linux-kernel - Re: [v11 2/2] misc: pci_endpoint_test: Fix overflow of bar

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Message-ID: <72aa3800-320c-494b-ae46-b4f2ebf71e92@nvidia.com>
Date: Wed, 22 Jan 2025 11:18:44 +0000
From: Jon Hunter <jonathanh@...dia.com>
To: Arnd Bergmann <arnd@...db.de>, Niklas Cassel <cassel@...nel.org>
Cc: Hans Zhang <18255117159@....com>,
 Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
 Krzysztof Wilczyński <kw@...ux.com>,
 Kishon Vijay Abraham I <kishon@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>, linux-pci@...r.kernel.org,
 linux-kernel@...r.kernel.org, rockswang7@...il.com,
 "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [v11 2/2] misc: pci_endpoint_test: Fix overflow of bar_size


On 22/01/2025 10:26, Arnd Bergmann wrote:
> On Wed, Jan 22, 2025, at 11:07, Niklas Cassel wrote:
>> On Tue, Jan 21, 2025 at 05:46:43PM +0000, Jon Hunter wrote:
>>>> diff --git a/drivers/misc/pci_endpoint_test.c b/drivers/misc/>>
>>>
>>> This change breaks building the kernel with GCC v7 and I see ...
>>>
>>> ERROR: modpost: "__aeabi_uldivmod" [drivers/misc/pci_endpoint_test.ko]
>>> undefined!
>>> ERROR: modpost: "__aeabi_ldivmod" [drivers/misc/pci_endpoint_test.ko]
>>> undefined!
>>>
>>> I know that this is an old GCC version, but this is a farm builder and the
>>> kernel still indicates that GCC v5.1 is still supported [0].
>>
>> do you have any idea what is going on here?
>>
>> I'm a bit puzzled, since looking at other reports of this error,
>> e.g.:
>> https://lore.kernel.org/all/20241018151016.3496613-1-arnd@kernel.org/
>>
>> using div_u64() is usually the solution for this problem (for things that
>> are not performance critical), not the cause of it... any ideas?
> 
> I have tried to look at the email thread, but not tried to reproduce
> it yet. I have two ideas about what might be happening:
> 
> a) something causes a /different/ division to call into
>     __aeabi_uldivmod(), not the one from div_u64().
> 
> b) the compiler notices one of the arguments to div_u64() being
>     constant in some cases and splits the calling function into
>     two special cases, for both the constant and non-constant
>     cases. This sometimes confuses the __builtin_constant_p()
>     in do_div() that decides to fall back to a 32-bit division.
> 
> Try looking at the .s file when you run
> 'make drivers/misc/pci_endpoint_test.s' to see where exactly it
> calls the two division functions, maybe you see the problem then,
> otherwise I can try to reproduce it here.


FWIW I have encountered similar problems with this before with this 
compiler ...

https://lore.kernel.org/linux-tegra/7733a4ca-330b-4127-af12-33f376fbbc47@nvidia.com/

Jon

-- 
nvpublic