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

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Date:   Mon, 15 Nov 2021 13:59:11 +0000
From:   Robin Murphy <robin.murphy@....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 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@...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);
>