Re: [PATCH 05/10] PCI: endpoint: pci-epf-vntb: Implement db_vector_count/mask for doorbells

On Wed, Feb 25, 2026 at 04:02:01PM -0500, Frank Li wrote:
> On Tue, Feb 24, 2026 at 10:34:54PM +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. While at it, use vntb_epf_db_vector_mask()
> > to simplify vntb_epf_db_valid_mask().
> >
> > Signed-off-by: Koichiro Den <[email protected]>
> > ---
> >  drivers/pci/endpoint/functions/pci-epf-vntb.c | 36 +++++++++++++++++--
> >  1 file changed, 34 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/pci/endpoint/functions/pci-epf-vntb.c b/drivers/pci/endpoint/functions/pci-epf-vntb.c
> > index 2eb3db035644..b651c54d6bef 100644
> > --- a/drivers/pci/endpoint/functions/pci-epf-vntb.c
> > +++ b/drivers/pci/endpoint/functions/pci-epf-vntb.c
> > @@ -1267,12 +1267,42 @@ 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);
> > +
> > +	/*
> > +	 * ndev->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 (ndev->db_count < 2)
> > +		return 0;
> > +
> > +	return ndev->db_count - 2;
> 
> return max(ndev->db_count - 2, 0);

db_count is u32, so it could underflow.
If a one-liner is preferred, something like:

   max_t(u32, ndev->db_count, 2U) - 2

would work. Personally, I think the original version is clearer.

Thanks,
Koichiro

> 
> Frank
> > +}
> > +
> >  static u64 vntb_epf_db_valid_mask(struct ntb_dev *ntb)
> >  {
> > -	if (ntb_ndev(ntb)->db_count < 2)
> > +	return BIT_ULL(vntb_epf_db_vector_count(ntb)) - 1;
> > +}
> > +
> > +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 - 2) - 1;
> > +	return BIT_ULL(db_vector);
> >  }
> >
> >  static int vntb_epf_db_set_mask(struct ntb_dev *ntb, u64 db_bits)
> > @@ -1512,6 +1542,8 @@ 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
> >