[PATCH] drivers/pci: Fix FIELD_PREP compilation error with gcc-8

[Anders Roxell <anders.roxell@linaro.org>]

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.

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@linaro.org>
Closes: https://lore.kernel.org/linux-pci/CA+G9fYuysVr6qT8bjF6f08WLyCJRG7aXAeSd2F7=zTaHHd7L+Q@mail.gmail.com/T/#u
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
---
 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);
-- 
2.50.1