USB DMA and RT Kernel

[Alexander Rössler]

I'm experiencing a problem with the RT kernel and USB DMA, or the lack of it when using a webcam on the BeagleBone Black.

I noticed that both, the CPU load and the RT performance degrades to an unusable level when I plug in and use USB webcam on the Debian Stretch image with RT kernel.

My kernel version is:

$ uname -a
Linux sandybox 4.9.108-bone-rt-r10 #1 PREEMPT RT Thu Jun 14 12:49:42 UTC 2018 armv7l GNU/Linux

I saw one the usb IRQs (irq/202-musb-hd or irq/203-musb-hd) rising to the top of top. Some researched told me that this could be the lack of the USB DMA option being enabled in the kernel.

Are there any options to fix this problem without re-compiling the Linux kernel?

Thanks,

Alexander

[Alexander Rössler]

Here is a snapshot of top:

  PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND                                                                                                                                                                                                                                                                                                                                                                      
12304 root      20   0   67924  62956  13936 S 40.2 12.7   0:22.13 rtapi:0                                                                                                                                                                                                                                                                                                                                                                      
12344 machine+  20   0   32256  21968  11776 R 17.6  4.4   0:07.81 mkwrapper                                                                                                                                                                                                                                                                                                                                                                    
  86 root     -51   0       0      0      0 S 17.4  0.0   7:22.63 irq/203-musb-hd                                                                                                                                                                                                                                                                                                                                                              
12324 machine+  20   0   18300  11136   8276 S  4.7  2.2   0:03.27 hal_temp_ads782                                                                                                                                                                                                                                                                                                                                                              
12329 machine+  20   0    7240   2596   1940 R  4.0  0.5   0:02.32 top                                                                                                                                                                                                                                                                                                                                                                          
12322 machine+  20   0   18228  10996   8188 S  3.7  2.2   0:02.87 hal_gpio_mcp230                                                                                                                                                                                                                                                                                                                                                              
12343 machine+  20   0   25632  15500  12956 R  3.7  3.1   0:02.38 milltask                                                                                                                                                                                                                                                                                                                                                                     
12230 machine+  20   0   50580  13708  13220 S  2.7  2.8   0:01.92 mjpg_streamer                                                                                                                                                                                                                                                                                                                                                                
12323 machine+  20   0   18292  10936   8132 S  1.2  2.2   0:01.30 hal_pwm_pca9685                                                                                                                                                                                                                                                                                                                                                              
11883 machine+  20   0    8500   2404   1812 S  1.0  0.5   0:00.57 sshd                                                                                                                                                                                                                                                                                                                                                                         
  33 root     -51   0       0      0      0 S  0.7  0.0  29:08.61 irq/164-4802a00

[Robert Nelson]

Sadly no.. You'll need to rebuild..

diff --git a/patches/defconfig b/patches/defconfig
index 65ce0ab..ca4814f 100644
--- a/patches/defconfig
+++ b/patches/defconfig
@@ -4316,7 +4316,8 @@ CONFIG_USB_MUSB_AM335X_CHILD=y
#
# MUSB DMA mode
#
-CONFIG_MUSB_PIO_ONLY=y
+# CONFIG_MUSB_PIO_ONLY is not set
+CONFIG_USB_TI_CPPI41_DMA=y
CONFIG_USB_DWC3=y
# CONFIG_USB_DWC3_HOST is not set
# CONFIG_USB_DWC3_GADGET is not set


Regards,

--
Robert Nelson
https://rcn-ee.com/

[Alexander Rössler]

Okay. What's the easiest way to build a custom kernel for the BB?

Regards,

Alexander