linux-kernel - Re: [PATCH 06/10] PCI/DPC: Use FIELD

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Message-ID: <20231011120739.00005d7d@Huawei.com>
Date:   Wed, 11 Oct 2023 12:07:39 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     <linux-pci@...r.kernel.org>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        <linux-kernel@...r.kernel.org>, Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 06/10] PCI/DPC: Use FIELD_GET()

On Tue, 10 Oct 2023 15:44:32 -0500
Bjorn Helgaas <helgaas@...nel.org> wrote:

> From: Bjorn Helgaas <bhelgaas@...gle.com>
> 
> Use FIELD_GET() to remove dependences on the field position, i.e., the
> shift value.  No functional change intended.
> 
> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
A question about what 'rules' you are applying for using these macros
vs choosing not not do so. Personally I prefer using them even for
flag fields mostly because it makes the code more consistent and
the compiler should remove any unnecessary shifts that result.

> ---

> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> index eeec1d6f9023..a9fdc2e3f110 100644
> --- a/drivers/pci/quirks.c
> +++ b/drivers/pci/quirks.c
> @@ -6154,7 +6154,7 @@ static void dpc_log_size(struct pci_dev *dev)
>  	if (!(val & PCI_EXP_DPC_CAP_RP_EXT))

This is what I'm commenting on below.

>  		return;
>  
> -	if (!((val & PCI_EXP_DPC_RP_PIO_LOG_SIZE) >> 8)) {
> +	if (FIELD_GET(PCI_EXP_DPC_RP_PIO_LOG_SIZE, val) == 0) {

Why do this one and not the one just above?
In both cases extracting a field then comparing it to 0, I'm not sure
it makes sense to care if that field is 1 bit or multiple bit.

>  		pci_info(dev, "Overriding RP PIO Log Size to 4\n");
>  		dev->dpc_rp_log_size = 4;
>  	}