1Introduction:
2=============
3
4This documentation entry describes the Common Clock Framework [CCF] port from
5Linux kernel (v5.1.12) to U-Boot.
6
7This code is supposed to bring CCF to IMX based devices (imx6q, imx7 imx8).
8Moreover, it also provides some common clock code, which would allow easy
9porting of CCF Linux code to other platforms.
10
11Design decisions:
12=================
13
14* U-Boot's driver model [DM] for clk differs from Linux CCF. The most notably
15  difference is the lack of support for hierarchical clocks and "clock as a
16  manager driver" (single clock DTS node acts as a starting point for all other
17  clocks).
18
19* The clk_get_rate() caches the previously read data if CLK_GET_RATE_NOCACHE is
20  not set (no need for recursive access).
21
22* On purpose the "manager" clk driver (clk-imx6q.c) is not using large table to
23  store pointers to clocks - e.g. clk[IMX6QDL_CLK_USDHC2_SEL] = .... Instead we
24  use udevice's linked list for the same class (UCLASS_CLK).
25
26  Rationale:
27  ----------
28    When porting the code as is from Linux, one would need ~1KiB of RAM to store
29    it. This is way too much if we do plan to use this driver in SPL.
30
31* The "central" structure of this patch series is struct udevice and its
32  uclass_priv field contains the struct clk pointer (to the originally created
33  one).
34
35* To keep things simple the struct udevice's uclass_priv pointer is used to
36  store back pointer to corresponding struct clk. However, it is possible to
37  modify clk-uclass.c file and add there struct uc_clk_priv, which would have
38  clock related members (like pointer to clk). As of this writing there is no
39  such need, so to avoid extra allocations (as it can be auto allocated by
40  setting .per_device_auto = sizeof(struct uc_clk_priv)) the
41  uclass_priv stores the pointer to struct clk.
42
43* Non-CCF clocks do not have a pointer to a clock in clk->dev->priv. In the case
44  of composite clocks, clk->dev->priv may not match clk. Drivers should always
45  use the struct clk which is passed to them, and not clk->dev->priv.
46
47* It is advised to add common clock code (like already added rate and flags) to
48  the struct clk, which is a top level description of the clock.
49
50* U-Boot's driver model already provides the facility to automatically allocate
51  (via private_alloc_size) device private data (accessible via dev->priv). It
52  may look appealing to use this feature to allocate private structures for CCF
53  clk devices e.g. divider (struct clk_divider *divider) for IMX6Q clock.
54
55  The above feature had not been used for following reasons:
56  - The original CCF Linux kernel driver is the "manager" for clocks - it
57    decides when clock is instantiated (and when memory for it is allocated).
58
59  - Using it would change the original structure of the CCF code.
60
61  - To bind (via clk_register()) the clock device with U-Boot driver model we
62    first need udevice for it (the "chicken and egg problem").
63
64* I've added the clk_get_parent(), which reads parent's dev->uclass_priv to
65  provide parent's struct clk pointer. This seems the easiest way to get
66  child/parent relationship for struct clk in U-Boot's udevice based clocks.  In
67  the future arbitrary parents may be supported by adding a get_parent function
68  to clk_ops.
69
70* Linux's CCF 'struct clk_core' corresponds to U-Boot's udevice in 'struct clk'.
71  Clock IP block agnostic flags from 'struct clk_core' (e.g. NOCACHE) have been
72  moved from this struct one level up to 'struct clk'. Many flags are
73  unimplemented at the moment.
74
75* For tests the new ./test/dm/clk_ccf.c and ./drivers/clk/clk_sandbox_ccf.c
76  files have been introduced. The latter setups the CCF clock structure for
77  sandbox by reusing, if possible, generic clock primitives - like divier and
78  mux. The former file provides code to tests this setup.
79
80  For sandbox new CONFIG_SANDBOX_CLK_CCF Kconfig define has been introduced.
81  All new primitives added for new architectures must have corresponding test in
82  the two aforementioned files.
83
84Testing (sandbox):
85==================
86
87make mrproper; make sandbox_defconfig; make -j4
88./u-boot -i -d arch/sandbox/dts/test.dtb
89=> ut dm clk
90
91or in a more "scriptable" way (with -v to print debug output):
92./u-boot --fdt arch/sandbox/dts/test.dtb --command "ut dm clk_ccf" -v
93
94To do:
95------
96
97* Use of OF_PLATDATA in the SPL setup for CCF - as it is now - the SPL grows
98  considerably and using CCF in boards with tiny resources (OCRAM) is
99  problematic.
100
101* On demand port other parts of CCF to U-Boot - as now only features _really_
102  needed by DM/DTS converted drivers are used.
103