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From:   Alex Deucher <alexdeucher@gmail.com>
Date:   Mon, 11 Dec 2023 10:17:57 -0500
Message-ID: <CADnq5_O+ozkwQAEn3K_=-pB2L0+pbc+tbPU0CEwOTa+QysogAg@mail.gmail.com>
Subject: Re: [PATCH 3/3] drm/amd/display: Support DRM_AMD_DC_FP on RISC-V
To:     Samuel Holland <samuel.holland@sifive.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Nathan Chancellor <nathan@kernel.org>,
        "Conor.Dooley" <conor.dooley@microchip.com>, 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,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>
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On Sun, Dec 10, 2023 at 5:10=E2=80=AFAM Samuel Holland
<samuel.holland@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 str=
ing
> >>>>> 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_v=
ba_32.c:58:13: warning: stack frame size (2416) exceeds limit (2048) in 'DI=
SPCLKDPPCLKDCFCLKDeepSleepPrefetchParametersWatermarksAndPerformanceCalcula=
tion' [-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 comm=
it
> >>> 6740ec97bcdb ("drm/amd/display: Increase frame warning limit with KAS=
AN
> >>> or KCSAN in dml2")?
> >>>
> >>> I am not really interested in playing whack-a-mole with these warning=
s
> >>> 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_recalc=
ulate'
> >> [-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, l=
ike 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/72ada8603e36291ad91e4f4=
0f10ef742ef79bc4e

Alex