September 2019 - kernelnewbies@kernelnewbies.org

alsa
by Nuno Sá Sept. 13, 2019

Sept. 13, 2019

Hi all, Im currently doing a driver for the alsa subsystem. This is my first interaction with it, so sorry if this is to trivial. Since this is my first time on the subsystem I'm first posting my question here before going to the alsa mailing list. So, I'm doing a codec driver for this device https://www.analog.com/media/en/technical-documentation/data-sheets/ADAU711… . The device is a converter from PDM to I2S/TDM and it has 4 PDM inputs which can be enabled/disabled via I2C. My question is, how can I add controls to enable/disable the inputs? I'm trying to use `SND_SOC_DAPM_SWITCH()` together with a `SOC_DAPM_SINGLE()` control. The thing is, when I use alsamixer this controls only come up on the playback tab which makes me feel that something is not ok (the device only supports Capture stream). Is this correct way to add controls for input's? Any inputs would be helpful... Thanks in advance, Nuno Sá

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how to get kernel crashdump
by Muni Sekhar Sept. 13, 2019

Sept. 13, 2019

Hi All, I’m having a Linux system, I’m seeing it gets hung during certain tests. When it hung, it does not even respond for SYSRQ button, only way to recover is power-button-only. No response for SYSRQ button means kernel crashed? After reboot I looked at the kern.log and most of the times it has “^@^@^@^ ...“ line just before reboot. Can someone clarify me what the kernel log entry “^@^@^@^ ...“ means? I suspect kernel is crashed, but it does give any crashdump in kern.log. Later I enabled the kernel crash dump(sudo apt install linux-crashdump) and rerun the test but still nothing copied to the disk(/var/crash/). I don’t have onboard serial port in my machine, so I tried get the crash dump via netconsole, but this method also does not able to catch the crash dump. Can someone help me how to debug in this scenario? And I'd like to know what other options available to get the crash dump? Any help will be greatly appreciated. -- Thanks, Sekhar

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Parent PID for kthreads?
by Martin Galvan Sept. 11, 2019

Sept. 11, 2019

Hi all, I'm looking for a way to determine the PID of the process on which kthread_create is invoked to spawn a new kthread. The newly created kthreads all show current->parent->pid == 2, which I understand corresponds to kthreadd. current->tgid doesn't seem to be set to the main process' PID either. Is there a way to determine this from within the kthread? Thanks!

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Re: perf_event wakeup_events = 0
by Valdis Klētnieks Sept. 7, 2019

Sept. 7, 2019

On Sat, 07 Sep 2019 09:14:49 -0700, Theodore Dubois said: Reading what it actually says rather than what I thought it said.. :) Events come in two flavors: counting and sampled. A counting event is one that is used for counting the aggregate number of events that occur. In general, counting event results are gathered with a read(2) call. A sampling event periodically writes measurements to a buffer that can then be accessed via mmap(2). For some reason, I was thinking counting events. -ENOCAFFEINE. :) > sample_freq is 4000 (and freq is 1). Here’s the man page on this field: > > sample_period, sample_freq > A "sampling" event is one that generates an overflow notifica‐ > tion every N events, where N is given by sample_period. A sam‐ > pling event has sample_period > 0. There's this part: > pling event has sample_period > 0. When an overflow occurs, > requested data is recorded in the mmap buffer. The sample_type > field controls what data is recorded on each overflow. So an entry is made in the buffer. It's not clear that this immediately triggers a signal... MMAP layout When using perf_event_open() in sampled mode, asynchronous events (like counter overflow or PROT_EXEC mmap tracking) are logged into a ring- buffer. This ring-buffer is created and accessed through mmap(2). The mmap size should be 1+2^n pages, where the first page is a metadata page (struct perf_event_mmap_page) that contains various bits of infor? mation such as where the ring-buffer head is. So you need to look at what size mmap buffer is being allocated. It's *probably* on the order of megabytes, so that you can buffer a fairly large number of entries and not take several user/kernel transitions on every single entry... > If I’m reading this right, this is a sampling event which overflows 4000 times a second. And 4,000 entries are made in the buffer per second.. > But perf then does a poll call which wakes up on this FD with POLLIN after > 1.637 seconds, instead of 0.00025 seconds At which point perf goes and looks at several thousand entries in the ring buffer...

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Re: perf_event wakeup_events = 0
by Valdis Klētnieks Sept. 7, 2019

Sept. 7, 2019

On Sat, 07 Sep 2019 09:14:49 -0700, Theodore Dubois said: > If I’m reading this right, this is a sampling event which overflows 4000 > times a second. But perf then does a poll call which wakes up on this FD with > POLLIN after 1.637 seconds, instead of 0.00025 seconds. No, it *takes a sample* 4,000 times a second. For instance, number of cache line misses since the last sample. You get an overflow when the counter wraps because there have been more than 2^32 events since you read the counter. At least that's my understanding of it.

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Re: perf_event wakeup_events = 0
by Valdis Klētnieks Sept. 7, 2019

Sept. 7, 2019

On Fri, 06 Sep 2019 16:28:24 -0700, Theodore Dubois said: > The man page for perf_event_open(2) says that recent kernels treat a 0 > value for wakeup_events the same as 1, which I believe means it will > notify after a single sample. However, strace on perf(1) shows that it > uses wakeup_events=0, and it's definitely not waking up on every > sample (it seems to be waking up every few seconds.) > tools/perf/design.txt says "Normally a notification is generated for > every page filled". Is the documentation wrong, or am I > misunderstanding something? wakeup_events, wakeup_watermark This union sets how many samples (wakeup_events) or bytes (wakeup_watermark) happen before an overflow notification happens. Which one is used is selected by the watermark bit flag. wakeup_events counts only PERF_RECORD_SAMPLE record types. To receive overflow notification for all PERF_RECORD types choose watermark and set wakeup_watermark to 1. Prior to Linux 3.0, setting wakeup_events to 0 resulted in no overflow notifications; more recent ker? nels treat 0 the same as 1. My reading of that is that in pre-3.0 kernels, you could choose to not get overflow notifications, and now you'll get them whether or not you wanted them. Under "overflow handling", we see: Overflows are generated only by sampling events (sample_period must have a nonzero value). So the reason strace says perf is only waking up every few seconds is probably because you either launched perf with options that only create trace events, or it takes several seconds for an overflow to happen on a sampling event. A lot of those fields are u64 counters, and won't overflow anytime soon. Even the u32 counters can take a few seconds to overflow....

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remote kernel developing with arm soc board eval kit
by Oliver Graute Sept. 7, 2019

Sept. 7, 2019

Hello list, I've a soc board on my desk in the office and would like to know what is the best practice to do kernel development remotely with such a board. In the office I can plug in sd-cards or switch power off which I can't do if I'am not physically present. But I think with a tftp/nfs setup and ssh/minicom I can do something in that direction. So I would like to know how the real kernel developers build such a setup and how they handle kernel panics without driving back to office or call a collegue. Best regards, Oliver

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perf_event wakeup_events = 0
by Theodore Dubois Sept. 6, 2019

Sept. 6, 2019

The man page for perf_event_open(2) says that recent kernels treat a 0 value for wakeup_events the same as 1, which I believe means it will notify after a single sample. However, strace on perf(1) shows that it uses wakeup_events=0, and it's definitely not waking up on every sample (it seems to be waking up every few seconds.) tools/perf/design.txt says "Normally a notification is generated for every page filled". Is the documentation wrong, or am I misunderstanding something? ~Theodore

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is there a macro symbol for strlen($LINUX_SOURCE_PATH)
by jim.cromie＠gmail.com Sept. 5, 2019

Sept. 5, 2019

in the kernel, there are a lot of usages of __FILE__ this means that there are a lot of copies of fully rooted paths, including all the varying ~/src/linux prefixes used by individual builders. Id hazard a guess that something like __RELPATH_FILE__ would work in many cases and perhaps even be more readable (by reducing long strings) in the generated output Do the macros exist to cobble this together for compile-time ? Does it warrant a short name ? either to wrap __FILE__ uses or replace them ?

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[OSSNA] Intro to kernel hacking tutorial
by Tobin C. Harding Sept. 4, 2019

Sept. 4, 2019

Hi, I am doing a tutorial at OSSNA in San Diego on getting into kernel hacking. I'm only a couple of years deep into kernel hacking so I wanted to reach out to those more experienced than myself (and those less experienced). Is there any thing that you would really like to see covered in this tutorial? If you are a grey beard is there anything that you have been lamenting us newbies not knowing/doing? If you are a newbie is there anything that you are struggling with that you really want to learn? Current format/content: the tutorial will attempt to bridge the gap in the learning process between the 'first patch' page on kernelnewbies.org wiki and being 'comfortable' patching the kernel via LKML. Outcome will (hopefully) be a small patch set into drivers/staging/. (Don't worry Greg only one group got to this stage last time, you won't get flooded with patches :) Thanks, Tobin.

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