Re: [PATCH v4 07/12] PCI: endpoint: pci-epf-vntb: Implement db_vector_count/mask for doorbells

[Frank Li]

On Wed, May 13, 2026 at 11:49:18AM +0900, Koichiro Den wrote:
> Implement .db_vector_count and .db_vector_mask so ntb core/clients can map
> doorbell events to per-vector work and avoid the thundering-herd behavior.
>
> pci-epf-vntb reserves two slots in db_count: slot 0 for link events and
> slot 1 which is historically unused. Therefore the number of doorbell
> vectors is (db_count - 2).
>
> Report vectors as 0..N-1 and return BIT_ULL(db_vector) for the
> corresponding doorbell bit. Build db_valid_mask from a validated vector
> count so out-of-range db_count values cannot create invalid shifts.
>
> Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
> ---

Reviewed-by: Frank Li <Frank.Li@xxxxxxx>

> Changes since v3:
>   - Tighten db_count-derived mask handling after Sashiko pointed out
>     invalid-shift cases. The new configfs guards should prevent them, but
>     keep the helpers robust on their own.
>   - Drop Reviewed-by tag due to the added validation.
>
>  drivers/pci/endpoint/functions/pci-epf-vntb.c | 42 ++++++++++++++++++-
>  1 file changed, 40 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pci/endpoint/functions/pci-epf-vntb.c b/drivers/pci/endpoint/functions/pci-epf-vntb.c
> index 58e41d95d029..c3caec927d74 100644
> --- a/drivers/pci/endpoint/functions/pci-epf-vntb.c
> +++ b/drivers/pci/endpoint/functions/pci-epf-vntb.c
> @@ -1362,12 +1362,48 @@ static int vntb_epf_peer_mw_count(struct ntb_dev *ntb)
>  	return ntb_ndev(ntb)->num_mws;
>  }
>
> +static int vntb_epf_db_vector_count(struct ntb_dev *ntb)
> +{
> +	struct epf_ntb *ndev = ntb_ndev(ntb);
> +	u32 db_count = READ_ONCE(ndev->db_count);
> +
> +	/*
> +	 * db_count is the total number of doorbell slots exposed to
> +	 * the peer, including:
> +	 *   - slot #0 reserved for link events
> +	 *   - slot #1 historically unused (kept for protocol compatibility)
> +	 *
> +	 * Report only usable per-vector doorbell interrupts.
> +	 */
> +	if (db_count < MIN_DB_COUNT || db_count > MAX_DB_COUNT)
> +		return 0;
> +
> +	return db_count - EPF_IRQ_DB_START;
> +}
> +
>  static u64 vntb_epf_db_valid_mask(struct ntb_dev *ntb)
>  {
> -	if (ntb_ndev(ntb)->db_count < EPF_IRQ_DB_START)
> +	int nr_vec = vntb_epf_db_vector_count(ntb);
> +
> +	if (!nr_vec)
> +		return 0;
> +
> +	return GENMASK_ULL(nr_vec - 1, 0);
> +}
> +
> +static u64 vntb_epf_db_vector_mask(struct ntb_dev *ntb, int db_vector)
> +{
> +	int nr_vec;
> +
> +	/*
> +	 * Doorbell vectors are numbered [0 .. nr_vec - 1], where nr_vec
> +	 * excludes the two reserved slots described above.
> +	 */
> +	nr_vec = vntb_epf_db_vector_count(ntb);
> +	if (db_vector < 0 || db_vector >= nr_vec)
>  		return 0;
>
> -	return BIT_ULL(ntb_ndev(ntb)->db_count - EPF_IRQ_DB_START) - 1;
> +	return BIT_ULL(db_vector);
>  }
>
>  static int vntb_epf_db_set_mask(struct ntb_dev *ntb, u64 db_bits)
> @@ -1617,6 +1653,8 @@ static const struct ntb_dev_ops vntb_epf_ops = {
>  	.spad_count		= vntb_epf_spad_count,
>  	.peer_mw_count		= vntb_epf_peer_mw_count,
>  	.db_valid_mask		= vntb_epf_db_valid_mask,
> +	.db_vector_count	= vntb_epf_db_vector_count,
> +	.db_vector_mask		= vntb_epf_db_vector_mask,
>  	.db_set_mask		= vntb_epf_db_set_mask,
>  	.mw_set_trans		= vntb_epf_mw_set_trans,
>  	.mw_clear_trans		= vntb_epf_mw_clear_trans,
> --
> 2.51.0
>