[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

(*) 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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