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Re: [PATCH] Fix eisa_mmap evaluation, add memory existence check

From: Christian Franke
Subject: Re: [PATCH] Fix eisa_mmap evaluation, add memory existence check
Date: Fri, 09 Nov 2007 22:53:23 +0100
User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv: Gecko/20070802 SeaMonkey/1.1.4

Robert Millan wrote:
On Tue, Oct 23, 2007 at 09:06:16PM +0200, Christian Franke wrote:
+/* Check memory address */
+static int
+addr_is_valid (grub_addr_t addr)
+  volatile unsigned char * p = (volatile unsigned char *)addr;
+  unsigned char x, y;
+  x = *p;
+  *p = x ^ 0xcf;
+  y = *p;
+  *p = x;
+  return y == (x ^ 0xcf);

I have a feeling this might be dangerous.  Any comments on the warnings
listed here:


Specially the bit about memory-mapped PCI.

This code does not check any memory-mapped PCI. It does only check the boundary returned by the BIOS memory map evaluation code (which didn't work in the E801 case).

Definitely not mandatory, but a IMO recommended assert-type check for experimental code.

Besides, if we really want it, perhaps it should be in a separate file so that
other ports can use it if it's needed ?

Good point.

Ah, and why 0xcf instead of 0xff ?

... or 0xaa or 0x55.

c.f. :-)

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