linux-kernel - Re: [PATCH] ARM: Do not select ARCH_HAS_KERNEL_FPU

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Message-ID: <c65023fd-5f35-40f0-947d-377088919af3@sifive.com>
Date: Tue, 14 May 2024 17:48:38 -0500
From: Samuel Holland <samuel.holland@...ive.com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Thiago Jung Bauermann <thiago.bauermann@...aro.org>,
 kernel test robot <lkp@...el.com>, Andrew Davis <afd@...com>,
 Arnd Bergmann <arnd@...db.de>, Christian König
 <christian.koenig@....com>, Eric DeVolder <eric.devolder@...cle.com>,
 Rob Herring <robh@...nel.org>, Russell King <linux@...linux.org.uk>,
 Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] ARM: Do not select ARCH_HAS_KERNEL_FPU_SUPPORT

Hello Ard,

On 2024-05-09 2:39 AM, Ard Biesheuvel wrote:
> On Thu, 9 May 2024 at 03:37, Samuel Holland <samuel.holland@...ive.com> wrote:
>>
>> On 32-bit ARM, conversions between `double` and `long long` require
>> runtime library support. Since the kernel does not currently provide
>> this library support, the amdgpu driver fails to build:
>>
>>   ERROR: modpost: "__aeabi_l2d" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
>>   ERROR: modpost: "__aeabi_d2ulz" [drivers/gpu/drm/amd/amdgpu/amdgpu.ko] undefined!
>>
>> As Arnd reports, there are likely no 32-bit ARM platforms which can use
>> the amdgpu driver anyway, due to lack of features like 64-bit
>> prefetchable BARs. Since amdgpu is currently the only real user of
>> ARCH_HAS_KERNEL_FPU_SUPPORT, drop support for this option instead of
>> bothering to implement the library functions.
>>
>> Fixes: 12624fe2d707 ("ARM: implement ARCH_HAS_KERNEL_FPU_SUPPORT")
> 
> This commit is not in mainline yet. Could we just drop the original
> patch instead?

No, like I mentioned in the original thread, later patches in the series (for
example bbce5cac4f5a ("lib/raid6: use CC_FLAGS_FPU for NEON CFLAGS")) depend on
CC_FLAGS_FPU being defined for both arm and arm64.

arm can't select ARCH_HAS_KERNEL_FPU_SUPPORT because the contract of that option
is not fully implemented, but in my opinion it doesn't hurt to keep the part
that is implemented.

Regards,
Samuel

>> Reported-by: Thiago Jung Bauermann <thiago.bauermann@...aro.org>
>> Closes: https://lore.kernel.org/lkml/87wmp4oo3y.fsf@linaro.org/
>> Reported-by: kernel test robot <lkp@...el.com>
>> Closes: https://lore.kernel.org/oe-kbuild-all/202404042327.jRpt81kP-lkp@intel.com/
>> Suggested-by: Ard Biesheuvel <ardb@...nel.org>
>> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
>> ---
>>
>>  arch/arm/Kconfig | 1 -
>>  1 file changed, 1 deletion(-)
>>
>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>> index b1751c2cab87..b14aed3a17ab 100644
>> --- a/arch/arm/Kconfig
>> +++ b/arch/arm/Kconfig
>> @@ -15,7 +15,6 @@ config ARM
>>         select ARCH_HAS_FORTIFY_SOURCE
>>         select ARCH_HAS_KEEPINITRD
>>         select ARCH_HAS_KCOV
>> -       select ARCH_HAS_KERNEL_FPU_SUPPORT if KERNEL_MODE_NEON
>>         select ARCH_HAS_MEMBARRIER_SYNC_CORE
>>         select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>>         select ARCH_HAS_PTE_SPECIAL if ARM_LPAE
>> --
>> 2.44.0
>>