[U-Boot-Users] Data Chache enable
Wolfgang Denk
wd at denx.de
Wed May 3 00:52:07 CEST 2006
Dear Jerry,
in message <44579D00.3060802 at smiths-aerospace.com> you wrote:
> > Processor: MPC8260
> > Purpose: SDH unit
...
> You didn't answer Wolfgang's question "Why do you want to enable data
> cache in u-boot???" Normally there no need to do that, so you are
> either doing something very unusual or you don't understand the problem
> you are trying to solve. If you tell us the problem, we might be able
> to give better advice (or at least tell you why we are saying "don't do
> that").
Well, the indication was in the subject line, which in the first
posting included "(performance improvement)" - the big question like
with all optimizations is: is he really trying to optimize anything
that has a significant effect? I have seen a couple of "SDH units",
and have an idea about the typical time needed to boot such a box -
and I bet a couple of beer that the time spent in U-boot to load,
uncompress and start the Linux kernel will be less than 1 % (*) of
the total boot time (= time needed from power on until the
application startup has been completed).
So even if enabling the data cache would give us 10.000% performance
improvement (which it will not, 10% are more likely), we would still
only save less than 1% of the total time. My recommendation (and I
sent this to "Mr. PDF conversion" in a private message [because he
wrote off the list, too]) is always to slaughter the fat pigs first,
i. e. start optimization where most of the time is lost^H^H^H^Hspent.
And I bet another beer that this is neither U-Boot nor Linux kernel
startup.
(*) I explicitely exclude from this bet any completely braindead
design decisions like loading a compressed kernel image from a
huge JFFS2 file system, etc.
Best regards,
Wolfgang Denk
--
Software Engineering: Embedded and Realtime Systems, Embedded Linux
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
Knowledge, sir, should be free to all!
-- Harry Mudd, "I, Mudd", stardate 4513.3
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