Lu, Brent brent.lu@intel.com 於 2020年8月13日 週四 下午3:55寫道：
CRAS calls snd_pcm_hw_params_set_buffer_size_max() to use as large buffer as possible. So the period size is an arbitrary number in different platforms. Atom SST platform happens to be 256, and CML SOF platform is 1056 for example.
ok, but earlier in this thread it was mentioned that values such as 432 are not suitable. the statement above seems to mean the period actual value is a "don't care", so I don't quite see why this specific patch2 restricting the value to 240 is necessary. Patch1 is needed for sure, Patch2 is where Takashi and I are not convinced.
I have downloaded the patch1 but it does not work. After applying patch1, the default period size changes to 320. However, it also has the same issue with period size 320. (It can be verified by aplay.)
The period_size is related to the audio latency so it's decided by application according to the use case it's running. That's why there are concerns about patch 2 and also you cannot find similar constraints in other machine driver.
You're right. However, the problem here is the provided period size does not work. Like 256, setting the period size to 320 also makes users have big latency in the DSP ring buffer.
localhost ~ # aplay -Dhw:1,0 --period-size=320 --buffer-size=640 /dev/zero -d 1 -f dat --test-position Playing raw data '/dev/zero' : Signed 16 bit Little Endian, Rate 48000 Hz, Stereo Suspicious buffer position (1 total): avail = 0, delay = 2640, buffer = 640 Suspicious buffer position (2 total): avail = 0, delay = 2640, buffer = 640 Suspicious buffer position (3 total): avail = 0, delay = 2720, buffer = 640 ...
Another problem is the buffer size. Too large buffer is not just wasting memories. It also creates problems to memory allocator since continuous pages are not always there. Using a small period_count like 2 or 4 should be sufficient for audio data transfer.
buffer_size = period_size * period_count * 1000000 / sample_rate; snd_pcm_hw_params_set_buffer_time_near(mPcmDevice, params, &buffer_size, NULL);
And one more problem here: you need to decide period_size and period_count first in order to calculate the buffer size...
It's a good point. I will bring it up to our team and see whether we can use the smaller buffer size. Thanks!
Regards, Brent
Thanks, Yu-Hsuan