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

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Message-ID: <87h6kpdj20.fsf@mail.lhotse>
Date:   Mon, 11 Dec 2023 23:23:35 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Samuel Holland <samuel.holland@...ive.com>,
        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>
Cc:     linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        linux-arch@...r.kernel.org,
        Samuel Holland <samuel.holland@...ive.com>
Subject: Re: [RFC PATCH 10/12] drm/amd/display: Use ARCH_HAS_KERNEL_FPU_SUPPORT

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.

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.

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