103 lines
4.5 KiB
Plaintext
103 lines
4.5 KiB
Plaintext
Introduction:
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=============
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This documentation entry describes the Common Clock Framework [CCF] port from
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Linux kernel (v5.1.12) to U-Boot.
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This code is supposed to bring CCF to IMX based devices (imx6q, imx7 imx8).
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Moreover, it also provides some common clock code, which would allow easy
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porting of CCF Linux code to other platforms.
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Design decisions:
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=================
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* U-Boot's driver model [DM] for clk differs from Linux CCF. The most notably
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difference is the lack of support for hierarchical clocks and "clock as a
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manager driver" (single clock DTS node acts as a starting point for all other
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clocks).
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* The clk_get_rate() caches the previously read data if CLK_GET_RATE_NOCACHE is
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not set (no need for recursive access).
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* On purpose the "manager" clk driver (clk-imx6q.c) is not using large table to
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store pointers to clocks - e.g. clk[IMX6QDL_CLK_USDHC2_SEL] = .... Instead we
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use udevice's linked list for the same class (UCLASS_CLK).
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Rationale:
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----------
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When porting the code as is from Linux, one would need ~1KiB of RAM to store
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it. This is way too much if we do plan to use this driver in SPL.
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* The "central" structure of this patch series is struct udevice and its
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uclass_priv field contains the struct clk pointer (to the originally created
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one).
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* To keep things simple the struct udevice's uclass_priv pointer is used to
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store back pointer to corresponding struct clk. However, it is possible to
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modify clk-uclass.c file and add there struct uc_clk_priv, which would have
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clock related members (like pointer to clk). As of this writing there is no
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such need, so to avoid extra allocations (as it can be auto allocated by
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setting .per_device_auto_alloc_size = sizeof(struct uc_clk_priv)) the
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uclass_priv stores the pointer to struct clk.
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* Non-CCF clocks do not have a pointer to a clock in clk->dev->priv. In the case
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of composite clocks, clk->dev->priv may not match clk. Drivers should always
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use the struct clk which is passed to them, and not clk->dev->priv.
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* It is advised to add common clock code (like already added rate and flags) to
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the struct clk, which is a top level description of the clock.
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* U-Boot's driver model already provides the facility to automatically allocate
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(via private_alloc_size) device private data (accessible via dev->priv). It
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may look appealing to use this feature to allocate private structures for CCF
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clk devices e.g. divider (struct clk_divider *divider) for IMX6Q clock.
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The above feature had not been used for following reasons:
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- The original CCF Linux kernel driver is the "manager" for clocks - it
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decides when clock is instantiated (and when memory for it is allocated).
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- Using it would change the original structure of the CCF code.
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- To bind (via clk_register()) the clock device with U-Boot driver model we
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first need udevice for it (the "chicken and egg problem").
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* I've added the clk_get_parent(), which reads parent's dev->uclass_priv to
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provide parent's struct clk pointer. This seems the easiest way to get
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child/parent relationship for struct clk in U-Boot's udevice based clocks. In
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the future arbitrary parents may be supported by adding a get_parent function
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to clk_ops.
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* Linux's CCF 'struct clk_core' corresponds to U-Boot's udevice in 'struct clk'.
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Clock IP block agnostic flags from 'struct clk_core' (e.g. NOCACHE) have been
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moved from this struct one level up to 'struct clk'. Many flags are
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unimplemented at the moment.
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* For tests the new ./test/dm/clk_ccf.c and ./drivers/clk/clk_sandbox_ccf.c
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files have been introduced. The latter setups the CCF clock structure for
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sandbox by reusing, if possible, generic clock primitives - like divier and
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mux. The former file provides code to tests this setup.
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For sandbox new CONFIG_SANDBOX_CLK_CCF Kconfig define has been introduced.
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All new primitives added for new architectures must have corresponding test in
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the two aforementioned files.
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Testing (sandbox):
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==================
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make mrproper; make sandbox_defconfig; make -j4
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./u-boot -i -d arch/sandbox/dts/test.dtb
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=> ut dm clk
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or in a more "scriptable" way (with -v to print debug output):
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./u-boot --fdt arch/sandbox/dts/test.dtb --command "ut dm clk_ccf" -v
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To do:
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------
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* Use of OF_PLATDATA in the SPL setup for CCF - as it is now - the SPL grows
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considerably and using CCF in boards with tiny resources (OCRAM) is
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problematic.
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* On demand port other parts of CCF to U-Boot - as now only features _really_
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needed by DM/DTS converted drivers are used.
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