On Thu, 2020-06-04 at 10:25 +0200, David Hildenbrand wrote:

On 04.06.20 10:01, Miles Chen wrote:

quoted

To build this driver as a kernel module, we cannot use
the unexported symbol "max_pfn" to setup enable_4GB.

Use totalram_pages() instead to setup enable_4GB.

Suggested-by: Mike Rapoport <redacted>
Signed-off-by: Miles Chen <redacted>
Cc: David Hildenbrand <redacted>
Cc: Yong Wu <yong.wu@mediatek.com>
Cc: Chao Hao <redacted>
---
 drivers/iommu/mtk_iommu.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/iommu/mtk_iommu.c b/drivers/iommu/mtk_iommu.c
index 5f4d6df59cf6..c2798a6e0e38 100644
--- a/drivers/iommu/mtk_iommu.c
+++ b/drivers/iommu/mtk_iommu.c

@@ -3,7 +3,6 @@
  * Copyright (c) 2015-2016 MediaTek Inc.
  * Author: Yong Wu <yong.wu@mediatek.com>
  */
-#include <linux/memblock.h>
 #include <linux/bug.h>
 #include <linux/clk.h>
 #include <linux/component.h>

@@ -626,8 +625,8 @@ static int mtk_iommu_probe(struct platform_device *pdev)
 		return -ENOMEM;
 	data->protect_base = ALIGN(virt_to_phys(protect), MTK_PROTECT_PA_ALIGN);
 
-	/* Whether the current dram is over 4GB */
-	data->enable_4GB = !!(max_pfn > (BIT_ULL(32) >> PAGE_SHIFT));
+	/* Whether the current dram is over 4GB, note: DRAM start at 1GB  */
+	data->enable_4GB = !!(totalram_pages() > ((SZ_2G + SZ_1G) >> PAGE_SHIFT));

A similar thing seems to be done by
drivers/media/platform/mtk-vpu/mtk_vpu.c:
	vpu->enable_4GB = !!(totalram_pages() > (SZ_2G >> PAGE_SHIFT));

I do wonder if some weird memory hotplug setups might give you false
negatives.

E.g., start a VM with 1GB and hotplug 1GB - it will be hotplugged on
x86-64 above 4GB, turning max_pfn into 5GB. totalram_pages() should
return something < 2GB.

Same can happen when you have a VM and use ballooning to fake-unplug
memory, making totalram_pages() return something < 4GB, but leaving
usable pfns >= 4GB

Yes. Yingjoe also told me that this patch is not correct.

Thanks for pointing this out. totalram_pages() does not work 
for some cases:

e.g., DRAM start @0x4000_0000 and DRAM size is 0x1_0000_0000 but we
reserve large amount of memory, which makes totalram_pages() < 3GB but
it is possible to allocate a pfn >= 4GB.

I will discuss this internally.

Miles

.

but
... I don't know if I understood what "enable_4GB" needs/implies
... I don't know if this is applicable to VMs
 at all (on real HW such
    memory hotplug setups should not exist)
... I don't know how this code would react to memory hotplug, so if the
    condition changes after the driver loaded and enable_4GB would
    suddenly apply. Again, most probably not relevant on real HW, only
    for VMs.

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