Date: Sat, 28 Aug 2010 19:20:10 +0200
From: Sascha Hauer <s.hauer@pengutronix.de>
To: Marek Vasut <marek.vasut@gmail.com>
Cc: Linus Walleij <linus.ml.walleij@gmail.com>,
        Dan Williams <dan.j.williams@intel.com>, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        Uwe =?iso-8859-15?Q?Kleine-K=F6nig?= 
	<u.kleine-koenig@pengutronix.de>
Subject: Re: [PATCH 3/3 v2] dmaengine: Add Freescale i.MX SDMA support
Message-ID: <20100828172010.GJ1473@pengutronix.de>
References: <1281956870-12463-1-git-send-email-s.hauer@pengutronix.de> <201008281727.10913.marek.vasut@gmail.com> <20100828161858.GI1473@pengutronix.de> <201008281830.05208.marek.vasut@gmail.com>
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On Sat, Aug 28, 2010 at 06:30:05PM +0200, Marek Vasut wrote:
> Dne So 28. srpna 2010 18:18:58 Sascha Hauer napsal(a):
> > On Sat, Aug 28, 2010 at 05:27:10PM +0200, Marek Vasut wrote:
> > > Dne So 28. srpna 2010 17:18:17 Linus Walleij napsal(a):
> > > > 2010/8/23 Uwe Kleine-K?nig <u.kleine-koenig@pengutronix.de>:
> > > > >> +     evt = readl(SDMA_H_EVTOVR);
> > > > >> +     mcu = readl(SDMA_H_HOSTOVR);
> > > > >> +     dsp = readl(SDMA_H_DSPOVR);
> > > > > 
> > > > > __raw_readl?
> > > > 
> > > > Sorry I never understood this __raw_[read|write][b|w|l] vs.
> > > > plain read[b|w|l] proliferation in some drivers and code.
> > > > 
> > > > What's the reason for?
> > > 
> > > Hey,
> > > 
> > > this trick is, if you create the VA<->PA mapping at the kernel start (eg.
> > > see how pxa_map_io() is replacedon some devices for instance), you then
> > > use the VA address you specified and use __raw_{read,write}[b,w,l](). So
> > > use __raw_{read,write}[b,w,l]() on drivers specific for certain device
> > > and use {read,write}[b,w,l]() on ioremap()ed memory areas, aka. in
> > > common drivers.
> > 
> > Nope, this has nothing to do with static mappings vs. ioremap. The
> > difference is that read[b,w,l] do little endian accesses suitable for
> > PCI whereas the __raw_* functions do accesses in CPU endianess.
> > Peripherals integrated into a SoC like the SDMA engine here are
> > normally accessible in native endianess and thus need the __raw_*
> > functions. An external network controller (for example a LAN9117) will
> > probably need the non raw functions. Note that 99% of the arm users use
> > little endian only and thus cpu_to_le* is a noop, so both types will work
> > for most people.
> 
> Sorry, you got me here. Why do we have io{read,write} then btw. ? That's for 
> x86's io space, right ?

Looking at the implementation in lib/iomap.c it seems that
io{read,write} can do both PIO and MMIO accesses depending on the
address given to them.

Sascha

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