[U-Boot-Users] Data Chache enable
Processor: MPC8260 U-boot Version: 1.1.2 Project: Communication SDH
I am trying to avoid the D chache exception 200 when I enable D cache.
I know that the documentation indicates: "Data cache cannot be enabled on systems like 8xx or 8260(where access to IMMR region must be uncached). D cache cannot be diabled on other systems where we (mis-)use d-cache to hold an initial stach & some data..."
Are there any new commands I can include in the U-Boot boot loader?
Does any one know what the specific register settings are to avoid the data cache exception.
If there is any documentation I overlooked please indicate.
I am new to this.
Thank you, Regards,
-Om
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PDF conversion wrote:
Processor: MPC8260 U-boot Version: 1.1.2 Project: Communication SDH
I am trying to avoid the D chache exception 200 when I enable D cache.
I know that the documentation indicates: "Data cache cannot be enabled on systems like 8xx or 8260(where access to IMMR region must be uncached). D cache cannot be diabled on other systems where we (mis-)use d-cache to hold an initial stach & some data..."
Are there any new commands I can include in the U-Boot boot loader?
Does any one know what the specific register settings are to avoid the data cache exception.
If there is any documentation I overlooked please indicate.
I am new to this.
Thank you, Regards,
-Om
Exception 0200 is Machine Check, it has nothing directly to do with data caching except for the fact that you are enabling data cache inappropriately.
The simplest way to use data cache the way you appear to want to use it is to use the BATs to map your I/O memory (minimum: IMMR region) as uncached and the rest of your used memory (flash + RAM) as cached. If you don't do that, _everything_ is cached which is a Very Bad Thing[tm], as you found out. I am not aware of anybody having success enabling data cache without BATs/MMUs and successfully maintaining cache coherency by hand (which is what you are hinting at).
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").
gvb
Dear Jerry,
in message 44579D00.3060802@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
I asked you before: Please provide a real name. This is the last reply I send to somebody who hides behind a silly name like "PDF conversion". Please READ the netiquette.
In message 20060502170715.88092.qmail@web37103.mail.mud.yahoo.com you wrote:
I am trying to avoid the D chache exception 200 when I enable D cache.
The fix is simple: don't enable D-Cache.
I know that the documentation indicates: "Data cache cannot be enabled on systems like 8xx or 8260(where
What exatly is not clear in the word "cannot"?
Do you understand what "can not" means?
Are there any new commands I can include in the U-Boot boot loader?
There are many commands that can be enabled, but probably none of them will help you doing things that cannot be done.
Does any one know what the specific register settings are to avoid the data cache exception.
If there is any documentation I overlooked please indicate.
It seems you did not overlook something, but you obviously try to ignore a couple of things.
Best regards,
Wolfgang Denk
participants (3)
-
Jerry Van Baren -
PDF conversion -
Wolfgang Denk