Troubleshooting • RPI5 how to disable DMA on ttyAMA0 ?

Hello, For a long time (with pi2, pi3, pi4) I've been using the ttyAMA0 serial port to read information sent by an electric meter (at 1200 bps).
I recently bought a RPI5, but it no longer works with ttyAMA0.
However, I tried it with ttyAMA3 and it works.
I think the problem comes from the fact that ttyAMA0 is managed by DMA.

My question is how do I disable DMA on ttyAMA0?

Thanks in advance for your help and advice.

Some information:
root@PI5:~ # dmesg
4.011419] macb 1f00100000.ethernet: gem-ptp-timer ptp clock registered.
[ 8.099510] macb 1f00100000.ethernet eth0: Link is Up - 1Gbps/Full - flow control off
[ 8.672770] pl011-axi 1f00030000.serial: DMA channel TX dma2chan0
[ 8.672782] pl011-axi 1f00030000.serial: DMA channel RX dma2chan1

root@PI5:~ # uname –a
Linux PI5 6.12.18-v8-16k+ #1862 SMP PREEMPT Wed Mar 12 12:33:09 GMT 2025 aarch64 GNU/Linux

root@PI5:~ # cat /sys/kernel/debug/dmaengine/summary
dma0 (1000010000.dma): number of channels: 5

dma1 (1000010600.dma): number of channels: 5
dma1chan0 | 107d004000.spi:tx
dma1chan1 | 107d004000.spi:rx
dma1chan2 | 107c701400.hdmi:audio-rx
dma1chan3 | 107c706400.hdmi:audio-rx

dma2 (1f00188000.dma): number of channels: 8
dma2chan0 | 1f00030000.serial:tx
dma2chan1 | 1f00030000.serial:rx

Statistics: Posted by ernesto — Sun Mar 16, 2025 7:40 am

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