[PATCH v4 2/9] usb: xhci: Use only 32-bit accesses in xhci_writeq/xhci_readq

Simon Glass sjg at chromium.org
Mon May 25 19:04:02 CEST 2020

Hi Sylwester,

On Mon, 25 May 2020 at 10:57, Sylwester Nawrocki <s.nawrocki at samsung.com> wrote:
> Hi Simon,
> On 25.05.2020 16:57, Simon Glass wrote:
> > On Mon, 25 May 2020 at 05:40, Sylwester Nawrocki <s.nawrocki at samsung.com> wrote:
> >>
> >> There might be hardware configurations where 64-bit data accesses
> >> to XHCI registers are not supported properly.  This patch removes
> >> the readq/writeq so always two 32-bit accesses are used to read/write
> >> 64-bit XHCI registers, similarly as it is done in Linux kernel.
> >>
> >> This patch fixes operation of the XHCI controller on RPI4 Broadcom
> >> BCM2711 SoC based board, where the VL805 USB XHCI controller is
> >> connected to the PCIe Root Complex, which is attached to the system
> >> through the SCB bridge.
> >>
> >> Even though the architecture is 64-bit the PCIe BAR is 32-bit and likely
> >> the 64-bit wide register accesses initiated by the CPU are not properly
> >> translated to a sequence of 32-bit PCIe accesses.
> >> xhci_readq(), for example, always returns same value in upper and lower
> >> 32-bits, e.g. 0xabcd1234abcd1234 instead of 0x00000000abcd1234.
> > Then I think this should be done with a quirk flag, enabled for this
> > particular device via the compatible string. It should not be an #if,
> > but an if().
> Thanks for your comments. I will check and see how this could be done.
> It might not be so straightforward since the XHCI controller is a PCI
> device matched by the pci_device_id so we would need to be looking
> at the compatible string of the PCI controller to set the quirk in
> the xhci layer. It's the PCI bridge that introduces the limitation,
> not the VL805 XHCI controller chip.

OK then it should be modelled as such.

How is this done in Linux?

You can add a quirk in the PCI controller and then XHCI can check its
parent's platdata to see the flag, perhaps, since the parent will
always be UCLASS_PCI.

You can always add the device to the devicetree if needed, and then
you get a compatible string.


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