I would like to make the Allwinner A64's pll-mipi to keep its rate when its parent's (pll-video0) rate changes. Keeping pll-mipi's rate is required, to let the A64 drive both an LCD and HDMI display at the same time, because both have pll-video0 as an ancestor. PATCH 1 adds this functionality as a feature into the clk framework (new flag: CLK_KEEP_RATE). Cores that use this flag, store a rate as req_rate when it or one of its descendants requests a new rate. That rate is then restored in the clk_change_rate recursion, which walks through the tree. It will reach the flagged core (e.g. pll-mipi) after the parent's rate (e.g. pll-video0) has already been set to the new rate. It will then call determine_rate (which requests the parent's current, i.e. new, rate) to determine a rate that is close to the flagged core's previous rate. Afterward it will re-calculate the rates for the flagged core's subtree. PATCH 2 & 3 demonstrate how the new flag can be used for A64's pll-mipi. By setting this flag, it is no longer required to get an exclusive lock when setting tcon0's rate, because the rate will be restored when its parent's (pll-mipi) rate is restored. This work is inspired by an out-of-tree patchset [1] [2] [3]. Unfortunately, the patchset uses clk_set_rate() in a notifier callback, which the following comment on clk_notifier_register() forbids: "The callbacks associated with the notifier must not re-enter into the clk framework by calling any top-level clk APIs." [4] Furthermore, that out-of-tree patchset no longer works with the current linux-next, because setting pll-mipi is now also resetting pll-video0 [5]. Thank you for considering this contribution, Frank [1] https://github.com/megous/linux/commit/4124e115de82797f604808aaa5caad4512a9a1ed [2] https://github.com/megous/linux/commit/edc93fd70ee759fd989664fcb85996cb48a006e6 [3] https://github.com/megous/linux/commit/40f5fc5b08b21142931662147d039ec217c9ba2f [4] https://elixir.bootlin.com/linux/v6.4.11/source/drivers/clk/clk.c#L4578 [5] https://lore.kernel.org/linux-kernel/20230807-pll-mipi_set_rate_parent-v6-0-f173239a4b59@xxxxxxxxxxxx/ Signed-off-by: Frank Oltmanns <frank@xxxxxxxxxxxx> --- Frank Oltmanns (2): clk: keep clock rate when parent rate changes clk: sunxi-ng: a64: keep rate of pll-mipi stable across parent rate changes Icenowy Zheng (1): drm/sun4i: tcon: parent keeps TCON0 clock stable on A64 drivers/clk/clk.c | 48 ++++++++++++++++++++++++++++++++++- drivers/clk/sunxi-ng/ccu-sun50i-a64.c | 3 ++- drivers/gpu/drm/sun4i/sun4i_tcon.c | 15 +++++++++-- drivers/gpu/drm/sun4i/sun4i_tcon.h | 1 + include/linux/clk-provider.h | 2 ++ 5 files changed, 65 insertions(+), 4 deletions(-) --- base-commit: c539c5c0a7ccafe7169c02564cceeb50317b540b change-id: 20230824-pll-mipi_keep_rate-0a3a0d3574cf Best regards, -- Frank Oltmanns <frank@xxxxxxxxxxxx>
