[PATCH 3/3] drm/amd/display: Support DRM_AMD_DC_FP on RISC-V

[Samuel Holland]

Hi Alex,

On 2023-12-11 9:17 AM, Alex Deucher wrote:
> On Sun, Dec 10, 2023 at 5:10 AM Samuel Holland
> <samuel.holland at sifive.com> wrote:
>>
>> Hi Arnd,
>>
>> On 2023-12-09 2:38 PM, Arnd Bergmann wrote:
>>> On Fri, Dec 8, 2023, at 06:04, Samuel Holland wrote:
>>>> On 2023-11-29 6:42 PM, Nathan Chancellor wrote:
>>>>> On Thu, Nov 23, 2023 at 02:23:01PM +0000, Conor Dooley wrote:
>>>>>> On Tue, Nov 21, 2023 at 07:05:15PM -0800, Samuel Holland wrote:
>>>>>>> RISC-V uses kernel_fpu_begin()/kernel_fpu_end() like several other
>>>>>>> architectures. Enabling hardware FP requires overriding the ISA string
>>>>>>> for the relevant compilation units.
>>>>>>
>>>>>> Ah yes, bringing the joy of frame-larger-than warnings to RISC-V:
>>>>>> ../drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn32/display_mode_vba_32.c:58:13: warning: stack frame size (2416) exceeds limit (2048) in 'DISPCLKDPPCLKDCFCLKDeepSleepPrefetchParametersWatermarksAndPerformanceCalculation' [-Wframe-larger-than]
>>>>>
>>>>> :(
>>>>>
>>>>>> Nathan, have you given up on these being sorted out?
>>>>>
>>>>> Does your configuration have KASAN (I don't think RISC-V supports
>>>>> KCSAN)? It is possible that dml/dcn32 needs something similar to commit
>>>>> 6740ec97bcdb ("drm/amd/display: Increase frame warning limit with KASAN
>>>>> or KCSAN in dml2")?
>>>>>
>>>>> I am not really interested in playing whack-a-mole with these warnings
>>>>> like I have done in the past for the reasons I outlined here:
>>>>>
>>>>> https://lore.kernel.org/20231019205117.GA839902@dev-arch.thelio-3990X/
>>>>
>>>> I also see one of these with clang 17 even with KASAN disabled:
>>>>
>>>> drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn32/display_mode_vba_32.c:37:6:
>>>> warning: stack frame size (2208) exceeds limit (2048) in 'dml32_recalculate'
>>>> [-Wframe-larger-than]
>>>> void dml32_recalculate(struct display_mode_lib *mode_lib)
>>>>
>>>>      ^
>>>> 1532/2208 (69.38%) spills, 676/2208 (30.62%) variables
>>>>
>>>> So I'm in favor of just raising the limit for these files for clang, like you
>>>> suggested in the linked thread.
>>>
>>> How about just adding a BUG_ON(IS_ENABLED(CONFIG_RISCV))
>>> in that function? That should also avoid the build failure
>>> but give a better indication of where the problem is
>>> if someone actually runs into that function and triggers
>>> a runtime stack overflow.
>>
>> Won't that break actual users of the driver, trading an unlikely but
>> theoretically possible stack overflow for a guaranteed crash? The intent of this
>> series is that I have one of these GPUs plugged in to a RISC-V board, and I want
>> to use it.
> 
> Does this patch address the issue?
> https://gitlab.freedesktop.org/agd5f/linux/-/commit/72ada8603e36291ad91e4f40f10ef742ef79bc4e

No, I get the warning without any of these debugging options enabled. I can
reproduce with just defconfig + CONFIG_DRM_AMDGPU=m when built with clang 17.

Regards,
Samuel