Looping back on this, after digging a bit deeper... On Fri, Feb 14, 2020 at 9:38 AM Nick Fan (范哲維) <Nick.Fan@xxxxxxxxxxxx> wrote: > [snip] > > > Another thing that I'm not implementing is the dance that Mediatek > > > does in their kbase driver when changing the clock (described in > > > patch > > > 2/7): > > > "" > > > The binding we use with out-of-tree Mali drivers includes more > > > clocks, this is used for devfreq: the out-of-tree driver switches > > > clk_mux to clk_sub_parent (26Mhz), adjusts clk_main_parent, then > > > switches clk_mux back to clk_main_parent: > > > (see > > > https://chromium.googlesource.com/chromiumos/third_party/kernel/+/ch > > > romeos-4.19/drivers/gpu/arm/midgard/platform/mediatek/mali_kbase_run > > > time_pm.c#423) > > > clocks = > > > <&topckgen CLK_TOP_MFGPLL_CK>, > > > <&topckgen CLK_TOP_MUX_MFG>, > > > <&clk26m>, > > > <&mfgcfg CLK_MFG_BG3D>; > > > clock-names = > > > "clk_main_parent", > > > "clk_mux", > > > "clk_sub_parent", > > > "subsys_mfg_cg"; > > > "" > > > Is there a clean/simple way to implement this in the clock > > > framework/device tree? Or should we implement something in the > > > panfrost driver? > > > > Putting parent clocks into 'clocks' for a device is a pretty common > > abuse. The 'assigned-clocks' binding is what's used for parent clock > > setup. Not sure that's going to help here though. Is this dance > > because the parent clock frequency can't be changed cleanly? > > Nick/Weiyi, any idea why we do that dance in the first place? (maybe the PLL clock is unstable while it's being changed?) > > Clock source may become unstable during clock frequency changes, so it is always safer to switch to a more reliable clock source. > Otherwise, it may cause some problem in some corner case. > I would suggest to keep it. The Mediatek CPUfreq driver actually does a very similar dance: https://github.com/torvalds/linux/blob/master/drivers/cpufreq/mediatek-cpufreq.c#L249 What they have in the device tree is the main clock, and the "intermediate" clock that is required during switching: clocks = <&mcucfg CLK_MCU_MP0_SEL>, <&topckgen CLK_TOP_ARMPLL_DIV_PLL1>; clock-names = "cpu", "intermediate"; The topology looks like this: clk26m 15 15 1 26000000 0 0 50000 armpll_ll 1 1 0 1417000000 0 0 50000 mcu_mp0_sel 0 0 0 1417000000 0 0 50000 And device tree provides mcu_mp0_sel as "cpu", and the armpll_div_pll1 as "intermediate". The driver looks up armpll_ll by calling get_parent, then: - set_parent(mcu_mp0_sel, armpll_div_pll1) - set_rate(armpll_ll, new_rate) - set_parent(mcu_mp0_sel, armpll_ll) On MT8183's GPU, the topology is a little bit more complicated (but I think there should be a way to merge mfg_bg3d an mfg_sel in the clock core) clk26m 15 15 1 26000000 0 0 50000 mfgpll 1 1 0 419999817 0 0 50000 mfgpll_ck 2 2 0 419999817 0 0 50000 mfg_sel 3 3 0 419999817 0 0 50000 mfg_bg3d 1 1 0 419999817 0 0 50000 We're going to need a special panfrost devfreq driver for mt8183 anyway (to handle the 2 regulators), so it would be easy to take a similar approach: - Add "intermediate" clock in the device tree (clk26m) - Find mfg_sel/mfgpll_ck using 1/2 clk_get_parent calls. - Switch mfg_sel to clk26m, set mfgpll_ck rate, switch mfg_sel back to mfgpll_ck. (BTW, I tried to look, and couldn't find examples or reparenting during clock changes in drivers/clk, are there existing drivers doing similar things? Or this would be new?).
