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- 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).
- * 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.
- * 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.
- 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. 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'. 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.
- 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.
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