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Message-ID: <4B2CC54A.9020507@gmail.com>
Date: Sat, 19 Dec 2009 13:21:30 +0100
From: Martin Fuzzey <mfuzzey@gmail.com>
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To: linux-kernel@vger.kernel.org
Subject: Portable drivers for 8 bit devices
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Hi,

I recently posted a patch for the sc26xx serial driver (which is
currently only enabled for SNI_RM 32 bit MIPS), enabling it for all
architectures (and adding support for 268x devices). I tested this on ARM.

However I was contacted (off list) by someone trying to use the driver
on 64 bit MIPS.
Turned out this didn't work because the driver (both the current main
line and my patched version) assume the UART chip (which is a 8 bit
device) is mapped into a 32 bit port (ie the first 3 8 bit register
offsets are defined as):

#define WR_PORT_MRx 0x00
#define WR_PORT_CSR 0x04
#define WR_PORT_CR  0x08

On the 64 bit MIPS system (a CISCO router..) these offsets needed to be
doubled.

What is the best way to fix this properly? Some ideas:
1) Compile time #ifdef based on CPU bus size
2) Add a driver specific KConfig option for the address shift
3) Add an address shift element to platform data

I don't think #1 is the right solution since the port size is often
configurable (especially on SoCs) independently of the CPU bus size.
My preference is #3

Cheers,

Martin