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Subject: Re: [PATCH] iommu/mediatek: Use totalram_pages to setup enable_4GB
To:     Miles Chen <miles.chen@mediatek.com>
Cc:     Joerg Roedel <joro@8bytes.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        iommu@lists.linux-foundation.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org,
        wsd_upstream@mediatek.com, Yong Wu <yong.wu@mediatek.com>,
        Chao Hao <chao.hao@mediatek.com>, yingjoe.chen@mediatek.com
References: <20200604080120.2628-1-miles.chen@mediatek.com>
 <55820901-430b-14c4-9426-7a4991ca0eed@redhat.com>
 <1591264174.12661.17.camel@mtkswgap22>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <f02c8c9d-ed75-6513-f8a9-a2fdbb11b750@redhat.com>
Date:   Thu, 4 Jun 2020 13:32:40 +0200
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On 04.06.20 11:49, Miles Chen wrote:
> On Thu, 2020-06-04 at 10:25 +0200, David Hildenbrand wrote:
>> On 04.06.20 10:01, Miles Chen wrote:
>>> 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 <rppt@linux.ibm.com>
>>> Signed-off-by: Miles Chen <miles.chen@mediatek.com>
>>> Cc: David Hildenbrand <david@redhat.com>
>>> Cc: Yong Wu <yong.wu@mediatek.com>
>>> Cc: Chao Hao <chao.hao@mediatek.com>
>>> ---
>>>  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:
> 

Just a thought: If memory hotplug is applicable as well, you might
either want to always assume data->enable_4GB, or handle memory hotplug
events from the memory notifier, when new memory gets onlined (not sure
how tricky that is).


-- 
Thanks,

David / dhildenb