July 2014 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

[PowerPC] - Define error in arch/arch/powerpc/boot/io.h ??
by Lucas Tanure July 24, 2014

July 24, 2014

Hi, Line 2 of arch/arch/powerpc/boot/io.h : 1 #ifndef _IO_H 2 #define __IO_H 3 4 #include "types.h" Should be _IO_H, not __IO_H. I'm right ? Thanks -- Lucas Tanure +55 (19) 988176559

4 4

[PATCH] powerpc: fix wrong defintion in boot/io.h
by Lucas Tanure July 24, 2014

July 24, 2014

Fix wrong __IO_H definition in boot/io.h Reported-by: Fernando Silveira <fsilveira(a)gmail.com> Signed-off-by: Lucas Tanure <tanure(a)linux.com> --- arch/powerpc/boot/io.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/powerpc/boot/io.h b/arch/powerpc/boot/io.h index 7c09f48..394da55 100644 --- a/arch/powerpc/boot/io.h +++ b/arch/powerpc/boot/io.h @@ -1,5 +1,5 @@ #ifndef _IO_H -#define __IO_H +#define _IO_H #include "types.h" -- 2.0.0

1 0

Re: Doubt Regarding Floating Point Arithmetic
by me storage July 24, 2014

July 24, 2014

Can you please explain me one scenario that lets suppose take calculator application in lower level it should interact with kernel code So here who is take care of Floating point calculations? Sorry if i am wrong Thanks for replying me On 23 July 2014 08:45, me storage <me.storage126(a)gmail.com> wrote: > Hi > I am reading LDD .In that i didn't understand one point .In Chapter > 2(Building and Running Modules) they mentioned that > " Kernel code cannot do floating point arithmetic" > .My doubt is which code is used for floating point arithmetic that means > at low level? > > Thank you >

2 1

question about kref API
by Jeff Haran July 23, 2014

July 23, 2014

Hi, I've been reading Documentation/kref.txt in order to understand the usage of this API. There is part of this documentation that I am having difficulty understanding and was hoping somebody on this list could clarify this. One of my assumptions in reading this is that the expected usage of this API is that for a given object embedding a kref, once the object has been initialized the number of calls to "put" a given instance of an object should never exceed the number of calls to "get" that same instance. Maybe that's the root of my misunderstanding, but my assumption is that calls to kref_get() and kref_put() are expected to be made in pairs for a given instance of struct kref. If this is wrong, please let me know. Kref.txt includes some sample code that discusses using a mutex to serialize the execution of its get_entry() and put_entry() functions: 146 static DEFINE_MUTEX(mutex); 147 static LIST_HEAD(q); 148 struct my_data 149 { 150 struct kref refcount; 151 struct list_head link; 152 }; 153 154 static struct my_data *get_entry() 155 { 156 struct my_data *entry = NULL; 157 mutex_lock(&mutex); 158 if (!list_empty(&q)) { 159 entry = container_of(q.next, struct my_data, link); 160 kref_get(&entry->refcount); 161 } 162 mutex_unlock(&mutex); 163 return entry; 164 } 165 166 static void release_entry(struct kref *ref) 167 { 168 struct my_data *entry = container_of(ref, struct my_data, refcount); 169 170 list_del(&entry->link); 171 kfree(entry); 172 } 173 174 static void put_entry(struct my_data *entry) 175 { 176 mutex_lock(&mutex); 177 kref_put(&entry->refcount, release_entry); 178 mutex_unlock(&mutex); 179 } The sample code does not show the creation of the link list headed by q, so it is unclear to me what the value of the reference counts in these my_data structures are at the time they are enqueued to the list. Put another way, it's not clear to me from reading this whether kref_init(&(entry->refcount)) was called before the instances of struct my_data were put into the list. So I have two interpretations of what is being illustrated with this sample code and neither makes much sense to me. 1) The krefs are initialized before they go into the list. If the krefs in the instances of struct my_data are initialized by kref_init() before they go into the list and thus start off with a 1, then it would seem to me that the mutex would not be necessary so long as the number of calls to put_entry() never exceeds the number of calls to get_entry(). However in this case, (so long as the number of calls to put_entry() never exceeds the number of calls to get_entry()), the release function would never get called and the entries would remain in the list because the reference count would never get down to 0. So this doesn't seem like a correct interpretation to me unless the assumption is that what is being illustrated is a case where the number of calls to put_entry() equals the number of calls to get_entry() plus 1, the final call to put_entry() being the one that causes the deletion of the entry from the list and its freeing, in which case the mutex is clearly necessary. But this usage (# of puts == # of gets + 1) seems somewhat artificial and contrary to my initial assumption. 2) The krefs are not initialized before they go into the list. On the other hand, if the krefs are not initialized by kref_init() before they go into the list and thus contain 0, then the first call to kref_get() to retrieve a given entry at line 160 would seem problematic since in recent kernels (going back at least 4 years near as I can tell), doing that would generate a warning: 41 static inline void kref_get(struct kref *kref) 42 { 43 /* If refcount was 0 before incrementing then we have a race 44 * condition when this kref is freeing by some other thread right now. 45 * In this case one should use kref_get_unless_zero() 46 */ 47 WARN_ON_ONCE(atomic_inc_return(&kref->refcount) < 2); 48 } It doesn't seem reasonable to me that sample code like this would be written such that it generated warnings when executed. So this doesn't seem like a valid interpretation either. If someone knowledgeable of this interface could explain whether in this sample code the krefs are initialized via kref_init() before they are put into the list, I think it would help a lot in my attempt to understand it. Thanks, Jeff Haran

4 9

wait_event_* and wake_up_*
by Pranith Kumar July 23, 2014

July 23, 2014

Hello, Could someone point me to good documentation about the wait_event_* and wake_up_* event family APIs? I tried searching and nothing concrete shows up. In particular: When do we use wake_up_interruptible()? Does wake_up(wq) wake up all the threads which are waiting on that work queue? Is the waking up of threads sequential? That should suffice for now I guess :) Regards, -- Pranith

2 2

usb serial programming
by Amadeus W.M. July 23, 2014

July 23, 2014

Not sure if this is the right venue for this question, please direct me to the right place if it's not. I have a C program that opens the serial port /dev/ttyS0 and sends commands (as strings) back and forth to a pan-tilt-zoom camera. That works very well, but I want to use the camera with a Raspberry Pi, which of course does not have a serial port (RS232). One option is to use a RS232 to usb adapter and rewrite the C code for the usb port. So how do I go about that? Is usb serial port programming possible and documented anywhere? Thanks!

4 11

Path of network packet in kernel
by Anil Joshi July 22, 2014

July 22, 2014

Hi All, I am just new to all this,just wanted to trace the path of the packet since it enter the system(inside the kernel (functions and system calls)) and reaches the destination application. How to do that. Regards

5 7

Confused by function names cpuidle_install_idle_handler/cpuidle_uninstall_idle_handler
by Mohammad Merajul Islam Molla July 22, 2014

July 22, 2014

Hello, In drivers/cpuidle/cpuidle.c, there are two functions cpuidle_install_idle_handler & cpuidle_uninstall_idle_handler. The names seem confusing to me as they don't install any handler, rather set 'initialized' variable to 1/0. In v3.0 kernel, these functions used to look as below where they installed and uninstalled some handler function - void cpuidle_install_idle_handler(void) 123 { 124 if (enabled_devices && (pm_idle != cpuidle_idle_call)) { 125 /* Make sure all changes finished before we switch to new idle */ 126 smp_wmb(); 127 pm_idle = cpuidle_idle_call; 128 } 129 } void cpuidle_uninstall_idle_handler(void) 135 { 136 if (enabled_devices && pm_idle_old && (pm_idle != pm_idle_old)) { 137 pm_idle = pm_idle_old; 138 cpuidle_kick_cpus(); 139 } 140 } Do these names (and corresponding comments) still hold? -- Thanks, -Meraj

3 5

Re: How to prevent a module from unloading when in used
by Chetan Nanda July 21, 2014

July 21, 2014

On Wed, Jul 16, 2014 at 8:49 PM, Chetan Nanda <chetannanda(a)gmail.com> wrote: > > > On Wed, Jul 16, 2014 at 6:39 PM, John de la Garza <john(a)jjdev.com> wrote: > >> On Wed, Jul 16, 2014 at 04:00:18PM +0530, Chetan Nanda wrote: >> > A depends on B, so B is automatically loaded when A is loaded. >> > B module is also directly being used by the user side code via misc >> > interface. >> >. >> > Now when I am unloading module A, via "modprobe -r A" it is also >> unloading >> > the module B which is being used by the application and resulting in the >> > kernel crash. >> >> You said that A depends on B, right? Why do you have A dependng on B? >> If it A needs to have B then it makes sense that you can not remove A >> while >> B is in use. If A doesn't need B, why not remove the dependency. > > A is calling few APIs defined by B. But why when user space application is already using module B. (it has already open its device fd) kernel allows to remove it. I tried with doing try_module_get() in the module's open function, it prevent module B unloading but cause thread doing modprobe -r to hang Is there any other way to mark module as busy when being used by user application?

2 8

How to automate checkpatch && get_maintainers && git send-email of commits range?
by Andrey Utkin July 20, 2014

July 20, 2014

Is there script for automated checkpatch.pl && get_maintainers.pl && git send-email for range of commits? I see none. Would it be welcome to submit such one to kernel tree? -- Andrey Utkin

6 5