Re: port-sgimips/38932: sgimips bus_dmamap_sync() is incorrect

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

The following reply was made to PR port-sgimips/38932; it has been noted by 
GNATS.

From: Manuel Bouyer <bouyer%antioche.eu.org@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: port-sgimips-maintainer%NetBSD.org@localhost, 
gnats-admin%NetBSD.org@localhost,
        netbsd-bugs%NetBSD.org@localhost
Subject: Re: port-sgimips/38932: sgimips bus_dmamap_sync() is incorrect
Date: Tue, 10 Jun 2008 21:59:36 +0200

 On Tue, Jun 10, 2008 at 07:35:02PM +0000, Izumi Tsutsui wrote:
 > The following reply was made to PR port-sgimips/38932; it has been noted by 
 > GNATS.
 > 
 > From: Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost>
 > To: gnats-bugs%NetBSD.org@localhost
 > Cc: port-sgimips-maintainer%NetBSD.org@localhost, 
 > gnats-admin%NetBSD.org@localhost,
 >         netbsd-bugs%NetBSD.org@localhost, tsutsui%ceres.dti.ne.jp@localhost
 > Subject: Re: port-sgimips/38932: sgimips bus_dmamap_sync() is incorrect
 > Date: Wed, 11 Jun 2008 04:31:35 +0900
 > 
 >  >   bus_dmamap_sync(POSTREAD) should do a cache invalidation.
 >  >   bus_dmamap_sync(PREREAD) should probably be turned to a NOP
 >  
 >  No, it won't work if specified region isn't cacheline aligned.
 >  
 >  We can't invalidate cache if specified region is not cacheline aligned
 >  (otherwise some necessary data in the same cacheline could be lost) and
 >  we have to use writeback and invalidate in that case, but
 >  such writeback should be done _before_ starting DMA otherwise
 >  transfered data via DMA will be lost by writeback.
 >  That's the reason why all POSTWRITE and POSTREAD are no-op.
 >  
 >  Drivers should call PREREAD after each polling instead.
 >  (see sys/dev/ic/rtl8169.c:re_rxeof() etc.)
 
 I see your point. But I'm not sure such devices can work reliably
 with a write-back cache if regions are not cacheline-aligned.
 For example, I'm almost sure an uchi/ehci can't work reliably on hosts
 with write-back caches (unless the descriptors are mappped uncached).
 I suspect re(4) has the same issue with re_desc->re_cmdstat (the updated
 re_cmdstat from a descriptor may be lost if the host writes another
 descriptor in the same cache line).
 
 -- 
 Manuel Bouyer <bouyer%antioche.eu.org@localhost>
      NetBSD: 26 ans d'experience feront toujours la difference
 --