Proper way to set a DMA_MASK on a USB device

Noam Liron <lnoam@xxxxxxxxxxx> · Fri, 25 Dec 2020 08:18:14 +0000

Hi All,

I am working on LK 4.14.76, on a SOC in which RAM starts at 0x200000000, and need to limit DMA buffer allocations to be below 0x220000000. 
I planned to do it by setting dma_mask to 0x21FFFFFFF.

But in drivers/usb/core/usb.c:590 I see the following:
Note: calling dma_set_mask() on a USB device would set the
           mask for the entire HCD, so don't do that.

I also found  related message:
https://marc.info/?l=linux-usb&m=160023769800697&w=2 : "... as the dma_mask is set by default for most busses".

So how can I set properly the dma_mask?