AMD Elan SC520 CPU speed problem w/patches

There appears to be cleverly hidden problem with the Elan SC520 CPU speed when running the Linux kernel. The speed is controlled by a memory-mapped registers that the CPU always offers at the physical address 0xfffef000. Some BIOSs--at least General Systems--enable an alias of these registers at 0x000df000 for the sake of real-mode programs. It appears that something in the kernel writes to 0x000df002 because the CPU speed at the start of kernel initialization is correct at 133MHz, but when user-mode applications start running the CPU speed has been reduced to 100MHz. Disabling the unnecessary aliasing solves the problem. Note that the BOGOMIPS calculation is early enough to preceed the speed change.

<http://www.buici.com/elan>

Here, you'll find source to a program that verifies the problem. You'll also find a patch for linux-2.2.22 that fixes this bug as well as programming the PIT timer for Elan CPUs. Last, there is a watchdog driver that works when the above mentioned aliasing is disabled.

Kindly copy replies to my email address.

Cheers,
Marc Singer

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For the speed problem, the important part of the patch reads as below. The code is executed at the top of init_amd().

@@ -662,6 +662,29 @@
        
        int r=get_model_name(c);
        
+#ifdef CONFIG_ELAN_COMPATIBILITY 
+       if (c->x86 == 4 && (c->x86_model == 9 || c->x86_model == 10)) {
+         /* There appears to be a driver that clobbers one of the
+            system control registers because these registers are
+            mapped to 0xdf000.  So, this code disables that mapping.
+            It isn't necessary anyway and is a legacy of the BIOS. */
+#define CBAR           (0xfffc) /* Configuration Base Address  (32-bit) */
+#define CBAR_ENB       (0x80000000)
+#define CBAR_KEY       (0X000000CB)
+               if (inl (CBAR) & CBAR_ENB)
+                       outl (0 | CBAR_KEY, CBAR);
 

My AMD Elan patch for the Linux kernel lives on

http://www.pengutronix.de/software/elan_en.html

If you have other stuff for it please send it to me (or port it back to your 2.2 kernel).

Robert

 Dipl.-Ing. Robert Schwebel | http://www.pengutronix.de
 Pengutronix - Linux Solutions for Science and Industry
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   Hall 5, Booth 154 +++ Please contact us for details.

I don't understand. Are you planning to add my fix to your patch? Please, if you do so would you put a link to my page so that updates to my elan stuff is easy to find?