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

[Sylwester Nawrocki]

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.

-- 
Regards,
Sylwester