linux-kernel - Re: [PATCH v2 1/1] PCI: brcmstb: Use BIT() as _

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Message-ID: <41b85802-6118-33a6-692a-043d74b82d8e@gmail.com>
Date:   Tue, 16 Nov 2021 12:38:39 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        bcm-kernel-feedback-list@...adcom.com,
        linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Jim Quinlan <jim2101024@...il.com>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2 1/1] PCI: brcmstb: Use BIT() as __GENMASK() is for
 internal use only

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@...ux.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