More DMA issues on RPI4b in 64-bit mode with >1G RAM

[yaroslavros]

It seems that there are still problems with DMA operation on RPI4 in 64-bit mode with > 1G RAM. This time on bcm2835-mmc.

When I try downloading large file via WiFi using scp, I run into reproducible swiotlb buffer issue at any points between 700Mb to 2Gb of downloaded data. Kernel logs are:

bcm2835-dma fe00700.dma: swiotlb buffer is full (sz: 1536 bytes)
bcm2835-dma fe00700.dma: DMA: Out of SW-IOMMU space for 1536 bytes
mmc-bcm2835 fe300000.mmcnr: dma_map_sg returned zero length
mmc1: Timeout waiting for hardware interrupt.

I have confirmed this at 2, 3 and 4Gb RAM. It does not occur on 1Gb RAM - I have downloaded 6Gb file via Wifi without any trouble.

This happens on the latest 4.19.y kernel

I'll dig more into this in upcoming days. I suspect there is a leak somewhere in the relevant driver stack that prevents swiotlb bounce buffers from being freed. This would explain normal behaviour on 1G RAM where bounce buffers are not used.

[lategoodbye]

Please provide the exact kernel version

[yaroslavros]

I see it on the latest 4.19.y
# uname -a
Linux rpi4a 4.19.64-v8 #34 SMP Sat Aug 17 13:51:51 BST 2019 aarch64 GNU/Linux

I strongly suspect that this issue is related to the way bcm2835-mmc works with swiotlb and affects all kernel versions.

I am pretty sure root cause is the same as in #3114

[yaroslavros]

Closing as the fix is committed