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how to return unneeded init-data to kernel ?
by jim.cromie＠gmail.com June 3, 2021

June 3, 2021

hi all, When one builds with CONFIG_DYNAMIC_DEBUG=y one consumes extra memory for kernel data Ive managed to coax dyndbg into "not needing" about 30KiB in a __dyndbg* section, how can I return it to the kernel ? https://lore.kernel.org/lkml/20210529200029.205306-1-jim.cromie@gmail.com/ b4: dyndbg: 284 modules, 3013 entries and 11360 bytes in ddebug tables, 168728 bytes in __dyndbg section after: dyndbg: 2900 prdebugs in 277 modules, 12 KiB in ddebug tables, 90 KiB in __dyndbg section, 67 KiB in __dyndbg_sites section about 30Kib of the 67 is recoverable patchset, the short version: A- split __dyndbg in 2, isolate repeating module,file,function data. reach by new .site ptr. B- be safe if !.site C- work towards replacing .site with indexing on vectors: __dyndbgs & __dyndbg_sites c1 add ._index c2 use .index to get to __dyndbgs[0] ( then -> sites[._index] ) c3 insert header record at __dyndbgs[0] c4 rework header to differentiate from struct _ddebug, keep .site c5 drop .site, use indirection to get sites[._index] D- split _index into 2 jobs; _back (to [0]) and _map (to [N]) E- find duplicate site recs e1 - use just 1st of duplicate blocks e2 - in modules, move new site recs into front of sites vector e3 - in builtins, pack the whole sites vector, not just within the module-blocks So that leaves about 40% of __dyndbg_sites unneeded. How to return this to free memory ? FWIW, I tried krealloc on a module's chunk of memory (as attached by module:load_info()) it might have worked if that chunk was kallocd separately by elf-loader code. It evidently was not. PS: questions welcome here, reviews probably best at lkml

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How should I update the screen in an fbdev driver?
by Dávid Virág May 30, 2021

May 30, 2021

Hi! I am working on trying to figure out how an Android phone's display works by modifying the original driver, so I can understand how it works to write a less cluttered, and more non-Android distro friendly driver. What I noticed is that a register needs to be set in order for the screen to update (I guess there's some kind of internal double buffering going on in the background). I couldn't find a way for it to update automatically, so for now I included the code that sets the register in a VSync thread created originally to let the android HAL know that a VSync IRQ has occurred or a timeout has passed. I feel like this isn't what the solution should be. While looking around I found the fb_sync function. I thought that sounds like that refresh register should be set there, so I moved the code in fb_sync and I added it to the fb_ops struct. Now the display only refreshes on the first frame of the fbcon, replacing the phone's bootloader logo with the console. What am I doing wrong? How is this supposed to be handled? Btw I am using a 4.4 kernel fork, since that's the only kernel that ever shipped with the phone or the SoC, and there's no mainline support for it yet.

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Performing pointer arithmetic on a null pointer?
by Hyeonggon Yoo May 27, 2021

May 27, 2021

Hello, I was compiling kernel with make CC=clang-10 W=1 -s there are some places that compiler complains about pointer arithmetic like below.it says it's undefined behavior. is it just OK to use UBs like this (I hope it's not), or am I missing something? fs/kernfs/file.c:128:15: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic] return NULL + !*ppos;

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What Device Deriver Developers Do?
by Hyeonggon Yoo May 25, 2021

May 25, 2021

Hello, I've read some books (Linux Device Drivers, or Linux Kernel Development) OK, So now I know a little bit more about linux internals.. Yeah, I thought I should know about the hardware to develop device driver. because I'm interested in hdd and ssd, so I tried to read ATA specification. but I don't understand EVERY sentence of that document... Is there something I'm missing, like I should learn electronic circuit first...? Did I try too tough thing as newbie? Or is there a good entrypoint for developing device driver? Thank you, Any tiny advice will be so much helpful...! Hyeonggon.

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How to choose format specifier for boolean variables in Linux kernel?
by 慕冬亮 May 24, 2021

May 24, 2021

Hi all, how do I choose format specifier for boolean variables in Linux kernel? For example, bool bup; pr_alert("%XXX", bup); What should "XXX" be in the Linux kernel? -- My best regards to you. No System Is Safe! Dongliang Mu

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Adding a sched_class after the removal of ".next" regarding priority
by J Mårtensson May 21, 2021

May 21, 2021

Hi, I have been trying to add a new scheduler to the Linux kernel. I have found that to add a sched_class, I need to add it to SCHED_DATA in vmlinux.lds.h. instead of editing the now removed .next variable. Depending on what order I put into the priority list, it will crash the kernel during the booting process after rebooting. Any tips on what could be causing this would be appreciated! Currently this works #define SCHED_DATA \ STRUCT_ALIGN(); \ __begin_sched_classes = .; \ *(__idle_sched_class) \ *(__my_sched_class) \ *(__fair_sched_class) \ *(__rt_sched_class) \ *(__dl_sched_class) \ *(__stop_sched_class) \ __end_sched_classes = .; While this does not #define SCHED_DATA \ STRUCT_ALIGN(); \ __begin_sched_classes = .; \ *(__idle_sched_class) \ *(__fair_sched_class) \ *(__my_sched_class) \ *(__rt_sched_class) \ *(__dl_sched_class) \ *(__stop_sched_class) \ __end_sched_classes = .; Regards Jacob

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How to test device driver?
by Hyeonggon Yoo May 18, 2021

May 18, 2021

Hello, there are lots of drivers in linux. My questions are: - how can we test driver? is it just using that device for a long time as a user? - what to do if there's a bug (like syzbot report), fixed it, but if we don't have that device?

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How to make BPF program which is attached to kprobe keep working after process exits?
by Junyeong Jeong May 15, 2021

May 15, 2021

Hello :) My BPF program is loaded by `bpf_load_program_xattr` of `libbpf` and then attached to kprobe event using `perf_event_open`, `ioctl(PERF_EVENT_IOC_SET_BPF)` and `ioctl(PERF_EVENT_IOC_ENABLE)`. Then the BPF program is working as I expected. And I wanted to make the BPF program keep working even though the userspace process exits. So I pinned it using `bpf_obj_pin()`. But unfortunately the BPF program is never called after the userspace process exits. I discovered the cause of the failure. The file descriptor of perf event is automatically closed when userspace process exits so objects related to perf event get destroyed. So I tried to find how to pin the fd of perf event, but I got to know that only BPF programs, BPF maps and BPF links are subject to pin. How can I make my BPF program which is attached to kprobe keep working after userspace process exits? regards, Junyeong

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Adding a new scheduler to the kernel
by J Mårtensson May 13, 2021

May 13, 2021

Hi, I am trying to develop a modified version of CFS. After following several examples online, I am wondering two things. First, how do I add the new scheduler after the removal of .next in the sched_class? I have tried adding it to the SCHED_DATA in /include/asm-generic/vmlinux.lds.h. Is this the correct place or do I need to make more changes? Second, the modified kernel will compile without a problem but will refuse to launch in grub, is there any way I can debug it on an older working kernel or use other appropriate methods? As it stands now it will just show a black screen after "Loading initial ramdisk ..." . I do not get any further information after removing "quiet splash". Regards Jacob

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Re: Does FreeRTOS works on Linux?
by Primoz Beltram May 13, 2021

May 13, 2021

On 6. 05. 21 18:00, kernelnewbies-request(a)kernelnewbies.org wrote: > Subject: > Does FreeRTOS works on Linux? > From: > Raul Piper <raulpblooper(a)gmail.com> > Date: > 5. 05. 21 20:07 > > To: > kernelnewbies <kernelnewbies(a)kernelnewbies.org> > > > Hello, > I am having an embedded linux system and wanted to use any RTOS on > it. Can I use Free RTOS? > My project has 2 tasks , one is producer and another is consumer > working parallely. > > I have seen Free RTOS used in microcontrollers handling this , how can > this be done in Linux(which such APIs if not Free RTOS). > > Regards, > R > > Open AMP project (www.openampproject.org) ? WBR Primoz

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