[PATCH 0/3] Make Allwinner A64's pll-mipi keep its rate when parent rate changes

[Frank Oltmanns]

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@oltmanns.dev/

Signed-off-by: Frank Oltmanns <frank at oltmanns.dev>
---
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 at oltmanns.dev>