linux-kernel - Re: [RFC PATCH 10/12] drm/amd/display: Use ARCH_HAS_KERNEL_FPU

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Message-ID: <7ed20fcf-8a9d-40d5-b913-b5d2da443cd6@sifive.com>
Date:   Wed, 13 Dec 2023 19:03:20 -0600
From:   Samuel Holland <samuel.holland@...ive.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        loongarch@...ts.linux.dev, linuxppc-dev@...ts.ozlabs.org,
        x86@...nel.org, linux-riscv@...ts.infradead.org,
        Christoph Hellwig <hch@...radead.org>,
        Timothy Pearson <tpearson@...torengineering.com>
Subject: Re: [RFC PATCH 10/12] drm/amd/display: Use
 ARCH_HAS_KERNEL_FPU_SUPPORT

On 2023-12-11 6:23 AM, Michael Ellerman wrote:
> Hi Samuel,
> 
> Thanks for trying to clean all this up.
> 
> One problem below.
> 
> Samuel Holland <samuel.holland@...ive.com> writes:
>> Now that all previously-supported architectures select
>> ARCH_HAS_KERNEL_FPU_SUPPORT, this code can depend on that symbol instead
>> of the existing list of architectures. It can also take advantage of the
>> common kernel-mode FPU API and method of adjusting CFLAGS.
>>
>> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> ...
>> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c b/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c
>> index 4ae4720535a5..b64f917174ca 100644
>> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c
>> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c
>> @@ -87,20 +78,9 @@ void dc_fpu_begin(const char *function_name, const int line)
>>  	WARN_ON_ONCE(!in_task());
>>  	preempt_disable();
>>  	depth = __this_cpu_inc_return(fpu_recursion_depth);
>> -
>>  	if (depth == 1) {
>> -#if defined(CONFIG_X86) || defined(CONFIG_LOONGARCH)
>> +		BUG_ON(!kernel_fpu_available());
>>  		kernel_fpu_begin();
>> -#elif defined(CONFIG_PPC64)
>> -		if (cpu_has_feature(CPU_FTR_VSX_COMP))
>> -			enable_kernel_vsx();
>> -		else if (cpu_has_feature(CPU_FTR_ALTIVEC_COMP))
>> -			enable_kernel_altivec();
>  
> Note altivec.
> 
>> -		else if (!cpu_has_feature(CPU_FTR_FPU_UNAVAILABLE))
>> -			enable_kernel_fp();
>> -#elif defined(CONFIG_ARM64)
>> -		kernel_neon_begin();
>> -#endif
>>  	}
>>  
>>  	TRACE_DCN_FPU(true, function_name, line, depth);
>> diff --git a/drivers/gpu/drm/amd/display/dc/dml/Makefile b/drivers/gpu/drm/amd/display/dc/dml/Makefile
>> index ea7d60f9a9b4..5aad0f572ba3 100644
>> --- a/drivers/gpu/drm/amd/display/dc/dml/Makefile
>> +++ b/drivers/gpu/drm/amd/display/dc/dml/Makefile
>> @@ -25,40 +25,8 @@
>>  # It provides the general basic services required by other DAL
>>  # subcomponents.
>>  
>> -ifdef CONFIG_X86
>> -dml_ccflags-$(CONFIG_CC_IS_GCC) := -mhard-float
>> -dml_ccflags := $(dml_ccflags-y) -msse
>> -endif
>> -
>> -ifdef CONFIG_PPC64
>> -dml_ccflags := -mhard-float -maltivec
>> -endif
> 
> And altivec is enabled in the flags there.
> 
> That doesn't match your implementation for powerpc in patch 7, which
> only deals with float.
> 
> I suspect the AMD driver actually doesn't need altivec enabled, but I
> don't know that for sure. It compiles without it, but I don't have a GPU
> to actually test. I've added Timothy on Cc who added the support for
> powerpc to the driver originally, hopefully he has a test system.

I tested this series on a POWER9 system with an AMD Radeon RX 6400 GPU (which
requires this FPU code to initialize), and got functioning graphics output.

> Anyway if that's true that it doesn't need altivec we should probably do
> a lead-up patch that drops altivec from the AMD driver explicitly, eg.
> as below.

That makes sense to me. Do you want to provide your Signed-off-by so I can send
this patch with your authorship?

Regards,
Samuel

> cheers
> 
> 
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c b/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c
> index 4ae4720535a5..0de16796466b 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/dc_fpu.c
> @@ -92,11 +92,7 @@ void dc_fpu_begin(const char *function_name, const int line)
>  #if defined(CONFIG_X86) || defined(CONFIG_LOONGARCH)
>  		kernel_fpu_begin();
>  #elif defined(CONFIG_PPC64)
> -		if (cpu_has_feature(CPU_FTR_VSX_COMP))
> -			enable_kernel_vsx();
> -		else if (cpu_has_feature(CPU_FTR_ALTIVEC_COMP))
> -			enable_kernel_altivec();
> -		else if (!cpu_has_feature(CPU_FTR_FPU_UNAVAILABLE))
> +		if (!cpu_has_feature(CPU_FTR_FPU_UNAVAILABLE))
>  			enable_kernel_fp();
>  #elif defined(CONFIG_ARM64)
>  		kernel_neon_begin();
> @@ -125,11 +121,7 @@ void dc_fpu_end(const char *function_name, const int line)
>  #if defined(CONFIG_X86) || defined(CONFIG_LOONGARCH)
>  		kernel_fpu_end();
>  #elif defined(CONFIG_PPC64)
> -		if (cpu_has_feature(CPU_FTR_VSX_COMP))
> -			disable_kernel_vsx();
> -		else if (cpu_has_feature(CPU_FTR_ALTIVEC_COMP))
> -			disable_kernel_altivec();
> -		else if (!cpu_has_feature(CPU_FTR_FPU_UNAVAILABLE))
> +		if (!cpu_has_feature(CPU_FTR_FPU_UNAVAILABLE))
>  			disable_kernel_fp();
>  #elif defined(CONFIG_ARM64)
>  		kernel_neon_end();
> diff --git a/drivers/gpu/drm/amd/display/dc/dml/Makefile b/drivers/gpu/drm/amd/display/dc/dml/Makefile
> index 6042a5a6a44f..554c39024a40 100644
> --- a/drivers/gpu/drm/amd/display/dc/dml/Makefile
> +++ b/drivers/gpu/drm/amd/display/dc/dml/Makefile
> @@ -31,7 +31,7 @@ dml_ccflags := $(dml_ccflags-y) -msse
>  endif
>  
>  ifdef CONFIG_PPC64
> -dml_ccflags := -mhard-float -maltivec
> +dml_ccflags := -mhard-float
>  endif
>  
>  ifdef CONFIG_ARM64
> diff --git a/drivers/gpu/drm/amd/display/dc/dml2/Makefile b/drivers/gpu/drm/amd/display/dc/dml2/Makefile
> index acff3449b8d7..7b51364084b5 100644
> --- a/drivers/gpu/drm/amd/display/dc/dml2/Makefile
> +++ b/drivers/gpu/drm/amd/display/dc/dml2/Makefile
> @@ -30,7 +30,7 @@ dml2_ccflags := $(dml2_ccflags-y) -msse
>  endif
>  
>  ifdef CONFIG_PPC64
> -dml2_ccflags := -mhard-float -maltivec
> +dml2_ccflags := -mhard-float
>  endif
>  
>  ifdef CONFIG_ARM64