[PATCH 0/5] clk: Use v2 rate calculation on i.MX8MP

[PATCH 0/5] clk: Use v2 rate calculation on i.MX8MP

[Miquel Raynal]

Hello,

Thanks to Brian's series, we can finally solve the problem of rate
calculation faced with the i.MX8MP LDB/PIX clocks. I've tested this
series using a v7.1 mainline kernel with a lot of unrelated DRM
patches. I then re-applied it on top of clk-next after fixing the few
conflicts when applying Brian's series, but that means I cannot (yet)
run a 7.2/7.3 kernel on the platform (and applying this series with b4
will fail due to Brian's series no longer applying on top of clk-next).

The DTS patch is there for reference, I don't mind if it's not picked
up.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
---
Miquel Raynal (5):
      clk: composite: Initialize inner hw clocks during registration
      clk: Fix v2 rate negotiation with composite clocks
      clk: imx8mp: Allow adjusting the video PLL rates for the LDB clock
      clk: imx8mp: Set v2 rate negociation to LDB and pixel clocks
      arm64: dts: imx8mp: Drop VIDEO_PLL1 assigned rate

 arch/arm64/boot/dts/freescale/imx8mp.dtsi |  6 ++----
 drivers/clk/clk-composite.c               | 34 ++++++++++++++-----------------
 drivers/clk/clk.c                         |  2 +-
 drivers/clk/imx/clk-imx8mp.c              |  6 +++---
 4 files changed, 21 insertions(+), 27 deletions(-)
---
base-commit: 31c632c62c5b98fd965ef5cff3c3fc1ffff02913
change-id: 20261021-ge-clk-lcm-b0d81fcaaa17
prerequisite-message-id: <20260327-clk-scaling-v8-0-86cd0aba3c5f@redhat.com>
prerequisite-patch-id: 8f4664c519b38f0c34e388104f9a75e0946c27f7
prerequisite-patch-id: 041b5fbc0014b25795f8eda6b89b6b61131d51ca
prerequisite-patch-id: 2b9d8038fb1733a94291248ce9042139275fb4d1
prerequisite-patch-id: dcf50f042320b672cabff74bcb861e5ddf26bf6b
prerequisite-patch-id: 3539877187131dae288e6bcfb204d047e5768f84
prerequisite-patch-id: e55a07d726e3caad4508b8f200a8a8c17327c8aa
prerequisite-patch-id: f09cbe9bc44cc58bf47aae239c3558b4e6f6233e
prerequisite-patch-id: 272e0710fba77dd768e6312aacef5a3e9addebc0

Best regards,
-- 
Miquel Raynal <miquel.raynal@bootlin.com>

[PATCH 1/5] clk: composite: Initialize inner hw clocks during registration

[Miquel Raynal]

The mux, rate and gate clk_hws of a composite are not registered as
clocks on their own. Only the ->clk pointer was set to the composite
clk, but their ->core remained unassigned.

Provider ops may call into clk_hw_* helpers, such as clk_hw_get_flags()
or clk_hw_get_rate() for instance. In these cases, hw->core will be
dereferenced.

The composite callbacks kind of papered with this situation by (almost)
always calling __clk_hw_set_clk() before
use. clk_composite_determine_rate() never did it, but it probably did
not fail because the core likely called callbacks like get_parent() or
recalc_rate() before.

Add a clk_composite_init() callback that sets up the inner core pointers
once for all just after the core clock registration, and before any
other callback call from the core. Thanks to this, each per-callback
assignement can be dropped.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
---
 drivers/clk/clk-composite.c | 34 +++++++++++++++-------------------
 1 file changed, 15 insertions(+), 19 deletions(-)

diff --git a/drivers/clk/clk-composite.c b/drivers/clk/clk-composite.c
index 11842bce5918..332b38a326fa 100644
--- a/drivers/clk/clk-composite.c
+++ b/drivers/clk/clk-composite.c
@@ -8,14 +8,26 @@
 #include <linux/err.h>
 #include <linux/slab.h>
 
+static int clk_composite_init(struct clk_hw *hw)
+{
+	struct clk_composite *composite = to_clk_composite(hw);
+
+	if (composite->mux_hw)
+		__clk_hw_set_clk(composite->mux_hw, hw);
+	if (composite->rate_hw)
+		__clk_hw_set_clk(composite->rate_hw, hw);
+	if (composite->gate_hw)
+		__clk_hw_set_clk(composite->gate_hw, hw);
+
+	return 0;
+}
+
 static u8 clk_composite_get_parent(struct clk_hw *hw)
 {
 	struct clk_composite *composite = to_clk_composite(hw);
 	const struct clk_ops *mux_ops = composite->mux_ops;
 	struct clk_hw *mux_hw = composite->mux_hw;
 
-	__clk_hw_set_clk(mux_hw, hw);
-
 	return mux_ops->get_parent(mux_hw);
 }
 
@@ -25,8 +37,6 @@ static int clk_composite_set_parent(struct clk_hw *hw, u8 index)
 	const struct clk_ops *mux_ops = composite->mux_ops;
 	struct clk_hw *mux_hw = composite->mux_hw;
 
-	__clk_hw_set_clk(mux_hw, hw);
-
 	return mux_ops->set_parent(mux_hw, index);
 }
 
@@ -37,8 +47,6 @@ static unsigned long clk_composite_recalc_rate(struct clk_hw *hw,
 	const struct clk_ops *rate_ops = composite->rate_ops;
 	struct clk_hw *rate_hw = composite->rate_hw;
 
-	__clk_hw_set_clk(rate_hw, hw);
-
 	return rate_ops->recalc_rate(rate_hw, parent_rate);
 }
 
@@ -126,10 +134,8 @@ static int clk_composite_determine_rate(struct clk_hw *hw,
 		req->rate = best_rate;
 		return 0;
 	} else if (rate_hw && rate_ops && rate_ops->determine_rate) {
-		__clk_hw_set_clk(rate_hw, hw);
 		return rate_ops->determine_rate(rate_hw, req);
 	} else if (mux_hw && mux_ops && mux_ops->determine_rate) {
-		__clk_hw_set_clk(mux_hw, hw);
 		return mux_ops->determine_rate(mux_hw, req);
 	} else {
 		pr_err("clk: clk_composite_determine_rate function called, but no mux or rate callback set!\n");
@@ -144,8 +150,6 @@ static int clk_composite_set_rate(struct clk_hw *hw, unsigned long rate,
 	const struct clk_ops *rate_ops = composite->rate_ops;
 	struct clk_hw *rate_hw = composite->rate_hw;
 
-	__clk_hw_set_clk(rate_hw, hw);
-
 	return rate_ops->set_rate(rate_hw, rate, parent_rate);
 }
 
@@ -161,9 +165,6 @@ static int clk_composite_set_rate_and_parent(struct clk_hw *hw,
 	struct clk_hw *mux_hw = composite->mux_hw;
 	unsigned long temp_rate;
 
-	__clk_hw_set_clk(rate_hw, hw);
-	__clk_hw_set_clk(mux_hw, hw);
-
 	temp_rate = rate_ops->recalc_rate(rate_hw, parent_rate);
 	if (temp_rate > rate) {
 		rate_ops->set_rate(rate_hw, rate, parent_rate);
@@ -182,8 +183,6 @@ static int clk_composite_is_enabled(struct clk_hw *hw)
 	const struct clk_ops *gate_ops = composite->gate_ops;
 	struct clk_hw *gate_hw = composite->gate_hw;
 
-	__clk_hw_set_clk(gate_hw, hw);
-
 	return gate_ops->is_enabled(gate_hw);
 }
 
@@ -193,8 +192,6 @@ static int clk_composite_enable(struct clk_hw *hw)
 	const struct clk_ops *gate_ops = composite->gate_ops;
 	struct clk_hw *gate_hw = composite->gate_hw;
 
-	__clk_hw_set_clk(gate_hw, hw);
-
 	return gate_ops->enable(gate_hw);
 }
 
@@ -204,8 +201,6 @@ static void clk_composite_disable(struct clk_hw *hw)
 	const struct clk_ops *gate_ops = composite->gate_ops;
 	struct clk_hw *gate_hw = composite->gate_hw;
 
-	__clk_hw_set_clk(gate_hw, hw);
-
 	gate_ops->disable(gate_hw);
 }
 
@@ -237,6 +232,7 @@ static struct clk_hw *__clk_hw_register_composite(struct device *dev,
 	hw = &composite->hw;
 
 	clk_composite_ops = &composite->ops;
+	clk_composite_ops->init = clk_composite_init;
 
 	if (mux_hw && mux_ops) {
 		if (!mux_ops->get_parent) {

-- 
2.56.0

[PATCH 2/5] clk: Fix v2 rate negotiation with composite clocks

[Miquel Raynal]

V2 rate negotiation walks the subtree using the composite hw clocks (the
ones that we have acces to while walking the clock tree), but compares
it internally with the inner hw clocks (typically the inner divider of a
composite clock). Since these may be different pointers in the case of
composite clocks, the comparison won't match for them, which fails the
whole v2 rate negociation logic.

Always use the top level core clock instead. Comparing the top level hw
clocks instead of the core clocks would also work, at the cost of an
extra pointer dereference.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
---
Brian, if you agree with this change, you may keep this fix or fold it
into your series, at your own preference.
---
 drivers/clk/clk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index 745b13a77c83..349c35ab8a5d 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -917,7 +917,7 @@ unsigned long clk_hw_get_children_lcm(struct clk_hw *hw, struct clk_hw *requesti
 
 	hlist_for_each_entry(child, &hw->core->children, child_node) {
 		/* Use requesting rate for the requesting child, current rate for others */
-		if (child->hw == requesting_hw) {
+		if (requesting_hw && child == requesting_hw->core) {
 			child_rate = requesting_rate;
 		} else {
 			if (!clk_hw_is_enabled(child->hw))

-- 
2.56.0

[PATCH 3/5] clk: imx8mp: Allow adjusting the video PLL rates for the LDB clock

[Miquel Raynal]

There must be a x7 or x14 relationship between the LDB clock and the
pixel clocks feeding the LCDIF blocks. There must be some negociation
happening between the two and that may only happen by using the upper
video PLLs. Allow this by setting the CLK_SET_RATE_PARENT flag to the
LDB clock.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
---
 drivers/clk/imx/clk-imx8mp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/clk/imx/clk-imx8mp.c b/drivers/clk/imx/clk-imx8mp.c
index fe6dac70f1a1..3540992b8940 100644
--- a/drivers/clk/imx/clk-imx8mp.c
+++ b/drivers/clk/imx/clk-imx8mp.c
@@ -753,7 +753,7 @@ static int imx8mp_clocks_probe(struct platform_device *pdev)
 	hws[IMX8MP_CLK_MEDIA_MIPI_PHY1_REF] = imx8m_clk_hw_composite("media_mipi_phy1_ref", imx8mp_media_mipi_phy1_ref_sels, ccm_base + 0xbd80);
 	hws[IMX8MP_CLK_MEDIA_DISP1_PIX] = imx8m_clk_hw_composite_bus_flags("media_disp1_pix", imx8mp_media_disp_pix_sels, ccm_base + 0xbe00, CLK_SET_RATE_PARENT);
 	hws[IMX8MP_CLK_MEDIA_CAM2_PIX] = imx8m_clk_hw_composite("media_cam2_pix", imx8mp_media_cam2_pix_sels, ccm_base + 0xbe80);
-	hws[IMX8MP_CLK_MEDIA_LDB] = imx8m_clk_hw_composite("media_ldb", imx8mp_media_ldb_sels, ccm_base + 0xbf00);
+	hws[IMX8MP_CLK_MEDIA_LDB] = imx8m_clk_hw_composite_bus_flags("media_ldb", imx8mp_media_ldb_sels, ccm_base + 0xbf00, CLK_SET_RATE_PARENT);
 	hws[IMX8MP_CLK_MEMREPAIR] = imx8m_clk_hw_composite_critical("mem_repair", imx8mp_memrepair_sels, ccm_base + 0xbf80);
 	hws[IMX8MP_CLK_MEDIA_MIPI_TEST_BYTE] = imx8m_clk_hw_composite("media_mipi_test_byte", imx8mp_media_mipi_test_byte_sels, ccm_base + 0xc100);
 	hws[IMX8MP_CLK_ECSPI3] = imx8m_clk_hw_composite("ecspi3", imx8mp_ecspi3_sels, ccm_base + 0xc180);

-- 
2.56.0

[PATCH 4/5] clk: imx8mp: Set v2 rate negociation to LDB and pixel clocks

[Miquel Raynal]

Both clocks are fed using the same PLL, but when modifying the PLL to
please one of the two, we must not break the other. Because of that, we
must enable the new negociation logic that does an LCM calculation and
preserves the existing rates while walking the subtree.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
---
 drivers/clk/imx/clk-imx8mp.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/clk/imx/clk-imx8mp.c b/drivers/clk/imx/clk-imx8mp.c
index 3540992b8940..96dc237c10ea 100644
--- a/drivers/clk/imx/clk-imx8mp.c
+++ b/drivers/clk/imx/clk-imx8mp.c
@@ -689,7 +689,7 @@ static int imx8mp_clocks_probe(struct platform_device *pdev)
 	hws[IMX8MP_CLK_AHB] = imx8m_clk_hw_composite_bus_critical("ahb_root", imx8mp_ahb_sels, ccm_base + 0x9000);
 	hws[IMX8MP_CLK_AUDIO_AHB] = imx8m_clk_hw_composite_bus("audio_ahb", imx8mp_audio_ahb_sels, ccm_base + 0x9100);
 	hws[IMX8MP_CLK_MIPI_DSI_ESC_RX] = imx8m_clk_hw_composite_bus("mipi_dsi_esc_rx", imx8mp_mipi_dsi_esc_rx_sels, ccm_base + 0x9200);
-	hws[IMX8MP_CLK_MEDIA_DISP2_PIX] = imx8m_clk_hw_composite_bus_flags("media_disp2_pix", imx8mp_media_disp_pix_sels, ccm_base + 0x9300, CLK_SET_RATE_PARENT);
+	hws[IMX8MP_CLK_MEDIA_DISP2_PIX] = imx8m_clk_hw_composite_bus_flags("media_disp2_pix", imx8mp_media_disp_pix_sels, ccm_base + 0x9300, CLK_SET_RATE_PARENT | CLK_V2_RATE_NEGOTIATION);
 
 	hws[IMX8MP_CLK_IPG_ROOT] = imx_clk_hw_divider2("ipg_root", "ahb_root", ccm_base + 0x9080, 0, 1);
 
@@ -751,9 +751,9 @@ static int imx8mp_clocks_probe(struct platform_device *pdev)
 	hws[IMX8MP_CLK_USDHC3] = imx8m_clk_hw_composite("usdhc3", imx8mp_usdhc3_sels, ccm_base + 0xbc80);
 	hws[IMX8MP_CLK_MEDIA_CAM1_PIX] = imx8m_clk_hw_composite("media_cam1_pix", imx8mp_media_cam1_pix_sels, ccm_base + 0xbd00);
 	hws[IMX8MP_CLK_MEDIA_MIPI_PHY1_REF] = imx8m_clk_hw_composite("media_mipi_phy1_ref", imx8mp_media_mipi_phy1_ref_sels, ccm_base + 0xbd80);
-	hws[IMX8MP_CLK_MEDIA_DISP1_PIX] = imx8m_clk_hw_composite_bus_flags("media_disp1_pix", imx8mp_media_disp_pix_sels, ccm_base + 0xbe00, CLK_SET_RATE_PARENT);
+	hws[IMX8MP_CLK_MEDIA_DISP1_PIX] = imx8m_clk_hw_composite_bus_flags("media_disp1_pix", imx8mp_media_disp_pix_sels, ccm_base + 0xbe00, CLK_SET_RATE_PARENT | CLK_V2_RATE_NEGOTIATION);
 	hws[IMX8MP_CLK_MEDIA_CAM2_PIX] = imx8m_clk_hw_composite("media_cam2_pix", imx8mp_media_cam2_pix_sels, ccm_base + 0xbe80);
-	hws[IMX8MP_CLK_MEDIA_LDB] = imx8m_clk_hw_composite_bus_flags("media_ldb", imx8mp_media_ldb_sels, ccm_base + 0xbf00, CLK_SET_RATE_PARENT);
+	hws[IMX8MP_CLK_MEDIA_LDB] = imx8m_clk_hw_composite_bus_flags("media_ldb", imx8mp_media_ldb_sels, ccm_base + 0xbf00, CLK_SET_RATE_PARENT | CLK_V2_RATE_NEGOTIATION);
 	hws[IMX8MP_CLK_MEMREPAIR] = imx8m_clk_hw_composite_critical("mem_repair", imx8mp_memrepair_sels, ccm_base + 0xbf80);
 	hws[IMX8MP_CLK_MEDIA_MIPI_TEST_BYTE] = imx8m_clk_hw_composite("media_mipi_test_byte", imx8mp_media_mipi_test_byte_sels, ccm_base + 0xc100);
 	hws[IMX8MP_CLK_ECSPI3] = imx8m_clk_hw_composite("ecspi3", imx8mp_ecspi3_sels, ccm_base + 0xc180);

-- 
2.56.0

[PATCH 5/5] arm64: dts: imx8mp: Drop VIDEO_PLL1 assigned rate

[Miquel Raynal]

This rate has never been assigned for good reasons. It was assigning a
frequency only adapted to the panel of some evaluation board, while
allowing a rate division by 7 and 14 for the needs of the LDB.

Linux now has an LCM-based divisor calculation that allows us from
dropping that inappropriate assignment.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
---

I believe most of the users of this DTS set their own frequency in their
downstream DTS, just because the existing assignment won't fit their
panels, so while in theory this change could break bootloaders/other OS
pulling this file, in practice it is unlikely it will. I don't know if
we really want to do that cleanup or not since at most it is an
incorrect and useless assignment, but at least one should know that it
can (should?) be removed.
---
 arch/arm64/boot/dts/freescale/imx8mp.dtsi | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/boot/dts/freescale/imx8mp.dtsi b/arch/arm64/boot/dts/freescale/imx8mp.dtsi
index 9cf9f6f08654..7691d917646e 100644
--- a/arch/arm64/boot/dts/freescale/imx8mp.dtsi
+++ b/arch/arm64/boot/dts/freescale/imx8mp.dtsi
@@ -1988,16 +1988,14 @@ media_blk_ctrl: blk-ctrl@32ec0000 {
 						  <&clk IMX8MP_CLK_MEDIA_APB>,
 						  <&clk IMX8MP_CLK_MEDIA_DISP1_PIX>,
 						  <&clk IMX8MP_CLK_MEDIA_DISP2_PIX>,
-						  <&clk IMX8MP_CLK_MEDIA_ISP>,
-						  <&clk IMX8MP_VIDEO_PLL1>;
+						  <&clk IMX8MP_CLK_MEDIA_ISP>;
 				assigned-clock-parents = <&clk IMX8MP_SYS_PLL2_1000M>,
 							 <&clk IMX8MP_SYS_PLL1_800M>,
 							 <&clk IMX8MP_VIDEO_PLL1_OUT>,
 							 <&clk IMX8MP_VIDEO_PLL1_OUT>,
 							 <&clk IMX8MP_SYS_PLL2_500M>;
 				assigned-clock-rates = <500000000>, <200000000>,
-						       <0>, <0>, <500000000>,
-						       <1039500000>;
+						       <0>, <0>, <500000000>;
 				#power-domain-cells = <1>;
 
 				lvds_bridge: bridge@5c {

-- 
2.56.0

Re: [PATCH 0/5] clk: Use v2 rate calculation on i.MX8MP

[Brian Masney]

Hi Miquel (and Jerome),

On Wed, Oct 21, 2026 at 03:09:56PM +0200, Miquel Raynal wrote:
> Hello,
> 
> Thanks to Brian's series, we can finally solve the problem of rate
> calculation faced with the i.MX8MP LDB/PIX clocks. I've tested this
> series using a v7.1 mainline kernel with a lot of unrelated DRM
> patches. I then re-applied it on top of clk-next after fixing the few
> conflicts when applying Brian's series, but that means I cannot (yet)
> run a 7.2/7.3 kernel on the platform (and applying this series with b4
> will fail due to Brian's series no longer applying on top of clk-next).
> 
> The DTS patch is there for reference, I don't mind if it's not picked
> up.
> 
> Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

Thanks for testing and posting this!

I know that Jerome would like to see some changes to my rate change
series before it can be merged. I'm going to probably poorly describe
his concerns here, so apologies in advance if I miscommunicate this
Jerome. I'd appreciate it if you could clarify.

- Currently the latest version posted (v8) to the list has the LCM
  helper (clk_hw_get_children_lcm) in drivers/clk/clk.c, and the LCM
  helper should be in clk-divider.c. I agree that it should be in the
  divider code, but we'd have to export some additional APIs to allow
  that. In the future if we went with what's in v8, and wanted to fix
  the fractional dividers, then it would need a similar type of helper
  in clk.c, which is not ideal. I initially went with this approach to
  limit the APIs that are exported from the clk core.

- I have a draft v9 series that I haven't posted yet that adds two new
  helpers: clk_hw_get_num_children() and clk_hw_get_child_by_index()
  similar to the existing functions for the parents. This allows moving
  the LCM logic into clk-divider.c where it belongs, however the
  downside to this approach is that it expands the clk exported APIs so
  that providers will be able to fully walk the clk tree. The concern is
  that providers may abuse this in the future and it may limit future
  changes / cleanups to the clk core. For example, if we wanted to clean
  up the way locking is done, this may potentially limit the available
  options.
  https://git.kernel.org/pub/scm/linux/kernel/git/clk/linux.git/log/?h=clk-divider-scaling-v9

- He would like to see it where each clk consumer sets the min/max rate
  that's acceptable, and the clk scaling in the provider takes those
  ranges into account when rates are negotiated.

Anyways, please chime in Jerome if I got this right / wrong, and any
additional concerns.

Brian

Re: [PATCH 0/5] clk: Use v2 rate calculation on i.MX8MP

[Jerome Brunet]

On mer. 21 oct. 2026 at 10:11, Brian Masney <bmasney@redhat.com> wrote:

> Hi Miquel (and Jerome),
>
> On Wed, Oct 21, 2026 at 03:09:56PM +0200, Miquel Raynal wrote:
>> Hello,
>> 
>> Thanks to Brian's series, we can finally solve the problem of rate
>> calculation faced with the i.MX8MP LDB/PIX clocks. I've tested this
>> series using a v7.1 mainline kernel with a lot of unrelated DRM
>> patches. I then re-applied it on top of clk-next after fixing the few
>> conflicts when applying Brian's series, but that means I cannot (yet)
>> run a 7.2/7.3 kernel on the platform (and applying this series with b4
>> will fail due to Brian's series no longer applying on top of clk-next).
>> 
>> The DTS patch is there for reference, I don't mind if it's not picked
>> up.
>> 
>> Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
>
> Thanks for testing and posting this!
>
> I know that Jerome would like to see some changes to my rate change
> series before it can be merged. I'm going to probably poorly describe
> his concerns here, so apologies in advance if I miscommunicate this
> Jerome. I'd appreciate it if you could clarify.
>
> - Currently the latest version posted (v8) to the list has the LCM
>   helper (clk_hw_get_children_lcm) in drivers/clk/clk.c, and the LCM
>   helper should be in clk-divider.c. I agree that it should be in the
>   divider code, but we'd have to export some additional APIs to allow
>   that. In the future if we went with what's in v8, and wanted to fix
>   the fractional dividers, then it would need a similar type of helper
>   in clk.c, which is not ideal. I initially went with this approach to
>   limit the APIs that are exported from the clk core.
>
> - I have a draft v9 series that I haven't posted yet that adds two new
>   helpers: clk_hw_get_num_children() and clk_hw_get_child_by_index()
>   similar to the existing functions for the parents. This allows moving
>   the LCM logic into clk-divider.c where it belongs, however the
>   downside to this approach is that it expands the clk exported APIs so
>   that providers will be able to fully walk the clk tree. The concern is
>   that providers may abuse this in the future and it may limit future
>   changes / cleanups to the clk core. For example, if we wanted to clean
>   up the way locking is done, this may potentially limit the available
>   options.
>   https://git.kernel.org/pub/scm/linux/kernel/git/clk/linux.git/log/?h=clk-divider-scaling-v9
>
> - He would like to see it where each clk consumer sets the min/max rate
>   that's acceptable, and the clk scaling in the provider takes those
>   ranges into account when rates are negotiated.
>
> Anyways, please chime in Jerome if I got this right / wrong, and any
> additional concerns.
>

Hi Brian, Miquel,

Thanks Brian for the summary, and Miquel for testing and posting this.
The i.MX8MP LDB/pixel case is a real problem and I'd like us to solve
it. My concerns are about how, more than whether.

You got most of it. The main nuance is I want to convey is where it
should live IMO: not in a divider helper, and not in providers at all,
but in the core. I'll try to explain.

So far, CCF has preserved the settings of each element, or more
honestly, left them alone: if a divider divides by 4, it keeps doing so
whatever happens to its parent. This behavior has been there for years
and many devices rely on it, even if unknowingly. Miquel, I understand
you would rather make the new behavior the default, but I think this one
is here to stay. That said, it would be nice to have a way to say that
the rate should be preserved rather than the setting. I'll come back
to that.

My first concern is about the responsibilities of each component. IMO a
provider should only describe the transfer function it implements:
"given this parent rate, here is what I can do". It should not walk
the tree or care about what its children are doing. Coordinating the
providers is the core's job, without assumptions about their transfer
functions.

Providers already query their parents: a mux asks each candidate
parent what it can provide, and a clock with CLK_SET_RATE_PARENT
forwards its request upward. That is the request path, and the core
stays in charge of it. Reading the state of children or siblings, and
deciding on their behalf, is a different thing. Once a provider is
allowed to do that, there is no good reason to deny it to the others,
and I think that would make the core much harder to maintain. It could
also get in the way of other pressing work, like the global lock problem,
where tree walking will certainly play a critical role. Miquel's
patch 2 is a good illustration: the helper ends up comparing the
clk_hw of the tree with the inner clk_hw of a composite, which is not
part of the tree.

My second concern is that it is tied to the integer divider, since it
relies on the LCM. If another element with a different transfer
function sits on the path, it does not work, as we discussed. We could
extend the principle to other types, but:
* I don't see how the different types would reconcile, since each one
  does its own thing in its own .determine_rate();
* the change is no longer simple and harmless if it has to spread to
  every provider to work.

The behavior is also hard for consumers to predict. For example, the
parent rate is only negotiated with the subtree of the divider's
direct parent. With a gate between the divider and the PLL, the other
branches of the PLL are left out. That could be fixed with more tree
walking in the provider, which brings us back to my first point.

If such a mechanism is needed, I would much prefer it done entirely in
the core, in a generic way, with no assumption about the transfer
functions involved. I realize this may go against a direction
suggested at a previous LPC, which this series followed, and I think the
points above are worth reconsidering it for.

I say "if" because I doubt clock trees are designed with runtime rate
negotiation in mind. So far, we have mostly dealt with these cases by
removing CLK_SET_RATE_PARENT and using assigned-clock-rates on the
shared ancestor. I understand that falls short when the PLL has to
follow the display mode, as here.

For rate negotiation, my suggestion would be to reuse what the
framework already has: clock ranges, and re-running .determine_rate()
on the children being dragged along, with the common ancestor's rate
pinned. When that rate does not suit a sibling's range, other ancestor
rates can be tried within the constraints, rather than giving up.

clk_set_rate() is a best effort function. It tries to get as close to
the request as possible, honoring the ranges of every clock along the
path, and it is up to the consumer to decide whether the result is
good enough. That decision is lost if the framework recomputes the
rates of a sibling branch on its own. Ranges could help it decide:
either "I'm fine with the new rate", or -EBUSY.

What I'm suggesting is to treat any clock with a range as one that
needs updating. Its consumer cared enough to tell us what it can and
cannot cope with. That removes the need for a specific flag, and puts
the rate policy in the consumer, which is where I'd prefer use case
choices to live, rather than in the provider or the core. It would
also fix clocks being dragged out of their range during an update.
Presumably, a clock without a range does not care enough to give us a
decider, so the framework has no basis to act on its behalf and keeps
today's behavior. I think this behavior will keep on being good enough
for the vast majority clocks out there and it is a lot lighter
on the math side (which is already quite severe as it is)

I've been checking whether the idea is remotely feasible. It involves
multiple planning stages and multiple tops. It is still very rough,
but there is the beginning of something:

https://github.com/jbrun3t/linux/commits/clk/clk-rate-range-update/

Miquel, if you fancy trying it, please let me know whether it works
for you. All you should need is to set a range on the clocks you care
about.

Sorry for this rather long explanation.

Brian, your series is what got this discussion going, I know a lot of
efforts have gone into it already. I'd be glad to work on this together.

> Brian
>

-- 
Jerome