Re: [PATCH v2 1/1] PCI: brcmstb: Use BIT() as __GENMASK() is for internal use only

Robin Murphy
2021-11-15T14:00:46Z

On 2021-11-15 11:20, Andy Shevchenko wrote:
> Use BIT() as __GENMASK() is for internal use only. The rationale
> of switching to BIT() is to provide better generated code. The
> GENMASK() against non-constant numbers may produce an ugly assembler
> code. On contrary the BIT() is simply converted to corresponding shift
> operation.

FWIW, If you care about code quality and want the compiler to do the
obvious thing, why not specify it as the obvious thing:

u32 val = ~0 << msi->legacy_shift;

Personally I don't think that abusing BIT() in the context of setting
multiple bits is any better than abusing __GENMASK()...

Robin.

> Note, it's the only user of __GENMASK() in the kernel outside of its own realm.
>
> Fixes: 3baec684a531 ("PCI: brcmstb: Accommodate MSI for older chips")
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
> v2: switched to BIT() and elaborated why, hence not included tag
> drivers/pci/controller/pcie-brcmstb.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/pci/controller/pcie-brcmstb.c b/drivers/pci/controller/pcie-brcmstb.c
> index 1fc7bd49a7ad..0c49fc65792c 100644
> --- a/drivers/pci/controller/pcie-brcmstb.c
> +++ b/drivers/pci/controller/pcie-brcmstb.c
> @@ -619,7 +619,7 @@ static void brcm_msi_remove(struct brcm_pcie *pcie)
>
> static void brcm_msi_set_regs(struct brcm_msi *msi)
> {
> - u32 val = __GENMASK(31, msi->legacy_shift);
> + u32 val = ~(BIT(msi->legacy_shift) - 1);
>
> writel(val, msi->intr_base + MSI_INT_MASK_CLR);
> writel(val, msi->intr_base + MSI_INT_CLR);