libata: implement and use ops inheritance

libata lets low level drivers build ata_port_operations table and
register it with libata core layer.  This allows low level drivers
high level of flexibility but also burdens them with lots of
boilerplate entries.

This becomes worse for drivers which support related similar
controllers which differ slightly.  They share most of the operations
except for a few.  However, the driver still needs to list all
operations for each variant.  This results in large number of
duplicate entries, which is not only inefficient but also error-prone
as it becomes very difficult to tell what the actual differences are.

This duplicate boilerplates all over the low level drivers also make
updating the core layer exteremely difficult and error-prone.  When
compounded with multi-branched development model, it ends up
accumulating inconsistencies over time.  Some of those inconsistencies
cause immediate problems and fixed.  Others just remain there dormant
making maintenance increasingly difficult.

To rectify the problem, this patch implements ata_port_operations
inheritance.  To allow LLDs to easily re-use their own ops tables
overriding only specific methods, this patch implements poor man's
class inheritance.  An ops table has ->inherits field which can be set
to any ops table as long as it doesn't create a loop.  When the host
is started, the inheritance chain is followed and any operation which
isn't specified is taken from the nearest ancestor which has it
specified.  This operation is called finalization and done only once
per an ops table and the LLD doesn't have to do anything special about
it other than making the ops table non-const such that libata can
update it.

libata provides four base ops tables lower drivers can inherit from -
base, sata, pmp, sff and bmdma.  To avoid overriding these ops
accidentaly, these ops are declared const and LLDs should always
inherit these instead of using them directly.

After finalization, all the ops table are identical before and after
the patch except for setting .irq_handler to ata_interrupt in drivers
which didn't use to.  The .irq_handler doesn't have any actual effect
and the field will soon be removed by later patch.

* sata_sx4 is still using old style EH and currently doesn't take
  advantage of ops inheritance.

Signed-off-by: Tejun Heo <htejun@gmail.com>

1 files changed, 18 insertions, 5 deletions

diff --git a/include/linux/libata.h b/include/linux/libata.h
index eccc38e1756..46aa4ab6489 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -433,7 +433,7 @@ struct ata_host {
 	void __iomem * const	*iomap;
 	unsigned int		n_ports;
 	void			*private_data;
-	const struct ata_port_operations *ops;
+	struct ata_port_operations *ops;
 	unsigned long		flags;
 #ifdef CONFIG_ATA_ACPI
 	acpi_handle		acpi_handle;
@@ -602,7 +602,7 @@ struct ata_link {
 
 struct ata_port {
 	struct Scsi_Host	*scsi_host; /* our co-allocated scsi host */
-	const struct ata_port_operations *ops;
+	struct ata_port_operations *ops;
 	spinlock_t		*lock;
 	unsigned long		flags;	/* ATA_FLAG_xxx */
 	unsigned int		pflags; /* ATA_PFLAG_xxx */
@@ -664,6 +664,13 @@ struct ata_port {
 	u8			sector_buf[ATA_SECT_SIZE]; /* owned by EH */
 };
 
+/* The following initializer overrides a method to NULL whether one of
+ * its parent has the method defined or not.  This is equivalent to
+ * ERR_PTR(-ENOENT).  Unfortunately, ERR_PTR doesn't render a constant
+ * expression and thus can't be used as an initializer.
+ */
+#define ATA_OP_NULL		(void *)(unsigned long)(-ENOENT)
+
 struct ata_port_operations {
 	/*
 	 * Command execution
@@ -733,6 +740,12 @@ struct ata_port_operations {
 	void (*phy_reset)(struct ata_port *ap);
 	void (*eng_timeout)(struct ata_port *ap);
 	irq_handler_t irq_handler;
+
+	/*
+	 * ->inherits must be the last field and all the preceding
+	 * fields must be pointers.
+	 */
+	const struct ata_port_operations	*inherits;
 };
 
 struct ata_port_info {
@@ -742,7 +755,7 @@ struct ata_port_info {
 	unsigned long		pio_mask;
 	unsigned long		mwdma_mask;
 	unsigned long		udma_mask;
-	const struct ata_port_operations *port_ops;
+	struct ata_port_operations *port_ops;
 	irq_handler_t		irq_handler;
 	void 			*private_data;
 };
@@ -765,7 +778,7 @@ extern const unsigned long sata_deb_timing_normal[];
 extern const unsigned long sata_deb_timing_hotplug[];
 extern const unsigned long sata_deb_timing_long[];
 
-extern const struct ata_port_operations ata_dummy_port_ops;
+extern struct ata_port_operations ata_dummy_port_ops;
 extern const struct ata_port_info ata_dummy_port_info;
 
 static inline const unsigned long *
@@ -812,7 +825,7 @@ extern int ata_host_activate(struct ata_host *host, int irq,
 			     struct scsi_host_template *sht);
 extern void ata_host_detach(struct ata_host *host);
 extern void ata_host_init(struct ata_host *, struct device *,
-			  unsigned long, const struct ata_port_operations *);
+			  unsigned long, struct ata_port_operations *);
 extern int ata_scsi_detect(struct scsi_host_template *sht);
 extern int ata_scsi_ioctl(struct scsi_device *dev, int cmd, void __user *arg);
 extern int ata_scsi_queuecmd(struct scsi_cmnd *cmd, void (*done)(struct scsi_cmnd *));