linux-kernel - Re: [PATCH v2] staging: vme_user: simplify bus number allocation using ffz()

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Message-ID: <20251207175018.43b29806@pumpkin>
Date: Sun, 7 Dec 2025 17:50:18 +0000
From: David Laight <david.laight.linux@...il.com>
To: Dharanitharan R <dharanitharan725@...il.com>
Cc: gregkh@...uxfoundation.org, linux-staging@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] staging: vme_user: simplify bus number allocation
 using ffz()

On Sun,  7 Dec 2025 16:43:17 +0000
Dharanitharan R <dharanitharan725@...il.com> wrote:

> David Laight pointed out that the loop in vme_register_bridge()
> is effectively searching for the first zero bit in vme_bus_numbers.
> 
> Replace the manual loop with ffz() and use BIT(n) for consistency.
> 
> Suggested-by: David Laight <david.laight.linux@...il.com>
> Signed-off-by: Dharanitharan R <dharanitharan725@...il.com>
> ---
>  drivers/staging/vme_user/vme.c | 15 +++++++++++++----------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/staging/vme_user/vme.c b/drivers/staging/vme_user/vme.c
> index 2095de72596a..45381e677475 100644
> --- a/drivers/staging/vme_user/vme.c
> +++ b/drivers/staging/vme_user/vme.c
> @@ -1765,16 +1765,19 @@ int vme_register_bridge(struct vme_bridge *bridge)
>  	int ret = -1;
>  
>  	mutex_lock(&vme_buses_lock);
> -	for (i = 0; i < sizeof(vme_bus_numbers) * 8; i++){
> -		if ((vme_bus_numbers & (1 << i)) == 0) {
> -			vme_bus_numbers |= (1 << i);
> -			bridge->num = i;
> -			INIT_LIST_HEAD(&bridge->devices);
> -			list_add_tail(&bridge->bus_list, &vme_bus_list);
> -			ret = 0;
> -			break;
> -		}
> +	
> +	/* Find the first clear bit */
> +	i = ffz(vme_bus_numbers);

You need to check for ~0u first:

 * ffz - find first zero in word.
 * @word: The word to search
 *
 * Undefined if no zero exists, so code should check against ~0UL first.

	David

> +
> +	/* Ensure we have a free slot */
> +	if (i < BITS_PER_LONG) {
> +		vme_bus_numbers |= BIT(i);
> +        bridge->num = i;
> +        INIT_LIST_HEAD(&bridge->devices);
> +        list_add_tail(&bridge->bus_list, &vme_bus_list);
> +        ret = 0;
> +	}
>  	mutex_unlock(&vme_buses_lock);
>  
>  	return ret;
> @@ -1787,7 +1790,7 @@ void vme_unregister_bridge(struct vme_bridge *bridge)
>  	struct vme_dev *tmp;
>  
>  	mutex_lock(&vme_buses_lock);
> -	vme_bus_numbers &= ~(1 << bridge->num);
> +	vme_bus_numbers &= ~BIT(bridge->num);
>  	list_for_each_entry_safe(vdev, tmp, &bridge->devices, bridge_list) {
>  		list_del(&vdev->drv_list);
>  		list_del(&vdev->bridge_list);