mtrr and ywrap

dtjohnst · 02-29-2008, 04:30 PM

I have a question which for the life of me I have never been able to find an answer for despite my scouring of the internet for long periods on several different occasions.

How can I find out if a system I'm on supports MTRR and/or YWRAP? I know you can access MTRR via /proc/mtrr, but I just read about how that information may be skewed by kernel/bios options. I know as well that MTRR will be listed in the flags section of /proc/cpuinfo, but that doesn't tell you what type of MTRR is used, which is what I'm looking for. That info is in /proc/mtrr but if it's skewed, that's no good. Also, I don't really know what /proc/mtrr is saying. For example, I have 2048MB of write-back starting at 1 address, 1MB uncachable after that, and 8MB write-back after that. Why is there 1MB uncachable in the middle and does that change anything?

For ywrap, I have NO idea where to find that info. I know if I set it on my machine in grub, my display usually doesn't display properly (though maybe 2% of the time it displays fine). I assume it doesn't support ywrap, but someone told me all modern PC's do.

I realize I could post my own hardware info here and someone might be able to tell me what my specific hardware can do, but I'm looking for more generic info that might be of use if/when I upgrade or am helping a friend install linux. There has to be somewhere to get the info, even if it means going to the manufacturers site and reading the detailed technical specifications, and if that's the answer, I'll give that another shot but I didn't find anything there last time.

Also, I assume ywrap is a video card setting, is it not? Or is it a generic memory setting? MTRR is processor/motherboard specific, correct? Or does it involve something else as well?

Thank you for your time.

Emerson · 03-01-2008, 09:25 AM

I think good start would be reading /usr/src/linux/Documentation/fb/vesafb.txt.

dtjohnst · 03-01-2008, 01:26 PM

Thanks. Checked that out. Also checked out uvesafb.txt since that's the framebuffer I'm using. Nothing says how to tell what my hardware can support.

I found out if you use the nomtrr grub option that you eliminate any inconsistencies in /proc/mtrr, so I did that I can see what my PC supports in terms of MTRR. I still have no idea to know if my system supports ywrap or ypan or any of that. Is it just trial and error to see what works?

Also, not that I learned I can see my actual MTRR capability if I use the nomtrr switch, I'm not quite sure mine is working. I have noticed that in mtrr if I use nomtrr I have:

Code:

reg00: base=0x00000000 (   0MB), size=2048MB: write-back, count=1
reg01: base=0x7ff00000 (2047MB), size=   1MB: uncachable, count=1

But if I set mtrr:2 (for write-back), I get:

Code:

reg00: base=0x00000000 (   0MB), size=2048MB: write-back, count=1
reg01: base=0x7ff00000 (2047MB), size=   1MB: uncachable, count=1
reg02: base=0xf9000000 (3984MB), size=   8MB: write-back, count=1

So I'm not sure what that means. Write-back enabled for the 2GB starting at address 0x00000000 or is it only enabled for the 8MB starting at 0xf9000000? And what's the 1937MB gap between the uncachable region and the final writeback all about? Perhaps this is why my ywrap always causes me problems?

/usr/src/linux/Documentation/mtrr.txt didn't really tell me anything useful either.

Emerson · 03-01-2008, 01:51 PM

You probably know more than I do, I never thought generic framebuffer has much to do with hardware, except it is using RAM of course.

dtjohnst · 03-01-2008, 05:04 PM

Oh. If you're right, that might change things. I was under the impression you could have accelerated framebuffer though, which would imply to me that it must be related to hardware.