ich9deblob: add more commenting

1 files changed, 102 insertions, 28 deletions

diff --git a/resources/utilities/ich9deblob/ich9deblob.c b/resources/utilities/ich9deblob/ich9deblob.c
index 0d1768d..a2819bf 100644
--- a/resources/utilities/ich9deblob/ich9deblob.c
+++ b/resources/utilities/ich9deblob/ich9deblob.c
@@ -1,5 +1,8 @@
 /*
- *  ich9deblob.c
+ *  main.c
+ *  This file is part of the ich9deblob utility from the libreboot project
+ * 
+ * Purpose: disable and remove the ME from ich9m/gm45 machines in coreboot.
  *
  *  Copyright (C) 2014 Steve Shenton <sgsit@libreboot.org>
  *                     Francis Rowe <info@gluglug.org.uk>
@@ -17,6 +20,25 @@
  *  You should have received a copy of the GNU General Public License
  *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
+ 
+/*
+ * Read a factory.rom dump (ich9m/gm45 machines) and 
+ * modify the flash descriptor to remove all regions except descriptor,
+ * Gbe and BIOS. Set BIOS region to full size of the ROM image (after
+ * the flash descriptor and gbe). Basically, deblob the descriptor.
+ * 
+ * This will will generate a concatenated descriptor+gbe dump suitable
+ * for use in libreboot. Currently tested: ThinkPad X200 (coreboot/libreboot)
+ */
+ 
+// See docs/hcl/x200_remove_me.html for info plus links to datasheet (also linked below)
+
+// Info about flash descriptor (read page 850 onwards):
+// * http://www.intel.co.uk/content/dam/doc/datasheet/io-controller-hub-9-datasheet.pdf
+
+// Info about Gbe region (read whole datasheet):
+// * http://www.intel.co.uk/content/dam/doc/application-note/i-o-controller-hub-9m-82567lf-lm-v-nvm-map-appl-note.pdf
+// * https://communities.intel.com/community/wired/blog/2010/10/14/how-to-basic-eeprom-checksums
 
 #include <stdio.h>
 #include <string.h>
@@ -25,16 +47,22 @@
 
 #define DESCRIPTORREGIONSIZE 0x1000 // 4096 bytes / 4KiB
 #define GBEREGIONSIZE 0x2000 // 8192 bytes / 8KiB
+// These will have a modified descriptor+gbe based on what's in the factory.rom
+// These will be joined into a single 12KiB buffer (descriptor, then gbe) and saved to a file
+// NOTE: The GBE region of 8K is actually 2x 4K regions in a single region; both 4K blocks can be identical (and by default, are)
+// The 2nd one is a "backup", but we don't know when it's used. perhaps it's used when the checksum on the first one does not match?
 
+// gbe checksum related functions
 unsigned short GetChecksum(char* buffer, unsigned short desiredValue); // for GBe region (checksum calculation)
 unsigned short GetRegionWord(int i, char* buffer); // used for getting each word needed to calculate said checksum
 
 int main(int argc, char *argv[])
 {
-	// check compiler bit-packs in a compatible way
+	
+	// descriptor region. Will have actual descriptor mapped to it (from the factory.rom dump)
 	struct DESCRIPTORREGIONRECORD descriptorRegion;
 	unsigned int descriptorRegionStructSize = sizeof(descriptorRegion);
-	// basically, it is expected that this code will be used on x86
+	// check compiler bit-packs in a compatible way basically, it is expected that this code will be used on x86
 	if (DESCRIPTORREGIONSIZE != descriptorRegionStructSize){
 		printf("\nerror: compiler incompatibility: descriptor struct length is %i bytes (should be %i)\n", descriptorRegionStructSize, DESCRIPTORREGIONSIZE);
 		return 1;
@@ -56,10 +84,11 @@ int main(int argc, char *argv[])
 	}
 	printf("\nfactory.rom opened successfully\n");
 	// -----------------------------------------------
+	
+	// Get the descriptor region dump from the factory.rom
 
 	// Create empty descriptor buffer (populated below)
 	char descriptorBuffer[DESCRIPTORREGIONSIZE];
-
 	// Extract the descriptor region from the factory.rom dump
 	// (goes in descriptorBuffer variable)
 	unsigned int readLen;
@@ -70,7 +99,6 @@ int main(int argc, char *argv[])
 		return 1;
 	}
 	printf("\ndescriptor region read successfully\n");
-
 	// copy descriptor buffer into descriptor struct memory
 	// descriptorRegion is an instance of a struct that actually
 	// defines the locations of all these variables in the descriptor,
@@ -78,16 +106,24 @@ int main(int argc, char *argv[])
 	// descriptor over the struct so that it can then be modified
 	// for libreboot's purpose
 	memcpy(&descriptorRegion, &descriptorBuffer, DESCRIPTORREGIONSIZE);
+	
+	// -----------------------------------------------------------------------------------------------
+	
+	// Get the gbe region dump from the factory.rom
 
 	// get original GBe region location
 	// (it will be moved to the beginning of the flash, after the descriptor region)
-	unsigned int flRegionBitShift = 12;
+	unsigned int flRegionBitShift = 12;// bits 12-24 are represented.
+	// note for example, gbeRegionLocation is set to <<12 of actual address (in C). this is how the addresses
+	// are stored in the descriptor.
 	unsigned int gbeRegionLocation = descriptorRegion.regionSection.flReg3.BASE << flRegionBitShift;
 
+	// Set offset so that we can read the data from
+	// the gbe region
 	fseek(fp, gbeRegionLocation, SEEK_SET);
-
+	// data will go in here
 	char gbeBuffer[GBEREGIONSIZE];
-
+	// Read the gbe data from the factory.rom and put it in gbeBuffer
 	readLen = fread(gbeBuffer, sizeof(char), GBEREGIONSIZE, fp);
 	if (GBEREGIONSIZE != readLen)
 	{
@@ -95,6 +131,8 @@ int main(int argc, char *argv[])
 		return 1;
 	}
 
+	// -----------------------------------------------------------------------------------------------
+
 	// Get size of ROM image
 	// This is needed for relocating the BIOS region (per descriptor)
 	fseek(fp, 0L, SEEK_END);
@@ -103,43 +141,69 @@ int main(int argc, char *argv[])
 	printf("\nfactory.rom size: [%i] bytes\n", romSize);
 
 	fclose(fp);
+	
+	// -----------------------------------------------------------------------------------------------
 
+	// debugging
 	printf("\nOriginal Descriptor start block: %08x ; Descriptor end block: %08x\n", descriptorRegion.regionSection.flReg0.BASE << flRegionBitShift, descriptorRegion.regionSection.flReg0.LIMIT << flRegionBitShift);
 	printf("Original BIOS start block: %08x ; BIOS end block: %08x\n", descriptorRegion.regionSection.flReg1.BASE << flRegionBitShift, descriptorRegion.regionSection.flReg1.LIMIT << flRegionBitShift);
 	printf("Original ME start block: %08x ; ME end block: %08x\n", descriptorRegion.regionSection.flReg2.BASE << flRegionBitShift, descriptorRegion.regionSection.flReg2.LIMIT << flRegionBitShift);
 	printf("Original GBe start block: %08x ; GBe end block: %08x\n", gbeRegionLocation, descriptorRegion.regionSection.flReg3.LIMIT << flRegionBitShift);
 
-	// set number of regions from 4 -> 2 (0 based)
+	// Now we need to modify the descriptor so that the ME can be excluded
+	// from the final ROM image (libreboot one) after adding the modified
+	// descriptor+gbe. Refer to libreboot docs for details: docs/hcl/x200_remove_me.html
+
+	// set number of regions from 4 -> 2 (0 based, so 4 means 5 and 2
+	// means 3. We want 3 regions: descriptor, gbe and bios, in that order)
 	descriptorRegion.flMaps.flMap0.NR = 2;
 
-	// make descriptor writable from OS.
+	// make descriptor writable from OS. This is that the user can run:
+	// sudo ./flashrom -p internal:laptop=force_I_want_a_brick 
+	// from the OS, without relying an an external SPI flasher, while
+	// being able to write to the descriptor region (locked by default,
+	// until making the change below):
 	descriptorRegion.masterAccessSection.flMstr1.fdRegionWriteAccess = 1;
 
 	// relocate BIOS region and increase size to fill image
-	descriptorRegion.regionSection.flReg1.BASE = 3;
-	descriptorRegion.regionSection.flReg1.LIMIT = ((romSize / 0x1000) - 1);
-
-	// set ME region size to 0
-	descriptorRegion.regionSection.flReg2.BASE = 0xFFF;
+	descriptorRegion.regionSection.flReg1.BASE = 3; // 3<<12 is 12KiB, which is where BIOS region is to begin (after descriptor and gbe)
+	descriptorRegion.regionSection.flReg1.LIMIT = ((romSize >> flRegionBitShift) - 1);
+	// ^ for example, 8MB ROM, that's 8388608 bytes.
+	// ^ 8388608>>12 (or 8388608/4096) = 2048 bytes
+	// 2048 - 1 = 2047 bytes. 
+	// This defines where the final 0x1000 (4KiB) page starts in the flash chip, because the hardware does:
+	// 2047<<12 (or 2047*4096) = 8384512 bytes, or 7FF000 bytes
+	// (it can't be 0x7FFFFF because of limited number of bits)
+
+	// set ME region size to 0 - the ME is a blob, we don't want it in libreboot
+	descriptorRegion.regionSection.flReg2.BASE = 0x1FFF; // setting 1FFF means setting size to 0. 1FFF<<12 is outside of the ROM image (8MB) size?
+	// ^ datasheet says to set this to 1FFF, but FFF was previously used and also worked.
 	descriptorRegion.regionSection.flReg2.LIMIT = 0;
-
-	// relocate Gbe region
-	descriptorRegion.regionSection.flReg3.BASE = 1;
+	// ^ 0<<12=0, so basically, the size is 0, and the base (1FFF>>12) is well outside the higher 8MB range. 
+	
+	// relocate Gbe region to begin at 4KiB (immediately after the flash descriptor)
+	descriptorRegion.regionSection.flReg3.BASE = 1; // 1<<12 is 4096, which is where the Gbe region is to begin (after the descriptor)
 	descriptorRegion.regionSection.flReg3.LIMIT = 2;
+	// ^ 2<<12=8192 bytes. So we are set it to size 8KiB after the first 4KiB in the flash chip.
 
-	// set Platform region size to 0
-	descriptorRegion.regionSection.flReg4.BASE = 0xFFF;
+	// set Platform region size to 0 - another blob that we don't want
+	descriptorRegion.regionSection.flReg4.BASE = 0x1FFF; // setting 1FFF means setting size to 0. 1FFF<<12 is outside of the ROM image (8MB) size?
+	// ^ datasheet says to set this to 1FFF, but FFF was previously used and also worked.
 	descriptorRegion.regionSection.flReg4.LIMIT = 0;
+	// ^ 0<<12=0, so basically, the size is 0, and the base (1FFF>>12) is well outside the higher 8MB range.
 
-	// disable ME in ICHSTRAP0
+	// disable ME in ICHSTRAP0 - the ME is a blob, we don't want it in libreboot
 	descriptorRegion.ichStraps.ichStrap0.meDisable = 1;
 
 	// disable ME and TPM in MCHSTRAP0
-	descriptorRegion.mchStraps.mchStrap0.meDisable = 1;
-	descriptorRegion.mchStraps.mchStrap0.tpmDisable = 1;
+	descriptorRegion.mchStraps.mchStrap0.meDisable = 1; // ME is a blob. not wanted in libreboot.
+	descriptorRegion.mchStraps.mchStrap0.tpmDisable = 1; // not wanted in libreboot
 
-	// disable ME, apart from chipset bugfixes (ME region still required)
-	//descriptorRegion.mchStraps.mchStrap0.meAlternateDisable = 1;
+	// disable ME, apart from chipset bugfixes (ME region should first be re-enabled above)
+	// This is sort of like the CPU microcode updates, but for the chipset
+	// (commented out below here, since blobs go against libreboot's purpose,
+	// but may be interesting for others)
+	// descriptorRegion.mchStraps.mchStrap0.meAlternateDisable = 1;
 
 	// debugging
 	printf("\nRelocated Descriptor start block: %08x ; Descriptor end block: %08x\n", descriptorRegion.regionSection.flReg0.BASE << flRegionBitShift, descriptorRegion.regionSection.flReg0.LIMIT << flRegionBitShift);
@@ -147,19 +211,27 @@ int main(int argc, char *argv[])
 	printf("Relocated ME start block: %08x ; ME end block: %08x\n", descriptorRegion.regionSection.flReg2.BASE << flRegionBitShift, descriptorRegion.regionSection.flReg2.LIMIT << flRegionBitShift);
 	printf("Relocated GBe start block: %08x ; GBe end block: %08x\n", descriptorRegion.regionSection.flReg3.BASE << flRegionBitShift, descriptorRegion.regionSection.flReg3.LIMIT << flRegionBitShift);
 
+	// ----------------------------------------------------------------------------------------------------------------
 
+	// Convert the descriptor and gbe back to byte arrays, so that they
+	// can more easily be written to files:
+	// deblobbed descriptor region
 	char deblobbedDescriptorBuffer[DESCRIPTORREGIONSIZE];
 	memcpy(&deblobbedDescriptorBuffer, &descriptorRegion, DESCRIPTORREGIONSIZE);
 
+	// delete old file before continuing
 	remove(deblobbedDescriptorFilename);
+	// open new file for writing the deblobbed descriptor+gbe
 	fp = fopen(deblobbedDescriptorFilename, "ab");
 
+	// write the descriptor region into the first part
 	if (DESCRIPTORREGIONSIZE != fwrite(deblobbedDescriptorBuffer, sizeof(char), DESCRIPTORREGIONSIZE, fp))
 	{
 		printf("\nerror: writing descriptor region failed\n");
 		return 1;
 	}
 
+	// add gbe to the end of the file
 	if (GBEREGIONSIZE != fwrite(gbeBuffer, sizeof(char), GBEREGIONSIZE, fp))
 	{
 		printf("\nerror: writing GBe region failed\n");
@@ -173,17 +245,20 @@ int main(int argc, char *argv[])
 	// calculate the 0x3F'th 16-bit uint to make the desired final checksum for GBe
 	// observed values (from actual factory.rom dumps) 0xBABA 0x3ABA 0x34BA. spec defined as 0xBABA.
 	// theoretically, this could be any l33t speak variation of BABA, eg 3434 or BA34, and so on, but this is untested. so far.
-	// 40BA was also observed in another factory dump for another X200 - this is not l33t speak. 
+	// 40BA was also observed in another factory dump for another X200 - this is l33t speak for aoba, not baba.... 
 	// maybe only the 8 least significant bits are checked? or something deeper than that
+	// it will need to be tested if those gbe regions that use something other than baba
+	// will also work with the checksum changed to match baba (per datasheets)
 	unsigned short gbeCalculatedChecksum = GetChecksum(gbeBuffer, 0xBABA);
 	// get the actual 0x3F'th 16-bit uint that was already in the supplied (pre-compiled) region data
-	unsigned short gbeChecksum = GetRegionWord(0x3F, gbeBuffer);
+	unsigned short gbeChecksum = GetRegionWord(0x3F, gbeBuffer); // from the original factory.rom
 
 	printf("\ncalculated Gbe checksum: 0x%hx actual GBe checksum: 0x%hx\n", gbeCalculatedChecksum, gbeChecksum);
 
 	return 0;
 }
 
+// checksum calculation for gbe region (algorithm based on datasheet)
 unsigned short GetChecksum(char* regionData, unsigned short desiredValue)
 {
 	unsigned short regionWord;
@@ -197,7 +272,6 @@ unsigned short GetChecksum(char* regionData, unsigned short desiredValue)
 	checksum = desiredValue - checksum;
 	return checksum;
 }
-
 // Read a 16-bit unsigned int from a supplied region buffer
 unsigned short GetRegionWord(int index, char* regionData)
 {