Re: [PATCH] drm/amdgpu: avoid integer overflow warning in amdgpu_device_resize_fb_bar()

[Christian König]

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@xxxxxxxx>
> > >
> > > 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