Introduction:
=============

This documentation entry describes the Common Clock Framework [CCF]
port from Linux kernel (v5.1.12) to U-Boot.

This code is supposed to bring CCF to IMX based devices (imx6q, imx7
imx8). Moreover, it also provides some common clock code, which would
allow easy porting of CCF Linux code to other platforms.

Design decisions:
=================

* U-Boot's driver model [DM] for clk differs from Linux CCF. The most
  notably difference is the lack of support for hierarchical clocks and
  "clock as a manager driver" (single clock DTS node acts as a starting
  point for all other clocks).

* The clk_get_rate() caches the previously read data if CLK_GET_RATE_NOCACHE
  is not set (no need for recursive access).

* On purpose the "manager" clk driver (clk-imx6q.c) is not using large
  table to store pointers to clocks - e.g. clk[IMX6QDL_CLK_USDHC2_SEL] = ....
  Instead we use udevice's linked list for the same class (UCLASS_CLK).

  Rationale:
  ----------
    When porting the code as is from Linux, one would need ~1KiB of RAM to
    store it. This is way too much if we do plan to use this driver in SPL.

* The "central" structure of this patch series is struct udevice and its
  uclass_priv field contains the struct clk pointer (to the originally created
  one).

* Up till now U-Boot's driver model (DM) CLK operates on udevice (main
  access to clock is by udevice ops)
  In the CCF the access to struct clk (embodying pointer to *dev) is
  possible via dev_get_clk_ptr() (it is a wrapper on dev_get_uclass_priv()).

* To keep things simple the struct udevice's uclass_priv pointer is used to
  store back pointer to corresponding struct clk. However, it is possible to
  modify clk-uclass.c file and add there struct uc_clk_priv, which would have
  clock related members (like pointer to clk). As of this writing there is no
  such need, so to avoid extra allocations (as it can be auto allocated by
  setting .per_device_auto_alloc_size = sizeof(struct uc_clk_priv)) the
  uclass_priv stores the pointer to struct clk.

* It is advised to add common clock code (like already added rate and flags) to
  the struct clk, which is a top level description of the clock.

* U-Boot's driver model already provides the facility to automatically allocate
  (via private_alloc_size) device private data (accessible via dev->priv).
  It may look appealing to use this feature to allocate private structures for
  CCF clk devices e.g. divider (struct clk_divider *divider) for IMX6Q clock.

  The above feature had not been used for following reasons:
  - The original CCF Linux kernel driver is the "manager" for clocks - it
    decides when clock is instantiated (and when memory for it is allocated).

  - Using it would change the original structure of the CCF code.

  - To bind (via clk_register()) the clock device with U-Boot driver model we
    first need udevice for it (the "chicken and egg problem").

* I've added the clk_get_parent(), which reads parent's dev->uclass_priv to
  provide parent's struct clk pointer. This seems the easiest way to get
  child/parent relationship for struct clk in U-Boot's udevice based clocks.

* Linux's CCF 'struct clk_core' corresponds to U-Boot's udevice in 'struct clk'.
  Clock IP block agnostic flags from 'struct clk_core' (e.g. NOCACHE) have been
  moved from this struct one level up to 'struct clk'.

* For tests the new ./test/dm/clk_ccf.c and ./drivers/clk/clk_sandbox_ccf.c
  files have been introduced. The latter setups the CCF clock structure for
  sandbox by reusing, if possible, generic clock primitives - like divier
  and mux. The former file provides code to tests this setup.

  For sandbox new CONFIG_SANDBOX_CLK_CCF Kconfig define has been introduced.
  All new primitives added for new architectures must have corresponding test
  in the two aforementioned files.


Testing (sandbox):
==================

make mrproper; make sandbox_defconfig; make -j4
./u-boot -i -d arch/sandbox/dts/test.dtb
=> ut dm clk

or in a more "scriptable" way (with -v to print debug output):
./u-boot --fdt arch/sandbox/dts/test.dtb --command "ut dm clk_ccf" -v

To do:
------

* Use of OF_PLATDATA in the SPL setup for CCF - as it is now - the SPL grows
  considerably and using CCF in boards with tiny resources (OCRAM) is
  problematic.

* On demand port other parts of CCF to U-Boot - as now only features _really_
  needed by DM/DTS converted drivers are used.