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