linux-kernel - Re: [PATCH v15 1/6] PCI: Clean up __pci_find_next_cap

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Message-ID: <353d1dc7e3c4e9b5f127ac9177a863d8a8cde39f.camel@linux.ibm.com>
Date: Wed, 20 Aug 2025 11:19:07 +0200
From: Gerd Bayer <gbayer@...ux.ibm.com>
To: Hans Zhang <18255117159@....com>, lpieralisi@...nel.org,
        kwilczynski@...nel.org, bhelgaas@...gle.com, helgaas@...nel.org,
        jingoohan1@...il.com, mani@...nel.org
Cc: robh@...nel.org, ilpo.jarvinen@...ux.intel.com, schnelle@...ux.ibm.com,
        lukas@...ner.de, arnd@...nel.org, geert@...ux-m68k.org,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v15 1/6] PCI: Clean up __pci_find_next_cap_ttl()
 readability

On Wed, 2025-08-13 at 22:45 +0800, Hans Zhang wrote:
> Refactor the __pci_find_next_cap_ttl() to improve code clarity:
> - Replace magic number 0x40 with PCI_STD_HEADER_SIZEOF.
> - Use ALIGN_DOWN() for position alignment instead of manual bitmask.
> - Extract PCI capability fields via FIELD_GET() with standardized masks.
> - Add necessary headers (linux/align.h).
> 
> No functional changes intended.
> 
> Signed-off-by: Hans Zhang <18255117159@....com>
> Acked-by: Manivannan Sadhasivam <mani@...nel.org>
> ---
>  drivers/pci/pci.c             | 9 +++++----
>  include/uapi/linux/pci_regs.h | 3 +++
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index b0f4d98036cd..40a5c87d9a6b 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -9,6 +9,7 @@
>   */
>  
>  #include <linux/acpi.h>
> +#include <linux/align.h>
>  #include <linux/kernel.h>
>  #include <linux/delay.h>
>  #include <linux/dmi.h>
> @@ -432,17 +433,17 @@ static u8 __pci_find_next_cap_ttl(struct pci_bus *bus, unsigned int devfn,
>  	pci_bus_read_config_byte(bus, devfn, pos, &pos);
>  
>  	while ((*ttl)--) {
> -		if (pos < 0x40)
> +		if (pos < PCI_STD_HEADER_SIZEOF)
>  			break;
> -		pos &= ~3;
> +		pos = ALIGN_DOWN(pos, 4);
>  		pci_bus_read_config_word(bus, devfn, pos, &ent);
>  
> -		id = ent & 0xff;
> +		id = FIELD_GET(PCI_CAP_ID_MASK, ent);
>  		if (id == 0xff)
>  			break;
>  		if (id == cap)
>  			return pos;
> -		pos = (ent >> 8);
> +		pos = FIELD_GET(PCI_CAP_LIST_NEXT_MASK, ent);
>  	}
>  	return 0;
>  }
> diff --git a/include/uapi/linux/pci_regs.h b/include/uapi/linux/pci_regs.h
> index f5b17745de60..1bba99b46227 100644
> --- a/include/uapi/linux/pci_regs.h
> +++ b/include/uapi/linux/pci_regs.h
> @@ -207,6 +207,9 @@
>  
>  /* Capability lists */
>  
> +#define PCI_CAP_ID_MASK		0x00ff	/* Capability ID mask */
> +#define PCI_CAP_LIST_NEXT_MASK	0xff00	/* Next Capability Pointer mask */
> +
>  #define PCI_CAP_LIST_ID		0	/* Capability ID */
>  #define  PCI_CAP_ID_PM		0x01	/* Power Management */
>  #define  PCI_CAP_ID_AGP		0x02	/* Accelerated Graphics Port */

Hi Hans,

I like your approach to replace the magic numbers here. If you went
further to replace the single pci_bus_read_config_word() with two
single-byte reads at the appropriate places - for CAP_ID and
CAP_LIST_NEXT - you could even go with the already existing offset
defines PCI_CAP_LIST_ID and PCI_CAP_LIST_NEXT from pci_regs.h.

But that might be a more intricate change and involves more HW accesses
than what it's worth.

So feel free to add my
Reviewed-by: Gerd Bayer <gbayer@...ux.ibm.com>

Thanks,
Gerd