DMA, CMA and SWIOTLB

[Jose Abreu]

Hi all,

I am writing a very simple KMS driver that uses Xilinx VDMA to
transfer data between the host and a FPGA. To handle memory
allocation for DMA I am using the CMA helpers available in the
DRM subsystem. When setting for low video modes (small memory
requirements) everything works fine, but if I try to set a large
video mode (specifically a 4k HDMI mode) I get SWIOTLB errors
saying the buffer is full.

The distinction between CMA and SWIOTLB is not clear to me, but
these are two different allocation engines, right?

One important aspect in my configuration is that the Xilinx VDMA
can only access up to 28 bits of the host memory so I set the DMA
mask to 28 bits and force the CMA region to be allocated bellow
these 28 bits. I also tried increasing the CMA size and the
SWIOTLB size without success.

Any comments on this?

Thanks in advance for your help.

Best regards,
Jose Miguel Abreu