Re: [PATCH 3/7] usb: gadget: pxa25x_udc: use readl/writel for mmio

[Sergei Shtylyov]

Hello.

On 01/29/2016 07:18 PM, Krzysztof HaÅasa wrote:

> > The unclear part here is for IXP4xx, which supports both big-endian
> > and little-endian configurations. So far, the driver has done
> > no byteswap in either case. I suspect that is wrong and it would
> > actually need to swap in one or the other case, but I don't know
> > which.
>
> If at all, I guess it should swap in LE mode. But it's far from certain.
>
> > It's also possible that there is some magic setting in
> > the chip that makes the endianess of the MMIO register match the
> > CPU, and in that case, the code actually does the right thing
> > for all configurations, both before and after this patch.
>
> This is IMHO most probable.
>
> Actually, the IXP4xx is "natural" in BE mode (except for PCI) and
> normally in LE mode it's order-coherent, meaning 32-bit "integer"
> accesses need no swapping, but 8-bit and (mostly unused) 16-bit
> transfers need swapping.
>
> Anyway, I think readl()/writel() do the right thing: in BE mode they
> swap PCI accesses and don't swap normal registers,

   Alas, readl()/writel() don't know what registers you are calling them for, 
they were designed for PCI which is little-endian, so they will swap in BE mode.

MBR, Sergei