Re: PCI<->PCI bridges, transparent resource fix

[Ivan Kokshaysky <ink@jurassic.park.msu.ru>]

On Wed, Aug 07, 2002 at 12:30:25PM -0600, Grant Grundler wrote:
> Send me a patch for 2.4.19 and I'll try it on the laptop.

Appended - please do.

> Ivan wrote:
> | ...subtractive decoding bridge _MUST_ have bit 0 in the ProgIf set to 1.
> 
> It sounds easy to check at the top and in that case DTRT.
> The "else" parts of later resource checks can go away.

Exactly.

> What you suggest implies the bridge waits for someone else to "claim"
> the transaction and I'm not convinced PCI spec would allow that.

It allows that as a matter of fact.

> Performance would certainly suffer if that were the case.

Sure, performance sucks, and there are other bad side effects,
like impossibility of the peer-to-peer DMA behind such bridge.

Ivan.

--- linux/drivers/pci/pci.c~	Fri Jun 28 14:46:21 2002
+++ linux/drivers/pci/pci.c	Thu Aug  8 14:57:25 2002
@@ -1073,6 +1073,14 @@ void __devinit pci_read_bridge_bases(str
 	if (!dev)		/* It's a host bus, nothing to read */
 		return;
 
+	if (dev->class & 1) {
+		printk("Subtractive decoding bridge %s -"
+			" assuming transparent\n", dev->name);
+		for(i = 0; i < 3; i++)
+			child->resource[i] = child->parent->resource[i];
+		return;
+	}
+
 	for(i=0; i<3; i++)
 		child->resource[i] = &dev->resource[PCI_BRIDGE_RESOURCES+i];
 
@@ -1095,13 +1103,6 @@ void __devinit pci_read_bridge_bases(str
 		res->start = base;
 		res->end = limit + 0xfff;
 		res->name = child->name;
-	} else {
-		/*
-		 * Ugh. We don't know enough about this bridge. Just assume
-		 * that it's entirely transparent.
-		 */
-		printk(KERN_ERR "Unknown bridge resource %d: assuming transparent\n", 0);
-		child->resource[0] = child->parent->resource[0];
 	}
 
 	res = child->resource[1];
@@ -1114,10 +1115,6 @@ void __devinit pci_read_bridge_bases(str
 		res->start = base;
 		res->end = limit + 0xfffff;
 		res->name = child->name;
-	} else {
-		/* See comment above. Same thing */
-		printk(KERN_ERR "Unknown bridge resource %d: assuming transparent\n", 1);
-		child->resource[1] = child->parent->resource[1];
 	}
 
 	res = child->resource[2];
@@ -1145,10 +1142,6 @@ void __devinit pci_read_bridge_bases(str
 		res->start = base;
 		res->end = limit + 0xfffff;
 		res->name = child->name;
-	} else {
-		/* See comments above */
-		printk(KERN_ERR "Unknown bridge resource %d: assuming transparent\n", 2);
-		child->resource[2] = child->parent->resource[2];
 	}
 }