linux-kernel - Re: [PATCH 3/3] drm/amd/display: Support DRM_AMD_DC

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Date:   Thu, 7 Dec 2023 22:49:53 -0600
From:   Samuel Holland <samuel.holland@...ive.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Harry Wentland <harry.wentland@....com>,
        Leo Li <sunpeng.li@....com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, David Airlie <airlied@...il.com>,
        Christian König <christian.koenig@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Pan Xinhui <Xinhui.Pan@....com>,
        Daniel Vetter <daniel@...ll.ch>, amd-gfx@...ts.freedesktop.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>,
        linux-kbuild@...r.kernel.org, Josh Poimboeuf <jpoimboe@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/3] drm/amd/display: Support DRM_AMD_DC_FP on RISC-V

Hi Christoph,

On 2023-11-22 2:40 AM, Christoph Hellwig wrote:
>> -	select DRM_AMD_DC_FP if (X86 || LOONGARCH || (PPC64 && ALTIVEC) || (ARM64 && KERNEL_MODE_NEON && !CC_IS_CLANG))
>> +	select DRM_AMD_DC_FP if ARM64 && KERNEL_MODE_NEON && !CC_IS_CLANG
>> +	select DRM_AMD_DC_FP if PPC64 && ALTIVEC
>> +	select DRM_AMD_DC_FP if RISCV && FPU
>> +	select DRM_AMD_DC_FP if LOONGARCH || X86
> 
> This really is a mess.  Can you add a ARCH_HAS_KERNEL_FPU_SUPPORT
> symbol that all architetures that have it select instead, and them
> make DRM_AMD_DC_FP depend on it?

Yes, I have done this for v2, which I will send shortly.

>> -#if defined(CONFIG_X86) || defined(CONFIG_LOONGARCH)
>> +#if defined(CONFIG_X86) || defined(CONFIG_LOONGARCH) || defined(CONFIG_RISCV)
>>  		kernel_fpu_begin();
>>  #elif defined(CONFIG_PPC64)
>>  		if (cpu_has_feature(CPU_FTR_VSX_COMP))
>> @@ -122,7 +124,7 @@ void dc_fpu_end(const char *function_name, const int line)
>>  
>>  	depth = __this_cpu_dec_return(fpu_recursion_depth);
>>  	if (depth == 0) {
>> -#if defined(CONFIG_X86) || defined(CONFIG_LOONGARCH)
>> +#if defined(CONFIG_X86) || defined(CONFIG_LOONGARCH) || defined(CONFIG_RISCV)
>>  		kernel_fpu_end();
>>  #elif defined(CONFIG_PPC64)
>>  		if (cpu_has_feature(CPU_FTR_VSX_COMP))
> 
> And then this mess can go away.  We'll need to decide if we want to
> cover all the in-kernel vector support as part of it, which would
> seem reasonable to me, or have a separate generic kernel_vector_begin
> with it's own option.

I think we may want to keep vector separate for performance on architectures
with separate FP and vector register files. For now, I have limited my changes
to FPU support only, which means I have removed VSX/Altivec from here; the
AMDGPU code doesn't need Altivec anyway.

>> diff --git a/drivers/gpu/drm/amd/display/dc/dml/Makefile b/drivers/gpu/drm/amd/display/dc/dml/Makefile
>> index ea7d60f9a9b4..5c8f840ef323 100644
>> --- a/drivers/gpu/drm/amd/display/dc/dml/Makefile
>> +++ b/drivers/gpu/drm/amd/display/dc/dml/Makefile
>> @@ -43,6 +43,12 @@ dml_ccflags := -mfpu=64
>>  dml_rcflags := -msoft-float
>>  endif
>>  
>> +ifdef CONFIG_RISCV
>> +include $(srctree)/arch/riscv/Makefile.isa
>> +# Remove V from the ISA string, like in arch/riscv/Makefile, but keep F and D.
>> +dml_ccflags := -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)([^v_]*)v?/\1\2/')
>> +endif
>> +
>>  ifdef CONFIG_CC_IS_GCC
>>  ifneq ($(call gcc-min-version, 70100),y)
>>  IS_OLD_GCC = 1
> 
> And this is again not really something we should be doing.
> Instead we need a generic way in Kconfig to enable FPU support
> for an object file or set of, that the arch support can hook
> into.

I've included this in v2 as well.

> Btw, I'm also really worried about folks using the FPU instructions
> outside the kernel_fpu_begin/end windows in general (not directly
> related to the RISC-V support).  Can we have objecttool checks
> for that similar to only allowing the unsafe uaccess in the
> uaccess begin/end pairs?

ARM partially enforces this at compile time: it disallows calling
kernel_neon_begin() inside a translation unit that has NEON enabled. That
doesn't prevent the programmer from calling a FPU-enabled function from outside
a begin/end section, but it does prevent the compiler from generating unexpected
FPU usage behind your back. I implemented this same functionality for RISC-V.

Actually tracking all possibly-FPU-tainted functions and their call sites is
probably possible, but a much larger task.

Regards,
Samuel