Delay in copying Linuxbios to ram

Eric W. Biederman ebiederman at lnxi.com
Mon Apr 28 15:17:01 CEST 2003


Steve Gehlbach <steve at nexpath.com> writes:

> Eric W. Biederman wrote:
> 
> > ``Write Protect'' in the mtrrs does not mean write protected.  It is a strange
> 
> > messed up form of write-through.  In particular it dumps the cache on writes
> > it does not forbid writes.
> > Unless you know of a situation where write protect is more appropriate please
> > use write-through.  It is less confusing, and since no one is writing to that
> > area anyway it gives the exact same result, reads are cached.
> > Eric
> 
> Good point, but do you think it would matter for reflashing?. 

Neither type will work reliably when reflashing.  The read caching prevents
polling of the status bits, to function properly.

> I don't know if
> Linux resets the MTRRs when booting, or assumes the BIOS has already done that.

LinuxBIOS does that.  And it is at least legal to manipulate that in an mtd
map driver if necessary.

> I got into the habit of using the WP setting because MS does it this way for the
> ROM areas for the Xbox (copying MS is probably not a good reason for doing
> anything!). 

WP is the least aggressive form of caching.  And in principle I don't have
any problems with it.  But it's name is very non-intuitive.

> For all to reference, Intel manual:
> • Write protected (WP)—Reads come from cache lines when possible, and read
> 
> misses cause cache fills. Writes are propagated to the system bus and cause
> corresponding cache lines on all processors on the bus to be
> invalidated. Speculative reads are allowed. This memory type is available in the
> 
> Pentium 4, Intel Xeon, and P6 family processors by
> programming the MTRRs (seeTable 10-6).
> ---------
> 
> -Steve




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