linux-kernel - [PATCH V2] x86/gart: Check for GART support before accessing GART registers

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Date:	Tue, 7 Apr 2015 16:46:37 -0500
From:	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>
To:	<mingo@...hat.com>, <bp@...e.de>, <linux-kernel@...r.kernel.org>
CC:	<Suravee.Suthikulpanit@....com>, Joerg Rodel <joro@...tes.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, x86-ml <x86@...nel.org>
Subject: [PATCH V2] x86/gart: Check for GART support before accessing GART registers

GART registers are not present in newer processors (Fam15h, Model 10h
and later). So, avoid accesses to GART registers in PCI config
space by returning early in early_gart_iommu_check() and
gart_iommu_hole_init() if GART is not available.

Current code doesn't break on existing processors but there are some
side effects:

We get bogus AGP aperture messages which are simply noise on
GART-less processors:

  AGP: Node 0: aperture [bus addr 0x00000000-0x01ffffff] (32MB)
  AGP: Your BIOS doesn't leave aperture memory hole
  AGP: Please enable the IOMMU option in the BIOS setup
  AGP: This costs you 64MB of RAM
  AGP: Mapping aperture over RAM [mem 0xd4000000-0xd7ffffff]

We can avoid calling allocate_aperture() and would not have to
wastefully reserve 64MB of RAM with memblock_reserve(). Also, we can
avoid having to loop through all PCI buses and devices twice, searching
for a non-existent AGP bridge if we bail out early.

Refactoring the family check used in amd_nb.c into an inline function
so we can use it here as well as in amd_nb.c

Fix some typos while at it.
Tested the patch on Fam10h and Fam15h Model 00h-fh and this code
runs fine. On Fam15h Model 60h-6fh and on Fam16h, we bail early
as they don't have GART.

Signed-off-by: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>
Reviewed-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Cc: Joerg Rodel <joro@...tes.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>
Cc: x86-ml <x86@...nel.org>
---
Changes in V2:
 - move amd_gart_present() before any other checks as it's least intrusive (per Boris)
 - fix typos in code and commit message (per Ingo)
 - minor change to subject line (per Boris' change)
 - Link to V1:http://marc.info/?l=linux-kernel&m=142792069809730&w=2

 arch/x86/include/asm/amd_nb.h | 11 +++++++++++
 arch/x86/kernel/amd_nb.c      |  4 +---
 arch/x86/kernel/aperture_64.c |  8 +++++++-
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/amd_nb.h b/arch/x86/include/asm/amd_nb.h
index aaac3b2..1a5da2e 100644
--- a/arch/x86/include/asm/amd_nb.h
+++ b/arch/x86/include/asm/amd_nb.h
@@ -98,11 +98,22 @@ static inline u16 amd_get_node_id(struct pci_dev *pdev)
 	return 0;
 }
 
+static inline bool amd_gart_present(void)
+{
+	/* GART present only on Fam15h, upto model 0fh */
+	if (boot_cpu_data.x86 == 0xf || boot_cpu_data.x86 == 0x10 ||
+	    (boot_cpu_data.x86 == 0x15 && boot_cpu_data.x86_model < 0x10))
+		return true;
+
+	return false;
+}
+
 #else
 
 #define amd_nb_num(x)		0
 #define amd_nb_has_feature(x)	false
 #define node_to_amd_nb(x)	NULL
+#define amd_gart_present(x)	false
 
 #endif
 
diff --git a/arch/x86/kernel/amd_nb.c b/arch/x86/kernel/amd_nb.c
index 5caed1d..29fa475 100644
--- a/arch/x86/kernel/amd_nb.c
+++ b/arch/x86/kernel/amd_nb.c
@@ -89,9 +89,7 @@ int amd_cache_northbridges(void)
 			next_northbridge(link, amd_nb_link_ids);
 	}
 
-	/* GART present only on Fam15h upto model 0fh */
-	if (boot_cpu_data.x86 == 0xf || boot_cpu_data.x86 == 0x10 ||
-	    (boot_cpu_data.x86 == 0x15 && boot_cpu_data.x86_model < 0x10))
+	if (amd_gart_present())
 		amd_northbridges.flags |= AMD_NB_GART;
 
 	/*
diff --git a/arch/x86/kernel/aperture_64.c b/arch/x86/kernel/aperture_64.c
index 76164e1..5c3e9b56 100644
--- a/arch/x86/kernel/aperture_64.c
+++ b/arch/x86/kernel/aperture_64.c
@@ -262,6 +262,9 @@ void __init early_gart_iommu_check(void)
 	u64 aper_base = 0, last_aper_base = 0;
 	int aper_enabled = 0, last_aper_enabled = 0, last_valid = 0;
 
+	if (!amd_gart_present())
+		return;
+
 	if (!early_pci_allowed())
 		return;
 
@@ -355,6 +358,9 @@ int __init gart_iommu_hole_init(void)
 	int fix, slot, valid_agp = 0;
 	int i, node;
 
+	if (!amd_gart_present())
+		return -ENODEV;
+
 	if (gart_iommu_aperture_disabled || !fix_aperture ||
 	    !early_pci_allowed())
 		return -ENODEV;
@@ -452,7 +458,7 @@ out:
 		   force_iommu ||
 		   valid_agp ||
 		   fallback_aper_force) {
-		pr_info("Your BIOS doesn't leave a aperture memory hole\n");
+		pr_info("Your BIOS doesn't leave aperture memory hole\n");
 		pr_info("Please enable the IOMMU option in the BIOS setup\n");
 		pr_info("This costs you %dMB of RAM\n",
 			32 << fallback_aper_order);
-- 
1.9.1

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