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Re: how to access structured registers correctly

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: how to access structured registers correctly
From: Dominic Sweetman <dom@mips.com>
Date: Wed, 27 Jul 2005 08:28:38 +0100
Cc: Hiroshi DOYU <Hiroshi_DOYU@montavista.co.jp>, linux-mips@linux-mips.org
In-reply-to: <20050726190643.GD7088@linux-mips.org>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <20050726182531.6341586f.Hiroshi_DOYU@montavista.co.jp> <20050726190643.GD7088@linux-mips.org>
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Ralf Baechle (ralf@linux-mips.org) writes:

> > In tx4938, every register access is done by using "volatile" like below.
> 
> Linus is right, volatile is a dangerous thing.  If you want to write
> portable code there's a bunch of things that are not being taken care of
> by plain C - even though in my opinion foo->somereg = 42 is more
> readable than writel(somereg, 42).  Among the things the pointer to
> volatile struct method doesn't catch are endianess conversion that might
> be necessary on some systems, write merging, dealing with write buffers
> or completly insane methods of attaching the bus such as the infamous
> ISA / EISA cage that's attached to the host system through a USB
> interface.

Yes, this is far outside the compiler's reach.

All of which suggests that it would make sense to define a standard function
which:

o will produce just one fixed-width write cycle to the destination;

o will deliver the data ordered so that the MSB of the C value is on
  the "most significant" bit of the device's data bus, usually the
  highest numbered bit (this doesn't solve all device endianess
  issues, but it gives you a well-defined place to start solving them);

o has a variant which returns only after some indication that the
  data was delivered;

The implementation of this function can then conceal the details of
the CPU and interconnect.

Such a function should probably not be called "writel()" because that
sounds like "write long", and "long" is not a fixed-size data type,
which undermines the promises above...  Tediously, you probably need
"writei32()", "writei16()", "writei8()"...

--
Dominic Sweetman
MIPS Technologies




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