Subject: Re: [PATCH 1/2] powerpc: add 16K/64K pages support for the 44x PPC32 architectures.

From: Hollis Blanchard <[email-masked]>
Date: Mon, 03 Nov 2008 13:55:21 -0600

On Mon, 2008-11-03 at 11:43 +1100, Benjamin Herrenschmidt wrote:
> > Cropping the size of the memory node.  That was simplest to do from the
> > cuboot wrapper at the time.  If marking it reserved via a reserve map
> > is more elegant and correct, we could do that.
> > 
> > But I will still like to know what about the other way is hairy please.
> 
> I don't like it :-) Bad feeling ... don't like having a memory
> node entry that isn't aligned to some large power of two typically.

More specifically, mm/bootmem.c seems to be making the implicit
assumption that memory size is an even multiple of PAGE_SIZE. With 4K
pages, 0xffff000 bytes of RAM fits; with 64K pages it does not.

Using the device tree reserve map stuff does indeed seem to solve the
problem. However, I really don't understand the layering in
arch/powerpc/boot at all, so I'll just put this patch out here and
people can play with wrappers and prototypes all they want:

powerpc/4xx: work around CHIP11 errata in a more PAGE_SIZE-friendly way

The current CHIP11 errata truncates the device tree memory node, and assumes a
4K page size. This breaks kernels with non-4K PAGE_SIZE.

Instead, use a device tree memory reservation to reserve only the 256 bytes
actually affected by the errata, leaving the total memory size unaltered.

Signed-off-by: Hollis Blanchard <[email-masked]>

diff --git a/arch/powerpc/boot/4xx.c b/arch/powerpc/boot/4xx.c
--- a/arch/powerpc/boot/4xx.c
+++ b/arch/powerpc/boot/4xx.c
@@ -21,7 +21,7 @@
 #include "reg.h"
 #include "dcr.h"
 
-static unsigned long chip_11_errata(unsigned long memsize)
+static void chip_11_errata(unsigned long memsize)
 {
 	unsigned long pvr;
 
@@ -31,13 +31,11 @@ static unsigned long chip_11_errata(unsi
 		case 0x40000850:
 		case 0x400008d0:
 		case 0x200008d0:
-			memsize -= 4096;
+			fdt_reserve_mem(memsize - 256, 256);
 			break;
 		default:
 			break;
 	}
-
-	return memsize;
 }
 
 /* Read the 4xx SDRAM controller to get size of system memory. */
@@ -53,7 +51,7 @@ void ibm4xx_sdram_fixup_memsize(void)
 			memsize += SDRAM_CONFIG_BANK_SIZE(bank_config);
 	}
 
-	memsize = chip_11_errata(memsize);
+	chip_11_errata(memsize);
 	dt_fixup_memory(0, memsize);
 }
 
@@ -219,7 +217,7 @@ void ibm4xx_denali_fixup_memsize(void)
 		bank = 4; /* 4 banks */
 
 	memsize = cs * (1 << (col+row)) * bank * dpath;
-	memsize = chip_11_errata(memsize);
+	chip_11_errata(memsize);
 	dt_fixup_memory(0, memsize);
 }
 
diff --git a/arch/powerpc/boot/libfdt-wrapper.c b/arch/powerpc/boot/libfdt-wrapper.c
--- a/arch/powerpc/boot/libfdt-wrapper.c
+++ b/arch/powerpc/boot/libfdt-wrapper.c
@@ -167,6 +167,11 @@ static unsigned long fdt_wrapper_finaliz
 	return (unsigned long)fdt;
 }
 
+int fdt_reserve_mem(unsigned long addr, unsigned long bytes)
+{
+	return fdt_add_mem_rsv(fdt, addr, bytes);
+}
+
 void fdt_init(void *blob)
 {
 	int err;
diff --git a/arch/powerpc/boot/ops.h b/arch/powerpc/boot/ops.h
--- a/arch/powerpc/boot/ops.h
+++ b/arch/powerpc/boot/ops.h
@@ -83,6 +83,7 @@ extern struct loader_info loader_info;
 
 void start(void);
 void fdt_init(void *blob);
+int fdt_reserve_mem(unsigned long addr, unsigned long bytes);
 int serial_console_init(void);
 int ns16550_console_init(void *devp, struct serial_console_data *scdp);
 int mpsc_console_init(void *devp, struct serial_console_data *scdp);

-- 
Hollis Blanchard
IBM Linux Technology Center