Re: [PATCH] drm/amd/display: Revert "add DCN support for aarch64"

From: Will Deacon <will@kernel.org>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: "Rob Herring" <robh@kernel.org>, "Leo Li" <sunpeng.li@amd.com>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	amd-gfx@lists.freedesktop.org,
	"Christian König" <christian.koenig@amd.com>,
	"David Airlie" <airlied@linux.ie>,
	dri-devel@lists.freedesktop.org,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Daniel Kolesa" <daniel@octaforge.org>,
	"Alex Deucher" <alexander.deucher@amd.com>,
	"Dave Martin" <dave.martin@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] drm/amd/display: Revert "add DCN support for aarch64"
Date: Mon, 14 Dec 2020 23:50:24 +0000	[thread overview]
Message-ID: <20201214235024.GB14575@willie-the-truck> (raw)
In-Reply-To: <20201214175225.38975-1-ardb@kernel.org>

On Mon, Dec 14, 2020 at 06:52:25PM +0100, Ard Biesheuvel wrote:
> This reverts commit c38d444e44badc557cf29fdfdfb823604890ccfa.
> 
> Simply disabling -mgeneral-regs-only left and right is risky, given that
> the standard AArch64 ABI permits the use of FP/SIMD registers anywhere,
> and GCC is known to use SIMD registers for spilling, and may invent
> other uses of the FP/SIMD register file that have nothing to do with the
> floating point code in question. Note that putting kernel_neon_begin()
> and kernel_neon_end() around the code that does use FP is not sufficient
> here, the problem is in all the other code that may be emitted with
> references to SIMD registers in it.
> 
> So the only way to do this properly is to put all floating point code in
> a separate compilation unit, and only compile that unit with
> -mgeneral-regs-only. But perhaps the use of floating point here is
> something that should be reconsidered entirely.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Dave Martin <dave.martin@arm.com>
> Cc: Rob Herring <robh@kernel.org>
> Cc: Leo Li <sunpeng.li@amd.com>
> Cc: Alex Deucher <alexander.deucher@amd.com>
> Cc: "Christian König" <christian.koenig@amd.com>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: Daniel Kolesa <daniel@octaforge.org>
> Cc: amd-gfx@lists.freedesktop.org
> Cc: dri-devel@lists.freedesktop.org
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  drivers/gpu/drm/amd/display/Kconfig                   |  2 +-
>  drivers/gpu/drm/amd/display/dc/calcs/Makefile         |  7 --
>  drivers/gpu/drm/amd/display/dc/clk_mgr/Makefile       |  7 --
>  drivers/gpu/drm/amd/display/dc/dcn10/Makefile         |  7 --
>  drivers/gpu/drm/amd/display/dc/dcn10/dcn10_resource.c | 81 ++++++++------------
>  drivers/gpu/drm/amd/display/dc/dcn20/Makefile         |  4 -
>  drivers/gpu/drm/amd/display/dc/dcn21/Makefile         |  4 -
>  drivers/gpu/drm/amd/display/dc/dml/Makefile           | 13 ----
>  drivers/gpu/drm/amd/display/dc/dsc/Makefile           |  5 --
>  drivers/gpu/drm/amd/display/dc/os_types.h             |  4 -
>  10 files changed, 32 insertions(+), 102 deletions(-)

I didn't notice we'd enabled this for arm64, but I agree with the reasoning
in the commit message, so:

Acked-by: Will Deacon <will@kernel.org>

The long and short of it is that it is not safe to compile kernel C code
without -mgeneral-regs-only on arm64.

Will

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