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Message-ID: <f80c7bf2-1d9e-40d7-d3bd-c8bbe548934b@linux.intel.com>
Date: Tue, 21 Jun 2022 08:11:48 -0500
From: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To: noman pouigt <variksla@...il.com>, Mark Brown <broonie@...nel.org>,
liam.r.girdwood@...ux.intel.com
Cc: alsa-devel@...a-project.org, peter.ujfalusi@...ux.intel.com,
ranjani.sridharan@...ux.intel.com, linux-kernel@...r.kernel.org,
yangyingliang@...wei.com, rander.wang@...el.com,
yung-chuan.liao@...ux.intel.com,
sound-open-firmware@...a-project.org
Subject: Re: [Sound-open-firmware] ASoC: SOF: Race condition in ipc.c
On 6/20/22 21:46, noman pouigt wrote:
> Folks,
>
> I have borrowed part of SOF architecture for my own DSP
> framework development as the memory on the DSP is
> extremely small and wouldn't be able to support SOF.
You're providing very little context here. Of course it's open-source
and you can do whatever you want with the code, but it's not clear if
the 'borrowed' code can deal with your specific case.
> Currently I am running into a race condition as below:
>
> CPU DSP
> PCM_TRIGGER_START
> sof_ipc_send_msg ---->
>
> <------Immediate ACK
> ipc3_wait_tx_done
> (wait_event_timeout)
> <------ POSITION update
>
> snd_pcm_period_elapsed
>
>
> As you can see TRIGGER_START didn't even finish
> and waiting for it to complete in ipc3_wait_tx_done
> function. However, before it could complete the position
> interrupt was issued which results in calling period_elapsed
> function.
>
> In order to fix this I assume below is called in SOF framework:
> schedule_work(&spcm->stream[substream->stream].period_elapsed_work);
>
> How is this design working? If the interrupt is coming too fast
> from the DSP than the associated function with this schedule_work
> will not get called as the scheduler will not get time to schedule the
> workqueue and elapsed function will not be called thereby not increasing
> the hw_ptr. How is the flow control for data transfer achieved?
The schedule_work was added by this commit, and the explanations are
rather straightforward:
commit e2803e610aecb36ea4fec5a04861547664580d0c
Author: Keyon Jie <yang.jie@...ux.intel.com>
Date: Tue Apr 30 18:09:25 2019 -0500
ASoC: SOF: PCM: add period_elapsed work to fix race condition in
interrupt context
"
The IPC implementation in SOF requires sending IPCs serially: we should
not send a new IPC command to the firmware before we get an ACK (or time
out) from firmware, and the IRQ processing is complete.
snd_pcm_period_elapsed() can be called in interrupt context before
IRQ_HANDLED is returned. When the PCM is done draining, a STOP
IPC will then be sent, which breaks the expectation that IPCs are
handled serially and leads to IPC timeouts.
This patch adds a workqueue to defer the call to snd_pcm_elapsed() after
the IRQ is handled.
"
I am not sure what the problem you have really is.
It's not really surprising that the first period is consumed
immediately, the flow will become more steady-state afterwards.
The DMAs should be primed to deal with more than one period, and the
schedule_work() will only signal that the application can refill the
ring buffer. There's all kinds of delays that will happen depending on
load and scheduling, and if the POSITION_UPDATE was received immediately
then there's should be still plenty of time to provide a new buffer.
>
> I am facing the above problem in my design.
>
> I am wondering if I can simply add one more IPC command(don't call
> wait_event_interruptible for this) after TRIGGER_START to start the
> streaming.This way schedule_work for updating period_elapsed function
> can be avoided and it can be called in an interrupt context.
See commit above, this won't work because you'll be sending an IPC while
dealing with an IPC.
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