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

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Date:	Tue, 16 Feb 2016 14:24:10 +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:

> Both writes leave the CPU core within the spinlock but are not serialized
> with anything else, so there is no ordering between the CPUs when they
> enter the shared bus, other than having address before data. You'd
> expect to see address0, data0, address1, data1, but it could also
> be e.g. address0, address1, data1, data0.

Ah, so it's a matter of flushing the write buffers before exiting the
spinlock-protected code.

> The point is more what the common case is. Almost all machines we support
> have fixed-endian devices, and the drivers are correct when using readl()
> or in_le32() but are not endian-safe when using __raw_readl().

Sure, e.g. PCI does it this way (eventually swapping the data lanes if
needed).

> Using __raw_readl() has the big danger of someone accidentally "fixing"
> the driver to work like all the others in order to solve a theoretical
> endian problem, while it should really be made obvious that the hardware
> actively swaps its data on the bus.

Sure - if this is the case. On-chip IXP4xx peripherals don't swap data
at all (i.e., they match CPU endianess) - accessing their registers is
like accessing a normal CPU register. That's why they don't use
PCI-style readl() etc. - however a better name than __raw_* would
probably help here.

Using __raw_* in a PCI driver would be generally wrong.
-- 
Krzysztof Halasa

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