Re: [PATCH] drm/[amdgpu|radeon]: fix memset on io mem

["Christian König" <christian.koenig@amd.com>]

Am 17.12.20 um 02:07 schrieb Chen Li:
> On Wed, 16 Dec 2020 22:19:11 +0800,
> Christian König wrote:
>> Am 16.12.20 um 14:48 schrieb Chen Li:
>>> On Wed, 16 Dec 2020 15:59:37 +0800,
>>> Christian König wrote:
>>>> [SNIP]
>>> Hi, Christian. I'm not sure why this change is a hack here. I cannot see the problem and wll be grateful if you give more explainations.
>> __memset is supposed to work on those addresses, otherwise you can't use the
>> e8860 on your arm64 system.
> If __memset is supposed to work on those adresses, why this commit(https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Ftorvalds%2Flinux%2Fcommit%2Fba0b2275a6781b2f3919d931d63329b5548f6d5f&data=04%7C01%7Cchristian.koenig%40amd.com%7C4ed3c075888746b7f41408d8a22811c5%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637437640274023350%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=HhWxUaLo3WpzoV6hjV%2BG1HICaIOXwsoNpzv5tNMNg8A%3D&reserved=0) is needed? (I also notice drm/radeon didn't take this change though) just out of curiosity.

We generally accept those patches as cleanup in the kernel with the hope 
that we can find a way to work around the userspace restrictions.

But when you also have this issue in userspace then there isn't much we 
can do for you.

>> Replacing the the direct write in the kernel with calls to writel() or
>> memset_io() will fix that temporary, but you have a more general problem here.
>   
> I cannot see what's the more general problem here :( u mean performance?

No, not performance. See standards like OpenGL, Vulkan as well as VA-API 
and VDPAU require that you can mmap() device memory and execute 
memset/memcpy on the memory from userspace.

If your ARM base board can't do that for some then you can't use the 
hardware with that board.

>>>> For amdgpu I suggest that we allocate the UVD message in GTT instead of VRAM
>>>> since we don't have the hardware restriction for that on the new generations.
>>>>
>>> Thanks, I will try to dig into deeper. But what's the "hardware restriction" meaning here? I'm not familiar with video driver stack and amd gpu, sorry.
>> On older hardware (AGP days) the buffer had to be in VRAM (MMIO) memory, but on
>> modern system GTT (system memory) works as well.
> IIUC, e8860 can use amdgpu(I use radeon now) beause its device id 6822 is in amdgpu's table. But I cannot tell whether e8860 has iommu, and I cannot find iommu from lspci, so graphics translation table may not work here?

That is not related to IOMMU. IOMMU is a feature of the CPU/motherboard. 
This is implemented using GTT, e.g. the VM page tables inside the GPU.

And yes it should work I will prepare a patch for it.

>>>> BTW: How does userspace work on arm64 then? The driver stack usually only works
>>>> if mmio can be mapped directly.
>>> I also post two usespace issue on mesa, and you may be interested with them:
>>>    https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.freedesktop.org%2Fmesa%2Fmesa%2F-%2Fissues%2F3954&data=04%7C01%7Cchristian.koenig%40amd.com%7C4ed3c075888746b7f41408d8a22811c5%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637437640274023350%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=ZR7pDS%2BCLUuMjCeKcMAXfHtbczt8WdUwSeLZCuHfCHw%3D&reserved=0
>>>    https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.freedesktop.org%2Fmesa%2Fmesa%2F-%2Fissues%2F3951&data=04%7C01%7Cchristian.koenig%40amd.com%7C4ed3c075888746b7f41408d8a22811c5%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637437640274033344%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=jAJo3aG2I1oIDTZXWhNgcKoKbd6tTdiAtc7vE4hJJPY%3D&reserved=0
>>> I paste some virtual memory map in userspace there. (and the two problems do bother me quite a long time.)
>> I don't really see a solution for those problems.
>>
>> See it is perfectly valid for an application to memset/memcpy on mmaped MMIO
>> space which comes from OpenGL or Vulkan.
>>
>> So your CPU simply won't work with the hardware. We could work around that with
>> a couple of hacks, but this is a pretty much general problem.
>>
>> Regards,
>> Christian.
>   
> Thanks! Can you provid some details about these hacks? Should I post another issue on the mail list?

Adjust the kernel and/or user space to never map VRAM to the CPU.

This violates the OpenGL/Vulkan specification in some ways. So not sure 
if that will work or not.

Regards,
Christian.
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