linux-kernel - Re: [v8] misc: pci_endpoint_test: Fix overflow of bar

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Message-ID: <Z36IJ6ql09I_dO98@ryzen>
Date: Wed, 8 Jan 2025 15:13:59 +0100
From: Niklas Cassel <cassel@...nel.org>
To: Arnd Bergmann <arnd@...db.de>
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
Subject: Re: [v8] misc: pci_endpoint_test: Fix overflow of bar_size

On Wed, Jan 08, 2025 at 03:10:03PM +0100, Arnd Bergmann wrote:
> On Tue, Jan 7, 2025, at 16:57, Niklas Cassel wrote:
> > On Tue, Jan 07, 2025 at 11:44:21PM +0800, Hans Zhang wrote:
> >> On 2025/1/7 19:47, Niklas Cassel wrote:
> >> 
> >> Hi Niklas,
> >> 
> >> > The error:
> >> > drivers/misc/pci_endpoint_test.c:315: undefined reference to `__udivmoddi4'
> >> > sounds like the compiler is using a specialized instruction to do both div
> >> > and mod in one. By removing the mod in patch 1/2, I expect that patch 2/2
> >> > will no longer get this error.
> >> 
> >> The __udivmoddi4 may be the way div and mod are combined.
> >> 
> >> Delete remain's patch 1/2 according to your suggestion. I compiled it as a
> >> KO module for an experiment.
> >> 
> >> There are still __udivdi3 errors, so the do_div API must be used.
> >
> > Ok. Looking at do_div(), it seems to be the correct API to use
> > for this problem. Just change bar_size type to u64 (instead of casting)
> > and use do_div() ? That is how it is seems to be used in other drivers.
> 
> I think using div_u64_rem() instead of do_div() would make this
> more readable as this is always an inline function, so the type can
> remain resource_size_t, and the division gets optimized well when
> that is a 32-bit type.

After patch 1/2, we no longer care about the remainder, so I guess
div64_u64() is the correct function to use then?


Kind regards,
Niklas