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

Florian Fainelli
2021-11-16T20:40:20Z

On 11/15/21 3:20 AM, 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.

The code is not necessarily any different on ARMv8 as far as I can tell,
before:

static void brcm_msi_set_regs(struct brcm_msi *msi)
{
u32 val = __GENMASK(31, msi->legacy_shift);
84: b9406402 ldr w2, [x0,#100]
88: d2800021 mov x1, #0x1
// #1
8c: 9ac22021 lsl x1, x1, x2
90: 4b0103e1 neg w1, w1

after:

static void brcm_msi_set_regs(struct brcm_msi *msi)
{
u32 val = ~(BIT(msi->legacy_shift) - 1);
84: b9406402 ldr w2, [x0,#100]
88: d2800021 mov x1, #0x1
// #1
8c: 9ac22021 lsl x1, x1, x2
90: 4b0103e1 neg w1, w1

and the usage of BIT() does not make this any clearer.

How about this instead:

diff --git a/drivers/pci/controller/pcie-brcmstb.c
b/drivers/pci/controller/pcie-brcmstb.c
index 1fc7bd49a7ad..f832c07337fa 100644
--- a/drivers/pci/controller/pcie-brcmstb.c
+++ b/drivers/pci/controller/pcie-brcmstb.c
@@ -144,6 +144,8 @@
#define BRCM_INT_PCI_MSI_NR 32
#define BRCM_INT_PCI_MSI_LEGACY_NR 8
#define BRCM_INT_PCI_MSI_SHIFT 0
+#define BRCM_INT_PCI_MSI_MASK GENMASK(BRCM_INT_PCI_MSI_NR - 1, 0)
+#define BRCM_INT_PCI_MSI_LEGACY_MASK GENMASK(31, 32 -
BRCM_INT_PCI_MSI_LEGACY_NR)

/* MSI target addresses */
#define BRCM_MSI_TARGET_ADDR_LT_4GB 0x0fffffffcULL
@@ -619,7 +621,8 @@ 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 = msi->legacy ? BRCM_INT_PCI_MSI_MASK :
+ BRCM_INT_PCI_MSI_LEGACY_MASK;

writel(val, msi->intr_base + MSI_INT_MASK_CLR);
writel(val, msi->intr_base + MSI_INT_CLR);

>
> 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);
>

--
Florian