RE: [PATCH] ASoC: fsl: imx-pcm-dma: Don't request dma channel in probe

[Robin Gong]

On 2021/04/19 17:46 Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote:
> Am Montag, dem 19.04.2021 um 07:17 +0000 schrieb Robin Gong:
> > Hi Lucas,
> >
> > On 2021/04/14 Lucas Stach <l.stach@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> > > > > 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.