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. M?rz 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 > > > ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/
