Re: [DVB patch 17/51] flexcop: add big endian register definitions

[Andrew Morton <akpm@osdl.org>]

Johannes Stezenbach <js@linuxtv.org> wrote:
>
> From: Patrick Boettcher <pb@linuxtv.org>
> 
> Add big-endian register definitions for running on a PowerPC.
> (Thanks to Paavo Hartikainen for testing.)
> 
> ...
> +	struct {
> +		u32 dma_address0                   :30;
> +		u32 dma_0No_update                 : 1;
> +		u32 dma_0start                     : 1;
> +	} dma_0x0;
>...
> +
> +	struct {
> +		u32 dma_0start                     : 1;
> +		u32 dma_0No_update                 : 1;
> +		u32 dma_address0                   :30;
> +	} dma_0x0;

Oh dear.  This is a good demonstration of the downside of trying to use
compiler bitfields to represent hardware registers.  I have vague memories
of writing BFINS and BFEXT in 3c59x to stomp this problem.

I don't think there's any guarantee that the code you have there will work
on all architectures/compiler versions btw.

Also...  The code appears to be assuming that BE architectures will
bit-reverse their bitfields.  Is that right?  I'd expect them to only
byte-reverse them?

IOW:

   31 30 ... 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

   |--------------- dma_address0 --------------------|
                                                       | dma_0No_update
                                                          | dma_0start

versus

   31 30 29 28 27 26 25 24 23 ... 08 07 06 05 04 03 02 01 00

   |--------------| dma_address0
                           |---- more of dma_address0 -----|
                     | dma_0No_update
                        | dma_0start

or something like that...  Brain hurts.
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