linux-kernel - Re: [PATCH] drm/amdgpu: avoid integer overflow warning in amdgpu_device_resize_fb

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Date:   Tue, 04 Jul 2023 16:31:25 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     Christian König <christian.koenig@....com>,
        "Arnd Bergmann" <arnd@...nel.org>,
        "Alex Deucher" <alexander.deucher@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        "Dave Airlie" <airlied@...il.com>,
        "Daniel Vetter" <daniel@...ll.ch>
Cc:     "Hawking Zhang" <Hawking.Zhang@....com>,
        "Lijo Lazar" <lijo.lazar@....com>,
        "Mario Limonciello" <mario.limonciello@....com>,
        "YiPeng Chai" <YiPeng.Chai@....com>, "Le Ma" <le.ma@....com>,
        "Bokun Zhang" <Bokun.Zhang@....com>,
        "Srinivasan Shanmugam" <srinivasan.shanmugam@....com>,
        "Shiwu Zhang" <shiwu.zhang@....com>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/amdgpu: avoid integer overflow warning in
 amdgpu_device_resize_fb_bar()

On Tue, Jul 4, 2023, at 14:33, Christian König wrote:
> Am 04.07.23 um 14:24 schrieb Arnd Bergmann:
>> On Tue, Jul 4, 2023, at 08:54, Christian König wrote:
>>> Am 03.07.23 um 14:35 schrieb Arnd Bergmann:
>> Not sure I understand the specific requirement. Do you mean the entire
>> amdgpu driver requires 64-bit BAR addressing, or just the bits that
>> resize the BARs?
>
> Well a bit of both.
>
> Modern AMD GPUs have 16GiB of local memory (VRAM), making those 
> accessible to a CPU which can only handle 32bit addresses by resizing 
> the BAR is impossible to begin with.
>
> But going a step further even without resizing it is pretty hard to get 
> that hardware working on such an architecture.

I'd still like to understand this part better, as we have a lot of
arm64 chips with somewhat flawed PCIe implementations, often with
a tiny 64-bit memory space, but otherwise probably capable of
using a GPU.

What exactly do you expect to happen here?

a) Use only part of the VRAM but otherwise work as expected
b) Access all of the VRAM, but at a performance cost for
   bank switching?
c) Require kernel changes to make a) or b) work, otherwise
   fail to load
d) have no chance of working even with driver changes

>>> It might be cleaner to just not build the whole driver on such systems
>>> or at least leave out this function.
>> How about this version? This also addresses the build failure, but
>> I don't know if this makes any sense:
>>
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
>> @@ -1325,6 +1325,9 @@ int amdgpu_device_resize_fb_bar(struct amdgpu_device *adev)
>>          u16 cmd;
>>          int r;
>>   
>> +       if (!IS_ENABLED(CONFIG_PHYS_ADDR_T_64BIT))
>> +               return 0;
>> +
>
> Yes, if that suppresses the warning as well then that makes perfect 
> sense to me.

Ok, I'll send that as a v2 then.

    Arnd