diff options
Diffstat (limited to 'doc/imx/clk/ccf.txt')
| -rw-r--r-- | doc/imx/clk/ccf.txt | 63 |
1 files changed, 32 insertions, 31 deletions
diff --git a/doc/imx/clk/ccf.txt b/doc/imx/clk/ccf.txt index 36b60dc438..e40ac360e8 100644 --- a/doc/imx/clk/ccf.txt +++ b/doc/imx/clk/ccf.txt @@ -1,42 +1,37 @@ Introduction: ============= -This documentation entry describes the Common Clock Framework [CCF] -port from Linux kernel (v5.1.12) to U-Boot. +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. +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). +* 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). +* 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). +* 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. + 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 @@ -45,13 +40,17 @@ Design decisions: setting .per_device_auto_alloc_size = sizeof(struct uc_clk_priv)) the uclass_priv stores the pointer to struct clk. +* Non-CCF clocks do not have a pointer to a clock in clk->dev->priv. In the case + of composite clocks, clk->dev->priv may not match clk. Drivers should always + use the struct clk which is passed to them, and not clk->dev->priv. + * 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. + (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 @@ -64,21 +63,23 @@ Design decisions: * 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. + child/parent relationship for struct clk in U-Boot's udevice based clocks. In + the future arbitrary parents may be supported by adding a get_parent function + to clk_ops. * 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'. + moved from this struct one level up to 'struct clk'. Many flags are + unimplemented at the moment. * 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. + 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. - + All new primitives added for new architectures must have corresponding test in + the two aforementioned files. Testing (sandbox): ================== |