Subject: Re: bus_dmamap_sync not enough ?
To: Jason R Thorpe <firstname.lastname@example.org>
From: Matt Thomas <email@example.com>
Date: 02/13/2003 08:39:39
At 08:39 AM 2/13/2003, Jason R Thorpe wrote:
> > c) Data in the bus_dmamap_sync footprint should not be accessed by the
> > processor
> > if a BUS_DMASYNC_PREREAD is in effect for the bus_dmamap_sync
> > ( Example: XSCALE architecture?)
> > Problems:
> > Reading data that falls in the bus_dmamap_sync footprint might
> cause a
> > cacheline to be loaded.
> > This cacheline might persist in the cache and present outdated memory
> > state to the CPU
> > even after a BUS_DMASYNC_POSTREAD sync operation.
> > The outdated state in the cacheline can be flushed by a
> > BUS_DMASYNC_PREREAD sync.
> > ( This could be fixed by flushing cache lines on
>No, I don't think this is a problem. If you determine that the device is
>going to update the memory location again, then you issue another PREREAD,
>which will do the necessary non-writeback-invalidate. This is what current
>Basically, the CPU should not even try to access DMA memory (read *or* write)
>while the device "owns" it (i.e. between PRE and POST ops).
Not always possible. Think mbufs with >1 reference counts.
Matt Thomas Internet: firstname.lastname@example.org
3am Software Foundry WWW URL: http://www.3am-software.com/bio/matt/
Cupertino, CA Disclaimer: I avow all knowledge of this message