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

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Date:	Mon, 15 Feb 2016 08:33:37 +0100
From:	khalasa@...p.pl (Krzysztof Hałasa)
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Felipe Balbi <balbi@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	Felipe Balbi <balbi@...com>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Daniel Mack <daniel@...que.org>,
	Imre Kaloz <kaloz@...nwrt.org>,
	Robert Jarzmik <robert.jarzmik@...e.fr>
Subject: Re: [PATCH 3/7] usb: gadget: pxa25x_udc: use readl/writel for mmio

Arnd Bergmann <arnd@...db.de> writes:

>> Anyway, I think readl()/writel() do the right thing: in BE mode they
>> swap PCI accesses and don't swap normal registers, in LE mode nothing is
>> swapped.
>
> This seems to be true when CONFIG_IXP4XX_INDIRECT_PCI is set, but
> not otherwise. For the indirect variant, writel() is a __raw_writel()
> without barrier or byteswap on non-PCI memory, while with that
> option disabled, we use the standard implementation that has both
> a byteswap and a barrier.
>
> According to your description, that would mean the version without
> indirect PCI access is broken and it appears to have been that way
> since before the start of git history in 2.6.12.
>
> It's possible that nobody cared because all drivers for non-PCI
> devices on ixp4xx (the on chip ones) just use __raw_readl or
> direct pointer references.

Well, it is possible. I recall I probably used __raw_* instead of
readl()/writel() in qmgr/npe/Ethernet drivers for this very reason.

I think Direct pointer references are only used for locations in RAM
(DMA buffers), they have their own share of problems because the
peripherals are always big endian.

I think I still have an early IXP425 board with UDC connector so I will
try to dig it up and test this code.

I wonder if we should switch the driver to use __raw_*, too. The problem
is almost nobody uses UDC with this CPU.
-- 
Krzysztof Halasa

Industrial Research Institute for Automation and Measurements PIAP
Al. Jerozolimskie 202, 02-486 Warsaw, Poland