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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Christian Lamparter <chunkeey@googlemail.com>,
	Arnd Bergmann <arnd@arndb.de>
Cc: Felipe Balbi <felipe.balbi@linux.intel.com>,
	linux-mips@linux-mips.org, johnyoun@synopsys.com,
	gregkh@linuxfoundation.org, linux-usb@vger.kernel.org,
	linux-kernel@vger.kernel.org, a.seppala@gmail.com,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: usb: dwc2: regression on MyBook Live Duo / Canyonlands since 4.3.0-rc4
Date: Fri, 13 May 2016 08:17:34 +1000	[thread overview]
Message-ID: <1463091454.3237.19.camel@kernel.crashing.org> (raw)
In-Reply-To: <7745292.ZB3149zIk7@debian64>

On Thu, 2016-05-12 at 11:58 +0200, Christian Lamparter wrote:
> 
> > http://www.linuxplumbersconf.org/2012/wp-content/uploads/2012/09/2012-lpc-ref-big-little-endian-herrenschmidt.odp
> > 
> > but there are at least two more twists that you completely missed here:
> > 
> > - Some architectures (e.g. ARMv5 "BE32" mode in IXP4xx, surely some others)
> >   do not implement big-endian mode by wiring up the data lines between the
> >   bus and the CPU differently between big- and little-endian mode like
> >   powerpc and armv7 "BE8" do, but instead they swizzle the *address* lines
> >   on 8-bit and 16-bit addresses. The effect of that is that normal RAM
> >   accesses work as expected both ways, and devices that are accessed using
> >   32-bit MMIO ops never need any byteswap (you actually get "native
> >   endian") while MMIO with 8 and 16 bit width does something completely
> >   unexpected and touches the wrong register. Having an explicit byteswap
> >   on the PCI host bridge gets you the expected addresses again for 8-bit
> >   cycles but it also means that readl()/writel() again need to swap the
> >   data.

Right. Old PowerPC did that too and it's completely stupid. Thankfully
most vendors grew a clue since then and this practice has slowly fallen
into oblivion.

> > - Some other architectures (e.g. Broadcom MIPS) apparently are even fancier
> >   and use a strapping pin on the SoC flips the endianess of the CPU core
> >   at the same time as all the peripheral MMIO registers, with the intention
> >   of never requiring any byte swaps. I believe they are implemented careful
> >   enough to actually get this right, but it confuses the heck out of
> >   Linux drivers that don't expect this.

Right. Drivers like that will need an explicit test in the accessors.

Cheers,
Ben.

  parent reply	other threads:[~2016-05-12 22:18 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-05-07 22:54 usb: dwc2: regression on MyBook Live Duo / Canyonlands since 4.3.0-rc4 Christian Lamparter
2016-05-08 10:40 ` Benjamin Herrenschmidt
2016-05-08 11:44   ` Christian Lamparter
2016-05-09  0:23     ` Benjamin Herrenschmidt
2016-05-09 10:36       ` Arnd Bergmann
2016-05-09 10:39         ` Felipe Balbi
2016-05-09 15:08           ` Arnd Bergmann
2016-05-09 19:06             ` Christian Lamparter
2016-05-09 20:10               ` Arnd Bergmann
2016-05-09 22:43               ` Benjamin Herrenschmidt
2016-05-09 22:37             ` Benjamin Herrenschmidt
2016-05-10  7:23               ` Arnd Bergmann
2016-05-12  9:58                 ` Christian Lamparter
2016-05-12 11:55                   ` Arnd Bergmann
2016-05-12 13:30                     ` Christian Lamparter
2016-05-12 18:40                       ` John Youn
2016-05-12 20:39                         ` Christian Lamparter
2016-05-12 20:50                           ` Arnd Bergmann
2016-05-12 20:55                           ` John Youn
2016-05-14 13:11                         ` Christian Lamparter
2016-05-14 19:45                           ` Arnd Bergmann
2016-05-17 23:50                           ` John Youn
2016-05-18 19:14                             ` Christian Lamparter
2016-05-18 21:09                               ` Arnd Bergmann
2016-05-19  0:36                               ` John Youn
2016-05-12 22:17                   ` Benjamin Herrenschmidt [this message]
2016-05-09 22:33           ` Benjamin Herrenschmidt
2016-05-09 14:02         ` Benjamin Herrenschmidt
2016-05-09 20:22         ` John Youn
2016-05-09 20:38           ` Arnd Bergmann
2016-05-09 21:11             ` John Youn
2016-05-09 21:30               ` Arnd Bergmann

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