 00:53:17 +0530 > From: Ritesh Harjani <ritesh.harjani(a)gmail.com> > Subject: Re: map bss to addr zero > To: Srinivas Ramanan <srinivas.ramanan(a)gmail.com>, > mulyadi.santosa(a)gmail.com > Cc: kernelnewbies(a)kernelnewbies.org > Message-ID: > < > CAD15agbM8jO%2B-KmrRByDpd0NRN3k_bcpEQi9p8W721c8_9Pefg(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > On Mon, Aug 27, 2012 at 2:01 AM, Srinivas Ramanan < > srinivas.ramanan(a)gmail.com> wrote: > > > Hi all, > > This is an interview question. > > "What is the implication of mapping BSS to the Virtual Memory Segment 0?" > > > > I was not able to find the answer from web. So posting here. > > thanks, > > srinivas > > > > _______________________________________________ > > Kernelnewbies mailing list > > Kernelnewbies(a)kernelnewbies.org > > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > > > > Hi, > > The bss section typically includes all uninitialized variables declared > with the static keyword. > The zero page which is a copy-on-write page that reads as zero is used to > map the BSS segment. Any process referencing the zero page sees the page > filled with zeos and if it tries to writes to the page, it ends up > modifying a private copy. (LDD ch15. page 429) > > Mulyadi, > > Why it will generate a page fault later when process will tries to > reference it, while it already has mapped with the zero page? > > > Sorry if i am wrong anywhere. > Ritesh >

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