[alsa-devel] [PATCH] ASoC: omap-mcbsp: Add PM QoS support for McBSP to prevent glitches

[Peter Ujfalusi]

On 09/08/16 17:48, Tony Lindgren wrote:
>>> My findings so far:
>>> w/o the QoS patch we will hit core OFF even if we are in element mode in McBSP:
>>> # cat /sys/devices/platform/68000000.ocp/49022000.mcbsp/dma_op_mode
>>> [element] threshold
>>>
>>> If I start the playback we will hit core OFF even if the we have DMA request
>>> coming at every sample (every 0.02ms).
> 
> I think that's because we still have windows without pm qos where dma
> is not active and we hit idle and there's nothing in hardware blocking
> off mode.

Hrm. Should we block idle when the FIFO threshold is 'low'. I don't think it
is a good thing to try to enter to lower C state to immediately jump out of
it. With 0.02ms there is not much time we can spend conserving power and the
enter/leave takes a bit more compared to staying wherever we were.
ON the other hand, if we have big ALSA buffer we can keep the MPU off for a
long time as we only need the DMA and the McBSP to run between user space
filling the ALSA buffer...

Oh, I have looked again to cpuidle34xx.c and we have two tables there:
omap3_idle_driver and omap3430_idle_driver.

The omap3430_idle_driver numbers are based on measurements while the
omap3_idle_driver probably contains safe numbers?

In any way, according to the numbers:
C7 is (7505 + 15274) vs (10000 + 30000)
C6 is (7580 + 4134) vs (3000 + 8500)
C5 is (855 + 1146) vs (2500 + 7500)
C4 is (121 + 3374) vs (1500 + 1800)
C3 is (107 + 410) vs (50 + 50)

with the 30ms QoS we set we will still hit OFF on OMAP3430, it should minimum
11.715ms for omap3430, but that will block C6 on omap36xx.

If we could have the data for the struct cpuidle_state coming from DT and not
wired in the cpuidle34xx/44xx files then the McBSP driver could look up the C7
number and block that...

>>> We hit C6, but when the DMA request comes we have 16 words in the FIFO (8
>>> samples, 0.16ms of audio). C6 should take 11.5ms to exit...
> 
> So maybe we have the C6 latency for 36xx wrong, and it's much faster than
> for 34xx? I wonder what happens on the original beagleboard, not the
> xm variant.. I don't have one though.

I have one working omap3 zoom2 board. AFAIK it has OMAP3430. I have
3.9.smthing kernel booting on it, but I'm not sure if mainline would even
work. Zoom3 and Zoom2 are mostly identical, but the SoC is different for sure
and probably memories, NAND, something else on the SOM? Can not find
information on this.

>>> Based on my experiments the FIFO threshold does not matter as even if we
>>> should not be able to leave from a C state in time, we do leave from the state :o
> 
> Yes that's a mystery :)
> 
>>> The patch generates the following warning when the playback is stopped:
> ...
>>> [  115.996002] [<c0155b74>] (__cancel_work_timer) from [<c0199ad8>]
>>> (pm_qos_remove_request+0x28/0x1c8)
>>> [  116.005523] [<c0199ad8>] (pm_qos_remove_request) from [<c0684634>]
>>> (omap_mcbsp_dai_trigger+0x70/0x8c)
>>> [  116.015197] [<c0684634>] (omap_mcbsp_dai_trigger) from [<c067baa8>]
>>> (soc_pcm_trigger+0xd0/0x11c)
>>> [  116.024414] [<c067baa8>] (soc_pcm_trigger) from [<c0663be0>]
>>> (snd_pcm_do_stop+0x50/0x54)
>>> [  116.032928] [<c0663be0>] (snd_pcm_do_stop) from [<c06639c4>]
>>> (snd_pcm_action_single+0x38/0x80)
> 
> Hmm OK I wonder why have not seen that one, I've been just hitting ctrl-c
> to stop the playback.

I also stopped the playback with ctrl-c :o

>>> There is one more issue: if we have playback and capture running at the same
>>> time and you stop one of them. It will remove the QoS and the remaining stream
>>> might break.
>>
>> it is better to place the QoS in omap_mcbsp_request() and remove it in
>> omap_mcbsp_free(). This way we retain the QoS for the time the McBSP is in
>> active use.
> 
> OK makes sense.
> 
> Regards,
> 
> Tony
> 


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