On Friday 29 January 2016 21:03:40 Sergei Shtylyov wrote: > On 01/29/2016 07:18 PM, Krzysztof Hałasa 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. > In general, that is correct, but as Krzysztof said, ixp4xx is rather special here, and it implements its own readl/writel functions that behave differently for PCI spaces than for on-chip addresses, See arch/arm/mach-ixp4xx/include/mach/io.h. This platform evidently does its own tricks with byte swapping so while a normal big-endian platform has to swap on readl (but not readsl), ixp4xx never does any swaps when CONFIG_IXP4XX_INDIRECT_PCI is set (neither readl nor readsl, neither PCI nor on-chip MMU) However if CONFIG_IXP4XX_INDIRECT_PCI is disabled (which I think is almost always the case), it basically behaves like all the other platforms, and in big-endian configurations performsm swaps for inl/readl/ioread32 but not readsl/ioread32_rep/insl (it swaps twice for the last one). Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
