+/**
+ * ata_slave_link_init - initialize slave link
+ * @ap: port to initialize slave link for
+ *
+ * Create and initialize slave link for @ap. This enables slave
+ * link handling on the port.
+ *
+ * In libata, a port contains links and a link contains devices.
+ * There is single host link but if a PMP is attached to it,
+ * there can be multiple fan-out links. On SATA, there's usually
+ * a single device connected to a link but PATA and SATA
+ * controllers emulating TF based interface can have two - master
+ * and slave.
+ *
+ * However, there are a few controllers which don't fit into this
+ * abstraction too well - SATA controllers which emulate TF
+ * interface with both master and slave devices but also have
+ * separate SCR register sets for each device. These controllers
+ * need separate links for physical link handling
+ * (e.g. onlineness, link speed) but should be treated like a
+ * traditional M/S controller for everything else (e.g. command
+ * issue, softreset).
+ *
+ * slave_link is libata's way of handling this class of
+ * controllers without impacting core layer too much. For
+ * anything other than physical link handling, the default host
+ * link is used for both master and slave. For physical link
+ * handling, separate @ap->slave_link is used. All dirty details
+ * are implemented inside libata core layer. From LLD's POV, the
+ * only difference is that prereset, hardreset and postreset are
+ * called once more for the slave link, so the reset sequence
+ * looks like the following.
+ *
+ * prereset(M) -> prereset(S) -> hardreset(M) -> hardreset(S) ->
+ * softreset(M) -> postreset(M) -> postreset(S)
+ *
+ * Note that softreset is called only for the master. Softreset
+ * resets both M/S by definition, so SRST on master should handle
+ * both (the standard method will work just fine).
+ *
+ * LOCKING:
+ * Should be called before host is registered.
+ *
+ * RETURNS:
+ * 0 on success, -errno on failure.
+ */
+int ata_slave_link_init(struct ata_port *ap)
+{
+ struct ata_link *link;
+
+ WARN_ON(ap->slave_link);
+ WARN_ON(ap->flags & ATA_FLAG_PMP);
+
+ link = kzalloc(sizeof(*link), GFP_KERNEL);
+ if (!link)
+ return -ENOMEM;
+
+ ata_link_init(ap, link, 1);
+ ap->slave_link = link;
+ return 0;
+}
+