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Subject: Re: [PATCH] ASoC: fsl: imx-pcm-dma: Don't request dma channel in
 probe
From:   Lucas Stach <l.stach@pengutronix.de>
To:     Robin Gong <yibin.gong@nxp.com>,
        Shengjiu Wang <shengjiu.wang@gmail.com>
Cc:     Nicolin Chen <nicoleotsuka@gmail.com>,
        Linux-ALSA <alsa-devel@alsa-project.org>,
        Liam Girdwood <lgirdwood@gmail.com>,
        "s.hauer@pengutronix.de" <s.hauer@pengutronix.de>,
        Timur Tabi <timur@kernel.org>, Xiubo Li <Xiubo.Lee@gmail.com>,
        "shawnguo@kernel.org" <shawnguo@kernel.org>,
        "S.j. Wang" <shengjiu.wang@nxp.com>,
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Date:   Tue, 20 Apr 2021 16:00:56 +0200
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Am Dienstag, dem 20.04.2021 um 13:47 +0000 schrieb Robin Gong:
> On 2021/04/19 17:46 Lucas Stach <l.stach@pengutronix.de> wrote:
> > Am Montag, dem 19.04.2021 um 07:17 +0000 schrieb Robin Gong:
> > > Hi Lucas,
> > > 
> > > On 2021/04/14 Lucas Stach <l.stach@pengutronix.de> wrote:
> > > > Hi Robin,
> > > > 
> > > > Am Mittwoch, dem 14.04.2021 um 14:33 +0000 schrieb Robin Gong:
> > > > > On 2020/05/20 17:43 Lucas Stach <l.stach@pengutronix.de> wrote:
> > > > > > Am Mittwoch, den 20.05.2020, 16:20 +0800 schrieb Shengjiu Wang:
> > > > > > > Hi
> > > > > > > 
> > > > > > > On Tue, May 19, 2020 at 6:04 PM Lucas Stach
> > > > > > > <l.stach@pengutronix.de>
> > > > > > wrote:
> > > > > > > > Am Dienstag, den 19.05.2020, 17:41 +0800 schrieb Shengjiu Wang:
> > > > > > > > > There are two requirements that we need to move the
> > > > > > > > > request of dma channel from probe to open.
> > > > > > > > 
> > > > > > > > How do you handle -EPROBE_DEFER return code from the channel
> > > > > > > > request if you don't do it in probe?
> > > > > > > 
> > > > > > > I use the dma_request_slave_channel or dma_request_channel
> > > > > > > instead of dmaengine_pcm_request_chan_of. so there should be
> > > > > > > not -EPROBE_DEFER return code.
> > > > > > 
> > > > > > This is a pretty weak argument. The dmaengine device might probe
> > > > > > after you try to get the channel. Using a function to request
> > > > > > the channel that doesn't allow you to handle probe deferral is
> > > > > > IMHO a bug and should be fixed, instead of building even more
> > > > > > assumptions on top
> > > > of it.
> > > > > > 
> > > > > > > > > - When dma device binds with power-domains, the power will
> > > > > > > > > be enabled when we request dma channel. If the request of
> > > > > > > > > dma channel happen on probe, then the power-domains will
> > > > > > > > > be always enabled after kernel boot up,  which is not good
> > > > > > > > > for power saving,  so we need to move the request of dma
> > > > > > > > > channel to .open();
> > > > > > > > 
> > > > > > > > This is certainly something which could be fixed in the
> > > > > > > > dmaengine driver.
> > > > > > > 
> > > > > > > Dma driver always call the pm_runtime_get_sync in
> > > > > > > device_alloc_chan_resources, the device_alloc_chan_resources
> > > > > > > is called when channel is requested. so power is enabled on
> > > > > > > channel
> > > > request.
> > > > > > 
> > > > > > So why can't you fix the dmaengine driver to do that RPM call at
> > > > > > a later time when the channel is actually going to be used? This
> > > > > > will allow further power savings with other slave devices than the audio
> > PCM.
> > > > > Hi Lucas,
> > > > >   Thanks for your suggestion. I have tried to implement runtime
> > > > > autosuspend in fsl-edma driver on i.mx8qm/qxp with delay time (2
> > > > > sec) for this feature as below (or you can refer to
> > > > > drivers/dma/qcom/hidma.c), and pm_runtime_get_sync/
> > > > > pm_runtime_put_autosuspend in all dmaengine driver interface like
> > > > > device_alloc_chan_resources/device_prep_slave_sg/device_prep_dma_c
> > > > > ycli
> > > > > c/
> > > > > device_tx_status...
> > > > > 
> > > > > 
> > > > >                 pm_runtime_use_autosuspend(fsl_chan->dev);
> > > > >                 pm_runtime_set_autosuspend_delay(fsl_chan->
> > dev,
> > > > 2000);
> > > > > 
> > > > > That could resolve this audio case since the autosuspend could
> > > > > suspend runtime after
> > > > > 2 seconds if there is no further dma transfer but only channel
> > > > request(device_alloc_chan_resources).
> > > > > But unfortunately, it cause another issue. As you know, on our
> > > > > i.mx8qm/qxp, power domain done by scfw
> > > > > (drivers/firmware/imx/scu-pd.c)
> > > > over mailbox:
> > > > >  imx_sc_pd_power()->imx_scu_call_rpc()->
> > > > > imx_scu_ipc_write()->mbox_send_message()
> > > > > which means have to 'waits for completion', meanwhile, some driver
> > > > > like tty will call dmaengine interfaces in non-atomic case as
> > > > > below,
> > > > > 
> > > > > static int uart_write(struct tty_struct *tty, const unsigned char
> > > > > *buf, int count) {
> > > > >    .......
> > > > > 	    port = uart_port_lock(state, flags);
> > > > >    ......
> > > > >         __uart_start(tty);  //call
> > start_tx()->dmaengine_prep_slave_sg...
> > > > >         uart_port_unlock(port, flags);
> > > > >         return ret;
> > > > > }
> > > > > 
> > > > > Thus dma runtime resume may happen in that timing window and cause
> > > > kernel alarm.
> > > > > I'm not sure whether there are similar limitations on other driver
> > > > > subsystem. But for me, It looks like the only way to resolve the
> > > > > contradiction between tty and scu-pd (hardware limitation on
> > > > > i.mx8qm/qxp) is to give up autosuspend and keep
> > > > > pm_runtime_get_sync
> > > > only in device_alloc_chan_resources because request channel is a
> > > > safe non-atomic phase.
> > > > > Do you have any idea? Thanks in advance.
> > > > 
> > > > If you look closely at the driver you used as an example (hidma.c)
> > > > it looks like there is already something in there, which looks very
> > > > much like what you need
> > > > here:
> > > > 
> > > > In hidma_issue_pending() the driver tries to get the device to runtime
> > resume.
> > > > If this doesn't work, maybe due to the power domain code not being
> > > > able to be called in atomic context, the actual work of waking up
> > > > the dma hardware and issuing the descriptor is shunted to a tasklet.
> > > > 
> > > > If I'm reading this right, this is exactly what you need here to be
> > > > able to call the dmaengine code from atomic context: try the rpm get
> > > > and issue immediately when possible, otherwise shunt the work to a
> > > > non- atomic context where you can deal with the requirements of scu-pd.
> > > Yes, I can schedule_work to worker to runtime resume edma channel by
> > calling scu-pd.
> > > But that means all dmaengine interfaces should be taken care, not only
> > > issue_pending() but also
> > > dmaengine_terminate_all()/dmaengine_pause()/dmaengine_resume()/
> > > dmaengine_tx_status(). Not sure why hidma only take care
> > > issue_pending. Maybe their user case is just for memcpy/memset so that
> > > no further complicate case as ALSA or TTY.
> > > Besides, for autosuspend in cyclic, we have to add pm_runtime_get_sync
> > > into interrupt handler as qcom/bam_dma.c. but how could resolve the
> > > scu-pd's non-atmoic limitation in interrupt handler?
> > 
> > Sure, this all needs some careful analysis on how those functions are called
> > and what to do about atomic callers, but it should be doable. I don't see any
> > fundamental issues here.
> > 
> > I don't see why you would ever need to wake the hardware in an interrupt
> > handler. Surely the hardware is already awake, as it wouldn't signal an
> > interrupt otherwise. And for the issue with scu-pd you only care about the
> > state transition of suspended->running. If the hardware is already
> > running/awake, the runtime pm state handling is nothing more than bumping
> > a refcount, which is atomic safe. Putting the HW in suspend is already handled
> > asynchronously in a worker, so this is also atomic safe.
> But with autosuspend used, in corner case, may runtime suspended before falling 
> Into edma interrupt handler if timeout happen with the delay value of
> pm_runtime_set_autosuspend_delay(). Thus, can't touch any edma interrupt
> status register unless runtime resume edma in interrupt handler while runtime
> resume function based on scu-pd's power domain may block or sleep.
> I have a simple workaround that disable runtime suspend in issue_pending worker
> by calling pm_runtime_forbid() and then enable runtime auto suspend in 
> dmaengine_terminate_all so that we could easily regard that edma channel is always
> in runtime resume between issue_pending and channel terminated and ignore the above
> interrupt handler/scu-pd limitation.

The IRQ handler is the point where you are informed by the hardware
that a specific operation is complete. I don't see any use-case where
it would be valid to drop the rpm refcount to 0 before the IRQ is
handled. Surely the hardware needs to stay awake until the currently
queued operations are complete and if the IRQ handler is the completion
point the IRQ handler is the first point in time where your autosuspend
timer should start to run. There should never be a situation where the
timer expiry can get between IRQ signaling and the handler code
running.

Regards,
Lucas