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

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Message-Id: <788147c6-defd-496a-8174-1571b73d1a71@app.fastmail.com>
Date:   Tue, 04 Jul 2023 14:24:09 +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 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?

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

     Arnd