linux-kernel - Re: [PATCH] PCI: Remove redundant macro

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Message-ID: <20241213173701.GA3419486@bhelgaas>
Date: Fri, 13 Dec 2024 11:37:01 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: zhangdongdong@...incomputing.com
Cc: alex.williamson@...hat.com, bhelgaas@...gle.com, yishaih@...dia.com,
	avihaih@...dia.com, yi.l.liu@...el.com, ankita@...dia.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH] PCI: Remove redundant macro

On Fri, Dec 13, 2024 at 05:46:17PM +0800, zhangdongdong@...incomputing.com wrote:
> From: Dongdong Zhang <zhangdongdong@...incomputing.com>
> 
> Removed the duplicate macro definition PCI_VSEC_HDR from
> pci_regs.h to avoid redundancy. Updated the VFIO PCI code
> to use the existing `PCI_VNDR_HEADER` macro for consistency,
> ensuring minimal changes to the codebase.
> 
> Signed-off-by: Dongdong Zhang <zhangdongdong@...incomputing.com>
> ---
>  drivers/vfio/pci/vfio_pci_config.c | 3 ++-
>  include/uapi/linux/pci_regs.h      | 1 -
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
> index ea2745c1ac5e..c30748912ff1 100644
> --- a/drivers/vfio/pci/vfio_pci_config.c
> +++ b/drivers/vfio/pci/vfio_pci_config.c
> @@ -1389,7 +1389,8 @@ static int vfio_ext_cap_len(struct vfio_pci_core_device *vdev, u16 ecap, u16 epo
>  
>  	switch (ecap) {
>  	case PCI_EXT_CAP_ID_VNDR:
> -		ret = pci_read_config_dword(pdev, epos + PCI_VSEC_HDR, &dword);
> +		ret = pci_read_config_dword(pdev, epos + PCI_VNDR_HEADER,
> +					    &dword);
>  		if (ret)
>  			return pcibios_err_to_errno(ret);
>  
> diff --git a/include/uapi/linux/pci_regs.h b/include/uapi/linux/pci_regs.h
> index 1601c7ed5fab..7b6cad788de3 100644
> --- a/include/uapi/linux/pci_regs.h
> +++ b/include/uapi/linux/pci_regs.h
> @@ -1001,7 +1001,6 @@
>  #define PCI_ACS_CTRL		0x06	/* ACS Control Register */
>  #define PCI_ACS_EGRESS_CTL_V	0x08	/* ACS Egress Control Vector */
>  
> -#define PCI_VSEC_HDR		4	/* extended cap - vendor-specific */
>  #define  PCI_VSEC_HDR_LEN_SHIFT	20	/* shift for length field */

We should resolve the duplication of PCI_VSEC_HDR and PCI_VNDR_HEADER,
but I don't like the fact that we're left with this dangling
PCI_VSEC_HDR_LEN_SHIFT.

That leaves vfio using PCI_VNDR_HEADER and PCI_VSEC_HDR_LEN_SHIFT,
which don't match at all.

I think you should remove PCI_VSEC_HDR_LEN_SHIFT as well and change
vfio to use PCI_VNDR_HEADER_LEN() instead.

It's somewhat dicey removing things from pci_regs.h since it's in
include/uapi/, but this is such a niche thing we might be able to get
away with it.

Bjorn