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Subject: Re: [PATCH V2 1/4] PCI/portdrv: Don't disable device during shutdown
To:     Huacai Chen <chenhuacai@gmail.com>,
        Bjorn Helgaas <helgaas@kernel.org>, huangshuai@loongson.cn
Cc:     Huacai Chen <chenhuacai@loongson.cn>,
        Bjorn Helgaas <bhelgaas@google.com>,
        linux-pci <linux-pci@vger.kernel.org>,
        Xuefeng Li <lixuefeng@loongson.cn>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Tiezhu Yang <yangtiezhu@loongson.cn>
References: <20210528071503.1444680-2-chenhuacai@loongson.cn>
 <20210528214309.GA1523480@bjorn-Precision-5520>
 <CAAhV-H4paNzoF4tEJd1_Z2VgBr64t3evfjdmrrA3CZMw=AXrGw@mail.gmail.com>
From:   Sinan Kaya <okaya@kernel.org>
Message-ID: <21b1495f-689f-2d75-d896-2a33c03b186b@kernel.org>
Date:   Mon, 7 Jun 2021 23:43:05 +0300
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On 6/4/2021 12:24 PM, Huacai Chen wrote:
>> So you need to explain why we need to allow DMA from those devices
>> even after we shutdown the port.  "It makes reboot work" is not a
>> sufficient explanation.
> I think only the designer of LS7A can tell us why. So, Mr. Shuai
> Huang, could you please explain this?
> 

Could there be some kind of a shutdown/init problem on your graphics
card driver?

During shutdown, remove() callback of all endpoints get called. This is
the opportunity for your graphics driver to put hardware into safe
state.

If there is a problem with the hardware/driver, it should be a quirk as
opposed to changing the default safe behavior for all devices.

The port driver here prevents memory from getting corrupted by rogue
hardware. There is a window during kexec where hardware can write to
system memory addresses if IOVA addresses and system memory addresses
overlap.