[coreboot] [PATCH] Avoid hang when 4GB or more DRAM is installed on AMD RS780 UMA systems

[Joseph Smith]

On 11/10/2010 07:30 PM, Scott Duplichan wrote:
> -----Original Message-----
> From: Joseph Smith [mailto:joe at settoplinux.org]
> Sent: Wednesday, November 10, 2010 05:42 PM
> To: Scott Duplichan
> Cc: 'Patrick Georgi'; coreboot at coreboot.org
> Subject: Re: [coreboot] [PATCH] Avoid hang when 4GB or more DRAM is installed on AMD RS780 UMA systems
>
> ]On 11/10/2010 06:18 PM, Scott Duplichan wrote:
> ]>  ] On i945, UMA is done by providing a fixed resource. I don't think any
> ]>  ] other changes were necessary (see src/northbridge/intel/i945/northbridge.c)
> ]>
> ]>  I took a look at the i945 code and found the AMD code also adds the exact
> ]>  same fixed resource for the UMA area. What I cannot figure out is how this
> ]>  can reduce the WB DRAM range so that the UMA memory ix excluded.
> ]>
> ]>  If I test with 2GB installed and a 256 MB frame buffer, function add_uma_resource
> ]>  is called with the expected arguments: Adding UMA memory area, base=70000000 size=10000000
> ]>
> ]>  Later, set_var_mtrr_resource is passed a range of c0000-7fffffff. The existing
> ]>  coreboot code assumes this range has already had the UMA part removed, and
> ]>  adds it back:
> ]>
> ]>         // Increase the base range and set up UMA as an UC hole instead
> ]>         var_state.range_sizek += (uma_memory_size>>   10);
> ]>
> ]>  What logic should deduct the reserved range from the DRAM range before
> ]>  this code runs?
> ]>
> ]Hmm. The UMA resource should be in high memory. And code should say if
> ]UMA high memory resource is used do not allocate c0000. Interesting.....
>
> I think I can explain this part. The message "Adding UMA memory area,
> base=70000000 size=10000000" refers to the physical address of the DRAM
> that becomes unavailable due to UMA. This DRAM is never accessed using
> this physical address. It is accessed through the frame buffer bar,
> which gets assigned an address between top of memory and 4GB, such as E8000000.
>
Not so sure about AMD chips but I know Intel chips reserve memory just 
below 4Gb for vga buffer. As for resource ranges Intel chips reserve a 
low memory range for VESA IO registers and a high memory range for GMCH 
MMIO registers. Hope that helps.


-- 
Thanks,
Joseph Smith
Set-Top-Linux
www.settoplinux.org