Re: [PATCH 1/1] PCI: tegra194: fix min() signedness when capping ASPM L1 entrance latency

[David Laight]

On Tue, 7 Apr 2026 20:27:49 +0530
Manikanta Maddireddy <mmaddireddy@xxxxxxxxxx> wrote:

> The DT property "aspm-l1-entry-delay-ns" is converted to microseconds,
> then encoded for the L1 entrance latency register field as ilog2(us) + 1,
> clamped to the hardware maximum of 7.
> 
> ilog2() returns int type, while the upper bound is 7U (unsigned int).
> The min() macro is implemented with __careful_cmp(), which rejects mixed
> signed and unsigned operands at compile time via BUILD_BUG_ON_MSG in
> minmax.h; that check trips on this pair, notably when building with W=1.
> 
> This combination fails to build (e.g. parisc allyesconfig, GCC 15, as
> reported by the 0-day bot).
> 
> Use min_t(u32, ilog2(us) + 1U, 7U) so both sides of the comparison are
> unsigned and consistent with aspm_l1_enter_lat.

Adding 1U (rather than 1) is enough to make everything signed.
Alternatively change the 7U to 7 and it will all be fine regardless of
whether ilog2() returns a signed or unsigned result.

Remember min_t(u32, x, y) is min((u32)x, (u32)y) and you wouldn't put in
casts like that for any other arithmetic operation.

Note that for the compile to fail there has to be a code path where
ilog2(us) isn't known to generate a non-negative value.
ilog2(us) (probably) ends up as 'fls(us) - 1'. If that is implemented using a
compiler builtin (because there is a single instruction) then gcc knows that
the input can't be zero (from the max()), so knows that fls() can't return 0
(which it does for 0), so knows it is never negative and the checks in min()
pass.

parisc may be one of the architectures that ends up with a real function
for fls() so the compiler doesn't know the result of ilog2() is
non-negative.

Just delete the U.

	David

> 
> Fixes: 4a44cd65c9dd ("PCI: tegra194: Use aspm-l1-entry-delay-ns DT property for L1 entrance latency")
> Signed-off-by: Manikanta Maddireddy <mmaddireddy@xxxxxxxxxx>
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202604051407.AODe3ddZ-lkp@xxxxxxxxx/
> ---
>  drivers/pci/controller/dwc/pcie-tegra194.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-tegra194.c b/drivers/pci/controller/dwc/pcie-tegra194.c
> index 393f75ce3df3..93d3452ac117 100644
> --- a/drivers/pci/controller/dwc/pcie-tegra194.c
> +++ b/drivers/pci/controller/dwc/pcie-tegra194.c
> @@ -1147,7 +1147,7 @@ static int tegra_pcie_dw_parse_dt(struct tegra_pcie_dw *pcie)
>  	if (!ret) {
>  		u32 us = max(val / 1000, 1U);
>  
> -		pcie->aspm_l1_enter_lat = min(ilog2(us) + 1, 7U);
> +		pcie->aspm_l1_enter_lat = min_t(u32, ilog2(us) + 1U, 7U);
>  	}
>  
>  	ret = of_property_read_u32(np, "num-lanes", &pcie->num_lanes);