This patch describes the design decisions considerations and taken approach for porting in a separate documentation entry.
Signed-off-by: Lukasz Majewski lukma@denx.de
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Changes in v6: None Changes in v5: - s/U-boot/U-Boot/g - Update Linux version to 5.1.12 - Add paragraph regarding sandbox CCF testing (common uclass code)
Changes in v4: - New patch
Changes in v3: None
doc/imx/clk/ccf.txt | 101 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 101 insertions(+) create mode 100644 doc/imx/clk/ccf.txt
diff --git a/doc/imx/clk/ccf.txt b/doc/imx/clk/ccf.txt new file mode 100644 index 0000000000..36b60dc438 --- /dev/null +++ b/doc/imx/clk/ccf.txt @@ -0,0 +1,101 @@ +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.