raspberrypi
Paced DMA on RP1 not possible
Describe the bug
Setting up paced DMA with PWM flow control (or any other flow control peripheral) fails to work on RP1. In dw-axi-dmac-platform.c the snps,sel-require entry is read from the device tree. However that entry does not exist. So in the dw_axi_dma_of_xlate function, the dw->sel_required values are always zero, resulting in a zero chan_mask. No channel can be selected.
Also the DMA handshake table in the RP1 Peripherals document is missing entirely, even though it is mentioned in the text.
Would adding a snps,sel-require entry in the device tree solve this problem? If so, what would be the appropriate values for DREQ setup?
Steps to reproduce the behaviour
Attempt to set up paced DMA using PWM on RP1
Device (s)
Raspberry Pi 5
System
Raspberry Pi reference 2025-05-13 Generated using pi-gen, https://github.com/RPi-Distro/pi-gen, 5dabc7dc940059dfbc46af5d97b60a1e812523dd, stage2
2025/05/08 15:13:17 Copyright (c) 2012 Broadcom version 69471177 (release) (embedded)
Linux precisionwall 6.12.47+rpt-rpi-2712 #1 SMP PREEMPT Debian 1:6.12.47-1+rpt1~bookworm (2025-09-16) aarch64 GNU/Linux
Logs
No response
Additional context
No response
However that entry does not exist.
Yes it does:
~$ od -Ax -tx4 --endian=big /proc/device-tree/axi/pcie@1000120000/rp1/dma@188000/snps,sel-require
000000 00000100 00000100 00000000 00000000
000010 00000000 00000000 00000000 00000000
000020
Unless, of course, you are getting your Device Tree from somewhere other than the one installed with the kernel, then loaded and passed on by the firmware.
So with those values, a kernel module with a device tree overlay having dmas = <&rp1_dma 0x118> should connect to the PWA flow control (assuming line 24), but it doesn't. Actually the DMA transfer doesn't run at all and gives no errors. Even with a dmas = <&rp1_dma 0> the transfer does not run. With dmas = <&rp1_dma 0xFFFFFF>, no errors occur and it does not run. Only a value 0x2XX give an error (precluded). Using dmas = <&dma40 0> or <&dma32 0> runs, but unpaced.
Wait, the no-preference fallback case doesn't look right, as you've discovered.
The 0x118 value comes from the sel-require value of 0x100 plus the PWM flow control value (from DREQSourcePWM0 =24). Since the RP1 documentation fails to provide any DREQ values, I cannot be sure this is correct. If someone knows the correct way to set the PWM DREQ line for the RP1 DMA, I'd really like to know because everything I've tried has failed. If someone has a working paced DMA example for the RP1, that would be awesome. Even better would be an example using the new Ticks hardware rather than PWM. It seems that none of this stuff connects properly within the RP1 DMA driver.
Sorry this took a while, but https://github.com/raspberrypi/linux/pull/7089 should fix the problem, and improve allocation in the event that the best channels are in use. In about half an hour you should be able to install a trial kernel (and DTBs) using sudo rpi-update pulls/7089. You should back up important data first, and if there's any possibility that you are relying on initramfs to boot you should first confirm that it still boots with auto_initramfs=0 before updating.