Re: [PATCH] clk: qcom: dispcc-sc8280xp: remove CLK_SET_RATE_PARENT from byte_div_clk_src dividers

[Brian Masney <bmasney@redhat.com>]

Hi Qualcomm Folks,

On Fri, Mar 06, 2026 at 06:27:20PM -0500, Brian Masney wrote:
> On Wed, Mar 4, 2026 at 10:08 AM Pengyu Luo <mitltlatltl@gmail.com> wrote:
> > On Wed, Mar 4, 2026 at 10:50 PM Brian Masney <bmasney@redhat.com> wrote:
> > > On Tue, Mar 03, 2026 at 01:10:43PM +0100, Konrad Dybcio wrote:
> > > > On 3/3/26 12:55 PM, Pengyu Luo wrote:
> > > > > From: White Lewis <liu224806@gmail.com>
> > > > >
> > > > > The four byte_div_clk_src dividers (disp{0,1}_cc_mdss_byte{0,1}_div_clk_src)
> > > > > had CLK_SET_RATE_PARENT set. When the DSI driver calls clk_set_rate() on
> > > > > byte_intf_clk, the rate-change propagates through the divider up to the
> > > > > parent PLL (byte_clk_src), halving the byte clock rate.
> > > > >
> > > > > A simiar issue had been also encountered on SM8750.
> > > > > b8501febdc51 ("clk: qcom: dispcc-sm8750: Drop incorrect CLK_SET_RATE_PARENT on byte intf parent").
> > > > >
> > > > > Likewise, remove CLK_SET_RATE_PARENT from all four byte divider clocks
> > > > > so that clk_set_rate() on the divider adjusts only the divider ratio,
> > > > > leaving the parent PLL untouched.
> > > > >
> > > > > Fixes: 4a66e76fdb6d ("clk: qcom: Add SC8280XP display clock controller")
> > > > > Signed-off-by: White Lewis <liu224806@gmail.com>
> > > > > [pengyu: reword]
> > > > > Signed-off-by: Pengyu Luo <mitltlatltl@gmail.com>
> > > > > ---
> > > >
> > > > Looks like more platforms have this issue.. thanks for fixing this
> > >
> > > I hope within the next week to post a new draft of my patch series to
> > > keep certain clk rates intact when sibling and parent rate changes occur
> > > [1]. This will be for certain critical clks, such as the ones used for
> > > DRM and sound. I have it working with kunit, and just need to clean up
> > > some of my new clk helpers and commit messages before I post a v5.
> > >
> > > I'd like to test this on some real hardware and I have the Thinkpad x13s
> > > with the sc8280xp SoC. Can this issue be reproduced on this hardware? If
> > > so, can you provide me detailed instructions about how to trigger this
> > > scenario?
> > >
> >
> > Quick answer, no, x13s uses a edp panel, not dsi. This issue is
> > related to dsi clks.
> 
> I posted my latest clk scaling work in an attempt to address the
> underlying issue here.

Ignore my previous patch set. In my v6 that I just posted, I updated
clk-divider.c to support the new v2 clk negotiation logic. The
clk_regmap_div_ops uses this driver, so you shouldn't have to make any
code changes.

Anyways, would someone from Qualcomm be willing to test this? The
procedure is fairly simple:

1) Back out the patch:
   clk: qcom: dispcc-sc8280xp: remove CLK_SET_RATE_PARENT from byte_div_clk_src dividers
   (or one of the similar patches posted for other SoCs)

2) Apply my v6 series:
   clk: add support for v1 / v2 clock rate negotiation and kunit tests
   https://lore.kernel.org/linux-clk/20260313-clk-scaling-v6-0-ce89968c5247@redhat.com/T/#

3) Boot... everything should behave the same as prior to my patch set.
   The clk crash will still occur.

4) Reboot, and boot the kernel this time with the clk_v2_rate_negotiation
   kernel parameter to globally opt into the v2 negotiation logic. The clk crash
   hopefully shouldn't occur.

Thanks,

Brian