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From: Ard Biesheuvel <ardb@kernel.org>
Date: Thu, 11 Apr 2024 09:15:43 +0200
Message-ID: <CAMj1kXGW5XQxXrYaPhT6sCjH7s0EwqzNjWies3b8UWnUBW5Ngw@mail.gmail.com>
Subject: Re: [PATCH v4 13/15] drm/amd/display: Use ARCH_HAS_KERNEL_FPU_SUPPORT
To: Samuel Holland <samuel.holland@sifive.com>, Arnd Bergmann <arnd@arndb.de>
Cc: Thiago Jung Bauermann <thiago.bauermann@linaro.org>, Andrew Morton <akpm@linux-foundation.org>, 
	linux-arm-kernel@lists.infradead.org, x86@kernel.org, 
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, 
	linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, 
	Christoph Hellwig <hch@lst.de>, loongarch@lists.linux.dev, amd-gfx@lists.freedesktop.org, 
	Alex Deucher <alexander.deucher@amd.com>
Content-Type: text/plain; charset="UTF-8"

(cc Arnd)

On Thu, 11 Apr 2024 at 03:11, Samuel Holland <samuel.holland@sifive.com> wrote:
>
> Hi Thiago,
>
> On 2024-04-10 8:02 PM, Thiago Jung Bauermann wrote:
> > Samuel Holland <samuel.holland@sifive.com> writes:
> >> On 2024-04-10 5:21 PM, Thiago Jung Bauermann wrote:
> >>>
> >>> Unfortunately this patch causes build failures on arm with allyesconfig
> >>> and allmodconfig. Tested with next-20240410.
> >
> > <snip>
> >
> >> In both cases, the issue is that the toolchain requires runtime support to
> >> convert between `unsigned long long` and `double`, even when hardware FP is
> >> enabled. There was some past discussion about GCC inlining some of these
> >> conversions[1], but that did not get implemented.
> >
> > Thank you for the explanation and the bugzilla reference. I added a
> > comment there mentioning that the problem came up again with this patch
> > series.
> >
> >> The short-term fix would be to drop the `select ARCH_HAS_KERNEL_FPU_SUPPORT` for
> >> 32-bit arm until we can provide these runtime library functions.
> >
> > Does this mean that patch 2 in this series:
> >
> > [PATCH v4 02/15] ARM: Implement ARCH_HAS_KERNEL_FPU_SUPPORT
> >
> > will be dropped?
>
> No, because later patches in the series (3, 6) depend on the definition of
> CC_FLAGS_FPU from that patch. I will need to send a fixup patch unless I can
> find a GPL-2 compatible implementation of the runtime library functions.
>

Is there really a point to doing that? Do 32-bit ARM systems even have
enough address space to the map the BARs of the AMD GPUs that need
this support?

Given that this was not enabled before, I don't think the upshot of
this series should be that we enable support for something on 32-bit
ARM that may cause headaches down the road without any benefit.

So I'd prefer a fixup patch that opts ARM out of this over adding
support code for 64-bit conversions.