linux-kernel - Re: [PATCH v10 2/6] PCI: Clean up __pci_find_next_cap

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Message-ID: <66cdb108-80f5-fe7c-329b-8c60caf55b64@linux.intel.com>
Date: Tue, 29 Apr 2025 18:17:57 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Hans Zhang <18255117159@....com>
cc: lpieralisi@...nel.org, bhelgaas@...gle.com, 
    manivannan.sadhasivam@...aro.org, kw@...ux.com, cassel@...nel.org, 
    robh@...nel.org, jingoohan1@...il.com, thomas.richard@...tlin.com, 
    linux-pci@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v10 2/6] PCI: Clean up __pci_find_next_cap_ttl()
 readability

On Tue, 29 Apr 2025, 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, uapi/linux/pci_regs.h).
> 
> The changes are purely non-functional cleanups, ensuring behavior remains
> identical to the original implementation.
> 
> Signed-off-by: Hans Zhang <18255117159@....com>
> ---
> Changes since v9:
> - None
> 
> Changes since v8:
> - Split into patch 1/6, patch 2/6.
> - The
>  drivers/pci/pci.c             | 10 ++++++----
>  include/uapi/linux/pci_regs.h |  2 ++
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 4d7c9f64ea24..1c29e8f20cb5 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>
> @@ -30,6 +31,7 @@
>  #include <asm/dma.h>
>  #include <linux/aer.h>
>  #include <linux/bitfield.h>
> +#include <uapi/linux/pci_regs.h>

linux/pci.h will pull this in through <uapi/linux/pci.h> so you don't need 
to add it (basically anywhere).

>  #include "pci.h"
>  
>  DEFINE_MUTEX(pci_slot_mutex);
> @@ -432,17 +434,17 @@ 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 ba326710f9c8..b59179e1210a 100644
> --- a/include/uapi/linux/pci_regs.h
> +++ b/include/uapi/linux/pci_regs.h
> @@ -206,6 +206,8 @@
>  /* 0x48-0x7f reserved */
>  
>  /* Capability lists */
> +#define PCI_CAP_ID_MASK		0x00ff
> +#define PCI_CAP_LIST_NEXT_MASK	0xff00

Consider adding a comment after the value as is done for most  defines in 
this file.

>  
>  #define PCI_CAP_LIST_ID		0	/* Capability ID */
>  #define  PCI_CAP_ID_PM		0x01	/* Power Management */
> 

-- 
 i.