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

[David Laight <david.laight.linux@gmail.com>]

On Fri, 26 Jun 2026 10:57:23 +0200
Michal Pecio <michal.pecio@gmail.com> wrote:

> 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.

Clearly the only hardware testing was 'does it work with 32bit windows' :-)

	David

> 
> Regards,
> Michal