Re: [PATCH] usb: xhci-pci: Disable 64-bit DMA for VIA VL805

[Michal Pecio]

On Fri, 26 Jun 2026 09:06:15 +0100, David Laight wrote:
> Can you do a read-back of one of the control registers that contains
> an address - if you are lucky the high bits will return zero.
> That might let the generic code enforce a limit.

Interesting idea, but nope. My VL805 with 36 bit 013600 firmware
is set up with 37 bit DMA mask right now. MMIO registers read:

DCBAAP_LOW = 0xffffe000
DCBAAP_HIGH = 0x0000001f
IR0_ERSTBA_LOW = 0xfffee000
IR0_ERSTBA_HIGH = 0x0000001f
IR0_ERDP_LOW = 0xffff4018
IR0_ERDP_HIGH = 0x0000001f

Nothing hints that the chip will truncate this to 36 bits when
DMA-ing. We learn about it when it happens.

Regards,
Michal