question on howto use invalidate_dcache_range

[Stephan Linke]

Hi Joke,

thanks for your explaination. Verry plausible to me now.
I guess we have to take care for the alignment anyway since we don't use sk buffer for some reason :).

Thanks, Stephan

> -----Original Message-----
> From: Joakim Tjernlund [mailto:joakim.tjernlund at lumentis.se]
> Sent: Donnerstag, 13. März 2003 08:41
> To: Stephan Linke; Linuxppc-Embedded
> Subject: RE: question on howto use invalidate_dcache_range
>
>
> > Hi,
> >
> > in November there was the discussion about Joakims patch to 8xx_io/enet.c.
> > (http://lists.linuxppc.org/linuxppc-embedded/200211/msg00122.html)
> >
> > Since the result of the discussion didn't seam that clear to me I still have a question related to this.
> >
> > After all is it realy neccessary to use invalidate_dcache range before giving the transmit buffer to the DMA?
> >   That's one of Joakims last modifications but what exactly is the reason? It's obvious for the flush command but I
> > couldn't figure
> > out the reason for invalidate.
>
> Yes, it is neccessary. When you allocate a new skb buffer it you may get a recently used
> buffer. The TCP/UPD/IP layer(s) may have modified some of the contents in that buffer before
> freeing it. Therefore can the dcache contain dirty(modified) cache lines which belong to the
> buffer. When the CPU needs more dcache, it will flush those dirty cache lines to memory which
> now belong to the CPM. By invalidating the buffer you tell the CPU to free those cachelines
> whithout writeing them to memory.
>
> > Beside the cache line alignment problem (causing nearby data to be flushed or invalidated) are there any other problems
> > to be taken
> > care of?
>
> There are no cache line alignment problems. There was some concern(expressed by Dan) that
> some vital data at the end of the skb buffer could be invalidated due to bad aligment.
> See http://lists.linuxppc.org/linuxppc-embedded/200302/msg00135.html
> These has been fixed in the skb layer long ago.
>
> The patch has been around for many months now and I have not received any problem
> reports.
>
>  Jocke
>
> > Thanks, Stephan
>
>
>


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