Re: [PATCH v2] PCI: Batch BAR sizing operations

Alex Williamson
2025-02-10T16:36:47Z

On Sun, 9 Feb 2025 16:45:12 +0100
Oleg Nesterov <oleg@redhat.com> wrote:

> On 01/20, Alex Williamson wrote:
> >
> > static void pci_read_bases(struct pci_dev *dev, unsigned int howmany, int rom)
> > {
> > + u32 rombar, stdbars[PCI_STD_NUM_BARS];
> > unsigned int pos, reg;
> > + u16 orig_cmd;
> > +
> > + BUILD_BUG_ON(howmany > PCI_STD_NUM_BARS);
>
> FYI, I can't build the kernel after this patch:
>
> $ make drivers/pci/probe.o
> UPD include/config/kernel.release
> UPD include/generated/utsrelease.h
> CALL scripts/checksyscalls.sh
> DESCEND objtool
> INSTALL libsubcmd_headers
> CC drivers/pci/probe.o
> In file included from <command-line>:0:0:
> drivers/pci/probe.c: In function ‘pci_read_bases’:
> ././include/linux/compiler_types.h:542:38: error: call to ‘__compiletime_assert_338’ declared with attribute error: BUILD_BUG_ON failed: howmany > PCI_STD_NUM_BARS
> _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> ^
> ././include/linux/compiler_types.h:523:4: note: in definition of macro ‘__compiletime_assert’
> prefix ## suffix(); \
> ^
> ././include/linux/compiler_types.h:542:2: note: in expansion of macro ‘_compiletime_assert’
> _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
> ^
> ./include/linux/build_bug.h:39:37: note: in expansion of macro ‘compiletime_assert’
> #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
> ^
> ./include/linux/build_bug.h:50:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
> BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
> ^
> drivers/pci/probe.c:348:2: note: in expansion of macro ‘BUILD_BUG_ON’
> BUILD_BUG_ON(howmany > PCI_STD_NUM_BARS);
>
> Yes, my gcc version 5.3.1 is very old, but according to Documentation/process/changes.rst
> it should still be supported, the minimal version is 5.1.

While I try to setup an environment with an old gcc, can we do something
with __builtin_constant_p() to only evaluate this on versions of gcc
that can determine howmany is constant? Ex.

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index b6536ed599c3..c70cb480125e 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -345,7 +345,8 @@ static void pci_read_bases(struct pci_dev *dev, unsigned int howmany, int rom)
unsigned int pos, reg;
u16 orig_cmd;

- BUILD_BUG_ON(howmany > PCI_STD_NUM_BARS);
+ if (__builtin_constant_p(howmany))
+ BUILD_BUG_ON(howmany > PCI_STD_NUM_BARS);

if (dev->non_compliant_bars)
return;

I welcome other suggestions too. Thanks,

Alex