linux-kernel - Re: [PATCH] drivers/block/xsysace - replace in(out)_8/in(out)_be16/in(out)

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Date:	Mon, 4 Feb 2013 17:24:47 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Michal Simek <monstr@...str.eu>
Cc:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
	linux-kernel@...r.kernel.org, grant.likely@...retlab.ca,
	benh@...nel.crashing.org, alan@...rguk.ukuu.org.uk,
	geert@...ux-m68k.org, dahinds@...rs.sourceforge.net
Subject: Re: [PATCH] drivers/block/xsysace - replace in(out)_8/in(out)_be16/in(out)_le16 with generic iowrite(read)8/16(be)

On Monday 04 February 2013, Michal Simek wrote:
> >
> > and select the CONFIG_FOO_BIG_ENDIAN and CONFIG_FOO_LITTLE_ENDIAN
> > symbols in Kconfig based on the system you are building for.
> 
> Using CONFIG_FOO_BIG/LITTLE is not good because it is just another
> Kconfig option.
> You can easily detect it at runtime and for dedicated hw with fixed
> endians you can just
> handle it in the driver and don't care about global setting.

The configuration option should not be visible, so it's
not something a user would have to worry about. It's just
sometimes nicer to express the configuration of the platform
in terms of Kconfig syntax than it is in C code if you have
complex platform dependencies.

> > This of course gets further complicated if you require different
> > accessors per architecture, like ARM wanting readl or ioread32
> > and PowerPC wanting in_le32 for a little-endian SoC component.
> 
> FYI: I have got two responses on linux-arch from Alan
> "Set the pointers up and pass them as data with your platform device, that
> way the function definitions are buried in your platform code where they
> depend."
> 
> and Geert:
> "Or embed a struct io_ops * in struct device, to be set up by the bus driver?
> 
> Wasn't David Hinds working on something like this in the context of PCMCIA
> a few decades ago?"
> 
> Based on their suggestions one way can be to pass it through void *platform_data
> which is probably not the best and then which make more sense to me is to extend
> struct dev_archdata archdata to add there native read/write functions.
> 
> What do you think?

I worry a little about code size if we have a lot of drivers that go
from one instruction to an indirect function call for each readl/writel.
Using platform_data ss also something that does not work too well with
device tree based platform configuration, which tries hard to leave
all run-time configuration inside of the driver.

	Arnd
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