On Thu, 28 Aug 2025 12:12:37 +0200
Anders Roxell <anders.roxell@xxxxxxxxxx> wrote:
> Commit cbc654d18d37 ("bitops: Add __attribute_const__ to generic
> ffs()-family implementations") causes a compilation failure on ARM
> footbridge_defconfig with gcc-8:
>
> FIELD_PREP: value too large for the field
>
> The error occurs in pcie_set_readrq() at:
> v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8);
>
> With __attribute_const__, gcc-8 now performs wrong compile-time
> validation in FIELD_PREP and cannot guarantee that ffs(rq) - 8 will
> always produce values that fit in the 3-bit PCI_EXP_DEVCTL_READRQ field.
Which is actually quite correct - in principle pcie_get_mps() could
return a small value.
What is probably happening is that two copies of the FIELD_PREP()
are being generated for ffs(rq) and ffs(mps).
The latter might be (mps ? asm_fun(mps) + 1 : 0) leading to an extra
copy for ffs(0) - which will cause the warning in FIELD_PREP.
An alternate fix you be to move the validation of rq below the
'performance' clamp.
David
>
> Avoid FIELD_PREP entirely by using direct bit manipulation. Replace
> FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8) with the equivalent
> manual bit operations: ((ffs(rq) - 8) << 12) & PCI_EXP_DEVCTL_READRQ.
>
> This bypasses the compile-time validation while maintaining identical
> runtime behavior and functionality.
>
> Fixes: cbc654d18d37 ("bitops: Add __attribute_const__ to generic ffs()-family implementations")
> Reported-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-pci/CA+G9fYuysVr6qT8bjF6f08WLyCJRG7aXAeSd2F7=zTaHHd7L+Q@xxxxxxxxxxxxxx/T/#u
> Signed-off-by: Anders Roxell <anders.roxell@xxxxxxxxxx>
> ---
> drivers/pci/pci.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index e698278229f2..9f9607bd9f51 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5893,7 +5893,8 @@ int pcie_set_readrq(struct pci_dev *dev, int rq)
> rq = mps;
> }
>
> - v = FIELD_PREP(PCI_EXP_DEVCTL_READRQ, ffs(rq) - 8);
> + /* Ideally we would used FIELD_PREP() but this is a work around for gcc-8 */
> + v = ((ffs(rq) - 8) << 12) & PCI_EXP_DEVCTL_READRQ;
>
> if (bridge->no_inc_mrrs) {
> int max_mrrs = pcie_get_readrq(dev);
