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41575d8e4c
This construct is quite long-winded. In earlier days it made some sense since auto-allocation was a strange concept. But with driver model now used pretty universally, we can shorten this to 'auto'. This reduces verbosity and makes it easier to read. Coincidentally it also ensures that every declaration is on one line, thus making dtoc's job easier. Signed-off-by: Simon Glass <sjg@chromium.org>
102 lines
4.5 KiB
Text
102 lines
4.5 KiB
Text
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 = 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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