Tejun Heo wrote:
libata lets low level drivers build scsi_host_template and
ata_port_operations tables and register them with upper layers. This
allows low level drivers high level of flexibility but also burdens
them with lots of boilerplate entries in thoes data structures.
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 SHT initializers and
ata_port_operations inheritance. SHT initializers can be used to
initialize all the boilerplate entries in a sht. Three variants of
them exist - BASE, BMDMA and NCQ - for different types of drivers.
Note that entries can be overriden by putting individual initializers
after the helper macro.
Ops handling is a bit more involved to allow LLDs to easily re-use
their own ops tables overriding only specific methods. This is
basically 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 sht and 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@xxxxxxxxx>
Two comments:
1) Please split into SHT and ops patches (SHT first, I presume)
2) It seems like inheritance would be easier and less error-prone if the
ops were copied, rather than modifying the structures in-place. Comments?
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