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

From: "Arnd Bergmann" <arnd@arndb.de>
To: "Samuel Holland" <samuel.holland@sifive.com>,
	"Nathan Chancellor" <nathan@kernel.org>,
	"Conor.Dooley" <conor.dooley@microchip.com>
Cc: llvm@lists.linux.dev, "Leo Li" <sunpeng.li@amd.com>,
	"Pan Xinhui" <Xinhui.Pan@amd.com>,
	"Rodrigo Siqueira" <Rodrigo.Siqueira@amd.com>,
	linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	amd-gfx@lists.freedesktop.org,
	"Alex Deucher" <alexander.deucher@amd.com>,
	linux-riscv@lists.infradead.org,
	"Christian König" <christian.koenig@amd.com>
Subject: Re: [PATCH 3/3] drm/amd/display: Support DRM_AMD_DC_FP on RISC-V
Date: Sat, 09 Dec 2023 21:38:45 +0100	[thread overview]
Message-ID: <5090a015-5b6f-44be-bb25-d2ca3fdf5d40@app.fastmail.com> (raw)
In-Reply-To: <4f277982-fffb-4fe1-bc02-007633400f31@sifive.com>

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.

      Arnd