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

[Patrick Boettcher <patrick.boettcher@desy.de>]

Hi,

On Tue, 28 Jun 2005, Johannes Stezenbach wrote:
> > > +	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?
>
> Probably the code should use __BIG_ENDIAN_BITFIELD /
> __LITTLE_ENDIAN_BITFIELD instead of __BIG_ENDIAN / __LITTLE_ENDIAN?
> Anyway, the comment from the CVS commit suggests that it was tested.

According to the user, it is tested and it is running flawlessly for
2 weeks now.

> I completely agree that this code is ugly as hell. It was the obvious, 
> simple fix to make the driver work on PowerPC (and a few users happy), 
> though. Rewriting the driver not to use bitfields seems to be quite a 
> bit of work. Blame me for not paying enough attention when the initial 
> flexcop driver was submitted ;-(

It's my fault. The driver (skystar2) which has been replaced by the 
flexcop-driver was so complicated to read, because of an IMHO abuse of 
bitmask and bit-shifting. I absolutely wanted to bring more readability to 
that driver without having the endian-issue in mind (rewrite was possible 
after the vendor released the data sheets). Sorry for that.

regards,
Patrick.

--
  Mail: patrick.boettcher@desy.de
  WWW:  http://www.wi-bw.tfh-wildau.de/~pboettch/
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