Megaraid Common Management Module

공통 management module의 목적

서로 다른 LSI Logic controller class는 같은 firmware control command를 이해하고 user application에 비슷하게 응답한다. Application도 controller class를 동일하게 다룰 수 있으므로 application과 여러 low-level driver 사이에 하나의 공통 module을 두는 것이 합리적이다.

Common Management Module은 low-level driver의 중복 code를 없애고, character device node export와 관련 처리를 driver에서 분리하며, policy를 한곳에서 구현하고, application이 여러 driver 대신 한 module에만 연결하게 한다.

현재 module은 ioctl command 발행에만 사용하지만 장차 모든 user-space interaction을 처리하도록 설계됐다. 따라서 `proc`와 `sysfs` 구현도 이 공통 module에 모은다.

.. SPDX-License-Identifier: GPL-2.0

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Megaraid Common Management Module
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Overview
--------

Different classes of controllers from LSI Logic accept and respond to the
user applications in a similar way. They understand the same firmware control
commands. Furthermore, the applications also can treat different classes of
the controllers uniformly. Hence it is logical to have a single module that
interfaces with the applications on one side and all the low level drivers
on the other.

The advantages, though obvious, are listed for completeness:

        i.        Avoid duplicate code from the low level drivers.
        ii.        Unburden the low level drivers from having to export the
                character node device and related handling.
        iii.        Implement any policy mechanisms in one place.
        iv.        Applications have to interface with only module instead of
                multiple low level drivers.

Currently this module (called Common Management Module) is used only to issue
ioctl commands. But this module is envisioned to handle all user space level
interactions. So any 'proc', 'sysfs' implementations will be localized in this
common module.

공통 helper module 제안의 배경

Jeff Garzik은 2004-02-25 LKML에서 공유 code를 세 번째 library module에 두는 방식을 받아들일 수 있다고 설명했다. `modprobe`가 dependency를 자동 load하므로 사용자가 `modprobe driver1` 또는 `modprobe driver2`를 실행하면 shared module도 자동으로 load된다.

Matt Domsch는 같은 날 MPT 기반 RAID controller와 기존 megaraid의 user space-driver API가 일관된다면 `lsiioctl` 같은 작은 helper module이 적합하다고 제안했다. `mptraid`와 `megaraid`가 발견한 adapter를 helper에 등록하고, helper가 `/dev/megaraid`를 동적으로 등록한 뒤 user-space ioctl을 해당 driver로 전환하는 구조다.

Credits
-------

::

        "Shared code in a third module, a "library module", is an acceptable
        solution. modprobe automatically loads dependent modules, so users
        running "modprobe driver1" or "modprobe driver2" would automatically
        load the shared library module."

- Jeff Garzik ([email protected]), 02.25.2004 LKML

::

        "As Jeff hinted, if your userspace<->driver API is consistent between
        your new MPT-based RAID controllers and your existing megaraid driver,
        then perhaps you need a single small helper module (lsiioctl or some
        better name), loaded by both mptraid and megaraid automatically, which
        handles registering the /dev/megaraid node dynamically. In this case,
        both mptraid and megaraid would register with lsiioctl for each
        adapter discovered, and lsiioctl would essentially be a switch,
        redirecting userspace tool ioctls to the appropriate driver."

- Matt Domsch, ([email protected]), 02.25.2004 LKML

megaraid_mm registry와 uioc_t 변환

Common Management Module은 `megaraid_mm.[ch]`에 구현되며 low-level HBA driver registry로 동작한다. 현재는 megaraid만 각 controller를 공통 module에 등록한다. Application은 module이 export한 character device node를 통해 접근한다.

Low-level driver는 개선된 ioctl packet `uioc_t`만 이해한다. Management module은 기존 application의 old ioctl packet을 `uioc_t`로 바꾸고 driver 처리 후 다시 old format으로 변환해 반환한다. 새 application이 기존 application을 대체하면 old packet format은 폐기될 예정이다.

Common module은 등록된 controller마다 `uioc_t` packet 하나를 전용으로 둔다. 여러 개로 늘릴 수 있지만 현재 유일한 low-level driver인 megaraid가 ioctl 하나만 처리하므로 더 둘 이유가 없다. 새 controller class가 추가되면 수를 조정할 수 있다.

Design
------

The Common Management Module is implemented in megaraid_mm.[ch] files. This
module acts as a registry for low level hba drivers. The low level drivers
(currently only megaraid) register each controller with the common module.

The applications interface with the common module via the character device
node exported by the module.

The lower level drivers now understand only a new improved ioctl packet called
uioc_t. The management module converts the older ioctl packets from the older
applications into uioc_t. After driver handles the uioc_t, the common module
will convert that back into the old format before returning to applications.

As new applications evolve and replace the old ones, the old packet format
will be retired.

Common module dedicates one uioc_t packet to each controller registered. This
can easily be more than one. But since megaraid is the only low level driver
today, and it can handle only one ioctl, there is no reason to have more. But
as new controller classes get added, this will be tuned appropriately.