libata-pmp-prep: implement ops->qc_defer()

Controllers which support PMP have various restrictions on which
combinations of commands are allowed to what number of devices
concurrently.  This patch implements ops->qc_defer() which determines
whether a qc can be issued at the moment or should be deferred.

If the function returns ATA_DEFER_LINK, the qc will be deferred until
a qc completes on the link.  If ATA_DEFER_PORT, until a qc completes
on any link.  The defer conditions are advisory and in general
ATA_DEFER_LINK can be considered as lower priority deferring than
ATA_DEFER_PORT.

ops->qc_defer() replaces fixed ata_scmd_need_defer().  For standard
NCQ/non-NCQ exclusion, ata_std_qc_defer() is implemented.  ahci and
sata_sil24 are converted to use ata_std_qc_defer().

ops->qc_defer() is heavier than the original mechanism because full qc
is prepped before determining to defer it, but various information is
needed to determine defer conditinos and fully translating a qc is the
only way to supply such information in generic manner.

IMHO, this shouldn't cause any noticeable performance issues as

* for most cases deferring occurs rarely (except for NCQ-aware
  cmd-switching PMP)
* translation itself isn't that expensive
* once deferred the command won't be repeated until another command
  completes which usually is a very long time cpu-wise.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

1 files changed, 31 insertions, 0 deletions

diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index 9467c2f6019..b666f51da7e 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -4346,6 +4346,36 @@ int ata_check_atapi_dma(struct ata_queued_cmd *qc)
 }
 
 /**
+ *      ata_std_qc_defer - Check whether a qc needs to be deferred
+ *      @qc: ATA command in question
+ *
+ *      Non-NCQ commands cannot run with any other command, NCQ or
+ *      not.  As upper layer only knows the queue depth, we are
+ *      responsible for maintaining exclusion.  This function checks
+ *      whether a new command @qc can be issued.
+ *
+ *      LOCKING:
+ *      spin_lock_irqsave(host lock)
+ *
+ *      RETURNS:
+ *      ATA_DEFER_* if deferring is needed, 0 otherwise.
+ */
+int ata_std_qc_defer(struct ata_queued_cmd *qc)
+{
+        struct ata_link *link = qc->dev->link;
+
+        if (qc->tf.protocol == ATA_PROT_NCQ) {
+                if (!ata_tag_valid(link->active_tag))
+                        return 0;
+        } else {
+                if (!ata_tag_valid(link->active_tag) && !link->sactive)
+                        return 0;
+        }
+
+        return ATA_DEFER_LINK;
+}
+
+/**
  *      ata_qc_prep - Prepare taskfile for submission
  *      @qc: Metadata associated with taskfile to be prepared
  *
@@ -7111,6 +7141,7 @@ EXPORT_SYMBOL_GPL(ata_interrupt);
 EXPORT_SYMBOL_GPL(ata_do_set_mode);
 EXPORT_SYMBOL_GPL(ata_data_xfer);
 EXPORT_SYMBOL_GPL(ata_data_xfer_noirq);
+EXPORT_SYMBOL_GPL(ata_std_qc_defer);
 EXPORT_SYMBOL_GPL(ata_qc_prep);
 EXPORT_SYMBOL_GPL(ata_dumb_qc_prep);
 EXPORT_SYMBOL_GPL(ata_noop_qc_prep);