On 2021-03-23 1:29 p.m., Takashi Iwai wrote:
On Tue, 23 Mar 2021 18:08:14 +0100, Andrey Grodzovsky wrote:
On 2021-03-23 12:50 p.m., Takashi Iwai wrote:
On Tue, 23 Mar 2021 17:11:20 +0100, Andrey Grodzovsky wrote:
On 2021-03-23 10:54 a.m., Takashi Iwai wrote:
On Tue, 23 Mar 2021 15:22:01 +0100, Andrey Grodzovsky wrote:
On 2021-03-23 7:39 a.m., Takashi Iwai wrote: > On Tue, 23 Mar 2021 12:23:16 +0100, > Andrey Grodzovsky wrote: >> >> Just an update, i found the issue which was actually to wake up the HW >> before doing stop/restart (using pm_runtime_get_sync), also handled >> protecting from concurrent snd_pcm_ioctls accessing the registers >> while the BAR is unmapped. Can go through BAR move while aplay is >> running now. >> >> Once again, would be happy for any comments on the code - >> https://nam11.safelinks.protection.outlook.com/?url=https:%2F%2Fcgit.freedes... > > Hrm, this kind of change is a bit scary; we have some mechanism to > stop the HD-audio hardware operation (e.g. for loading the DSP for > CA0132 codec), but it's pretty hackish and error-prone.
At least I didn't have issue with HW stopping, the problem is with unampping hdac_bus->remap_addr - as long as it's not iorampped back no code besides the PCI BAR move should run at all as any register access from this address will cause fatal page fault. I took care of most of the IOCTls i think, the interrupts are disabled in azx_stop_chip but I am probably still missing some stuff like some background work items or other IOCTls code path.
Are the all running processes stopped before entering rescan_prepare callback? Otherwise, e.g. a process accessing PCM stream via mmap can run without ioctl and this may trigger the PCM period elapsed eventually.
Is any of them remaps the BAR into user process VMA ? The only two I see in the sound subsystem doing remapping are are had_pcm_mmap and snd_pcm_lib_default_mmap and both of them remap dma buffers which are in system memory and not MMIO space and hence should not be impacted by the BAR move.
When a stream is running, it can cause an interrupt that may issue an unsolicited event that is deals with CORB/RIRB later in a work asynchronously.
On rescan_prepare I am calling snd_hdac_bus_stop_chip which in turn seems to disable interrupts - is this enough to guarantee all those won't be happening past that code ? Should I also flush any work queue items scheduled from within those ISRs before proceeding ?
Well, calling snd_hdac_bus_stop_chip() during operation itself isn't safe at all. It's called safely in the power management because we know all things have been already stopped. But calling it out of sudden is a completely different matter.
Or maybe I didn't get your concern correctly ? Why there would be a problem to access the stream otherwise during the BAR move ?
Similarly, a control interface may issue the verb that is
processed via CORB/RIRB, too. So what you need to cover is not only about PCM.
Can you point me to the code path for this - is this through another type of IOCTL or interrupt handlers ?
Yes, it's handled in the ALSA core control ioctls. Many of the control *_get()/*_put() callbacks in HD-audio are involved with the amp or other HD-audio verbs (typically calling snd_hda_codec_amp*(), etc). Those are the mixer elements and handled via regmap with HD-audio backend that deals with CORB/RIRB eventually.
> Do we really need the BAR remap while operating the sound streaming? > That is, can't we simply refuse the remap if it's in operation? > If it can be refused, we may avoid a big locking or such.
Problem here is that this feature is for the sake of hot-plug of a device, this supposed to give enough MMIO space to place all the BARs of newly plugging in device. Refusing to move BARs because there is some app using the sound card on the background might then lead to failure of plugging in the new device and this doesn't sound as a reasonable behavior to me. Also note that the lock's impact is minimal as it's read side lock only in the IOCTLs an so as long as hot plug not taking place it has no impact.
But a hot-plugged device can't have any running files, no? So at which timing is the BAR movement _must_ happen?
No, the devices this code is written for are those already in the system before the new device is plugged in, the new device needs enough free MMIO space for his own BARs and so this features enables existing devices to allow the PCI core to move their BARs within the physical address space such that enough free space will be available inside the MMIO window allocated for the new device's upstream PCI bridge.
Hmm, OK, so it's other way round as I thought of.
And, even if we need some synchronization, the current lock implementation looks fragile to me. We likely overlook something when trying to cover each branch code path.
Basically it needs to wait until all running processes via PCM or control API are released.
This approach brings a couple of hard questions -
How we make them stop at once - they might run for long time, you suggest something like invalidate all their mmaps and on next page fault return SIGBUS to terminate them ? Then I need also to know they actually are done - should I wait for their device file instance release? What if they don't access any mmapped are for a long time ? Sending SIGKILL explicitly ? But do I have the task id's of all of them ?
You shouldn't kill them but wait for them aborting gracefully, IMO. Essentially a kind of hot-unplug. Most of Linux drivers won't kill processes at the hot-unplug, AFAIK.
But see, the difference is such that on hot-plug I may just let those process pointing at zombie device to hang around indefently because I am not waiting for any output from them. That indeed what I am doing for amdgou hot-unplug work. Here i need to blocking wait for them since I need to finish the procedure so the PCI core can finish shuffling the existing BARs around and by this freeing enough space for the new device to be accepted. I user can't plug in his thunderbolt GPU and observe that nothing is happening actually because we are stuck waiting for some process just sitting idle.
Then we can see it from a different angle; read below.
But if it has to be done so, we have a list of file descriptors that are attached to the PCM and the control devices, too...
So you suggest iterate all of them, sigkill them and confirm all of them exit instead of using the rw_sem ?
Note that, for the sound applications, we don't kill at disconnection. But the ALSA core stuff shuts the all operations up but for closing (also some event notification for the disconnection was done), hence we just wait for applications releasing resources. In anyway...
This will cover IOCTLs and any mmapped accesses i guess. Interrupts we discussed above. What above any possible background kernel work going on in dedicated threads or work items ? Any pointers there what should be blocked and waited for ?
An alternative idea would be the analogy of the system suspend / resume. That is, we forcibly suspend the devices at first somehow, and also restricts the further accesses by some way. Then do remap,
But that the point I guess, how you block further accesses without those big locks, during S3 i believe user mode gets suspended before the driver and so you don't need to worry about concurrent IOCTLs when going through suspend sequence
Andrey
and resume. Not sure whether this will fly high, but if it works, it should be more systematic and more robust than papering over each piece manually, I suppose.
Takashi
Andrey
We had those scenarios discussed during my initial work on GPU hot unplug and came to a conclusssion that trying to wait for current processes to die is not a good approach - see the this respone I got at the time for example - https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.free...
But should this be seen rather as an analogy of hot-replug? That won't kill processes.
Takashi
Andrey
We have a mechanism to block the new openslike snd_hda_lock_devices(). We'll need an additional stuff to wait until all opened devices are released there, and this requires some help in the core code, I suppose. (Actually this hackish lock_devices/unlock_devices should have been in the core stuff from the beginning...)
thanks,
Takashi
Andrey
> > > thanks, > > Takashi > >> >> >> Andrey >> >> On 2021-03-19 5:22 p.m., Andrey Grodzovsky wrote: >>> Hi, I am working on adding AMD related drivers support for PCIe BARs >>> move feature developed by Sergei >>> (https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flinuxplumb...). >>> >>> His feature is still not upstream, all his code and mine on top can >>> be seen here - >>> https://nam11.safelinks.protection.outlook.com/?url=https:%2F%2Fcgit.freedes... >>> >>> I did basic implementation fro amdgpu driver and now I am doing the >>> same for snd_hda_intel to support our on GPU Azalia audio >>> chips. This relevant commit is here - >>> https://nam11.safelinks.protection.outlook.com/?url=https:%2F%2Fcgit.freedes... >>> >>> Any driver that wants to support movable BARs needs to implement >>> rescan_prepare, rescan_done and bar_fixed callbacks where >>> rescan_prepare needs to stop HW/SW and unamp all MMIO mappings and >>> rescan_done needs to ioremap the BAR from it's new MMIO location and >>> restart HW/SW. >>> >>> I am able currently to trigger BARs move by sysfs using >>> "/sys/bus/pci/rescan" and the driver will go through the sequence I >>> described above without hangs. Problem is that after this when i try >>> to use mplayer I am getting following errors: >>> >>> andrey@andrey-test:~$ sudo mplayer -ao alsa:device=hw=0.3 >>> Downloads/file_example_MP3_5MG.mp3 >>> MPlayer 1.3.0 (Debian), built with gcc-9 (C) 2000-2016 MPlayer Team >>> do_connect: could not connect to socket >>> connect: No such file or directory >>> Failed to open LIRC support. You will not be able to use your remote >>> control. >>> >>> Playing Downloads/file_example_MP3_5MG.mp3. >>> libavformat version 58.29.100 (external) >>> Audio only file format detected. >>> Load subtitles in Downloads/ >>> ========================================================================== >>> Opening audio decoder: [mpg123] MPEG 1.0/2.0/2.5 layers I, II, III >>> AUDIO: 44100 Hz, 2 ch, s16le, 320.0 kbit/22.68% (ratio: 40000->176400) >>> Selected audio codec: [mpg123] afm: mpg123 (MPEG 1.0/2.0/2.5 layers >>> I, II, III) >>> ========================================================================== >>> AO: [alsa] 44100Hz 2ch s16le (2 bytes per sample) >>> Video: no video >>> Starting playback... >>> A: 0.1 (00.0) of 132.0 (02:12.0) ??,?% >>> Audio device got stuck! >>> >>> and in dmesg I see >>> [ 365.355518] snd_hda_codec_hdmi hdaudioC0D0: Unable to sync >>> register 0x2f0d00. -5 >>> >>> Also I see 296.619014] snd_hda_intel 0000:0a:00.1: CORB reset >>> timeout#2, CORBRP = 65535 error during the rescan_done callback >>> execution >>> >>> I know it has to do with the move of BAR's MMIO address because when i >>> disallow BAR migration by returning true from bar_fixed callback I >>> have no such errors and mplayer works fine. >>> >>> I enabled MMIO trace and didn't see that post BAR move there is a >>> wrong MMIO access - all of them are from the new MMIO base offset - >>> 0xfcd80000 (trace attached including mmio trace and dmesg) >>> >>> I would be happy for any idea on this and any comment on the >>> correctness of my sequence in the code >>> >>> Andrey >>