linux-kernel - Re: [PATCH 3/7] usb: gadget: pxa25x

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Date:	Fri, 29 Jan 2016 21:03:40 +0300
From:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:	Krzysztof Hałasa <khalasa@...p.pl>,
	Arnd Bergmann <arnd@...db.de>
Cc:	Felipe Balbi <balbi@...nel.org>,
	linux-arm-kernel@...ts.infradead.org, Felipe Balbi <balbi@...com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Daniel Mack <daniel@...que.org>,
	Imre Kaloz <kaloz@...nwrt.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 3/7] usb: gadget: pxa25x_udc: use readl/writel for mmio

Hello.

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.

MBR, Sergei