Re: [PATCH 24/27] clk: rk3568: drop CLK_SET_RATE_PARENT from dclk_vop*

Heiko Stübner
2022-02-08T11:41:15Z

Am Montag, 31. Januar 2022, 09:10:42 CET schrieb Sascha Hauer:
> On Sat, Jan 29, 2022 at 06:48:13PM +0100, Heiko Stübner wrote:
> > Am Mittwoch, 26. Januar 2022, 15:55:46 CET schrieb Sascha Hauer:
> > > The pixel clocks dclk_vop[012] can be clocked from hpll, vpll, gpll or
> > > cpll. gpll and cpll also drive many other clocks, so changing the
> > > dclk_vop[012] clocks could change these other clocks as well. Drop
> > > CLK_SET_RATE_PARENT to fix that. With this change the VOP2 driver can
> > > only adjust the pixel clocks with the divider between the PLL and the
> > > dclk_vop[012] which means the user may have to adjust the PLL clock to a
> > > suitable rate using the assigned-clock-rate device tree property.
> > >
> > > Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
> > > ---
> > > drivers/clk/rockchip/clk-rk3568.c | 6 +++---
> > > 1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/clk/rockchip/clk-rk3568.c b/drivers/clk/rockchip/clk-rk3568.c
> > > index 9d889fc46811..7687c62d1fa8 100644
> > > --- a/drivers/clk/rockchip/clk-rk3568.c
> > > +++ b/drivers/clk/rockchip/clk-rk3568.c
> > > @@ -1044,13 +1044,13 @@ static struct rockchip_clk_branch rk3568_clk_branches[] __initdata = {
> > > RK3568_CLKGATE_CON(20), 8, GFLAGS),
> > > GATE(HCLK_VOP, "hclk_vop", "hclk_vo", 0,
> > > RK3568_CLKGATE_CON(20), 9, GFLAGS),
> > > - COMPOSITE(DCLK_VOP0, "dclk_vop0", hpll_vpll_gpll_cpll_p, CLK_SET_RATE_PARENT | CLK_SET_RATE_NO_REPARENT,
> > > + COMPOSITE(DCLK_VOP0, "dclk_vop0", hpll_vpll_gpll_cpll_p, CLK_SET_RATE_NO_REPARENT,
> >
> > hmm, I'm wondering about the use of having CLK_SET_RATE_NO_REPARENT here
> > (and even adding it below).
> >
> > Using SET_RATE_PARENT in the following patch for the hdmi-pll, should give
> > us at least a suitable rate for the hdmi output, so the vop using that
> > should already find a nice rate to use.
> >
> > The normal system-PLLs don't normally don't change their rate at runtime,
> > so I think we should liberate the dclks to select a PLL that best matches
> > their target rate - so drop the CLK_SET_RATE_NO_REPARENT as well.
> >
> > That way the DCLKs can change to another PLL source if that provides
> > a rate nearer to their target.
>
> The HDMI reference clock has the CLK_SET_RATE_PARENT flag set and we
> need that to program the HPLL clock to suitable rates for the HDMI
> output. Now any other display choosing HPLL as parent, because that
> provides the best rate in that point of time, hangs on a PLL which
> changes its rate whenever the resolution is changed on the HDMI output.

Ah, right ... the hpll is in the parent list, that changes things as you said.
I somehow only noticed the regular PLLs that normally have a constant
rate. So never mind ;-)

Heiko

> Changing parents on rate changes only works when all possible parents of
> all the children involved have a constant rate. IMO allowing reparenting
> on rate changes is a poorly chosen default because it's very unsafe. We
> should rather have a CLK_SET_RATE_ALLOW_REPARENT flag.
>
> Sascha
>
>