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

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Message-ID: <f1b190cb-3af7-178b-baeb-b59363868779@amd.com>
Date:   Tue, 4 Jul 2023 14:33:57 +0200
From:   Christian König <christian.koenig@....com>
To:     Arnd Bergmann <arnd@...db.de>, 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()

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:
>>> From: Arnd Bergmann <arnd@...db.de>
>>>
>>> On 32-bit architectures comparing a resource against a value larger than
>>> U32_MAX can cause a warning:
>>>
>>> drivers/gpu/drm/amd/amdgpu/amdgpu_device.c:1344:18: error: result of comparison of constant 4294967296 with expression of type 'resource_size_t' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
>>>                       res->start > 0x100000000ull)
>>>                       ~~~~~~~~~~ ^ ~~~~~~~~~~~~~~
>>>
>>> The compiler is right that this cannot happen in this configuration, which
>>> is ok, so just add a cast to shut up the warning.
>> Well it doesn't make sense to compile that driver on systems with only
>> 32bit phys_addr_t in the first place.
> 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.

>> 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.

Regards,
Christian.

>          /* Bypass for VF */
>          if (amdgpu_sriov_vf(adev))
>                  return 0;
>
>       Arnd