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@xxxxxxxxxx/ > > 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. Arnd
