Hi, On Thu, May 2, 2024 at 3:47 PM Stephen Boyd <swboyd@xxxxxxxxxxxx> wrote: > > There's two problems with shared RCGs. > > The first problem is that they incorrectly report the parent after > commit 703db1f5da1e ("clk: qcom: rcg2: Cache CFG register updates for > parked RCGs"). That's because the cached CFG register value needs to be > populated when the clk is registered. clk_rcg2_shared_enable() writes > the cached CFG register value 'parked_cfg'. This value is initially zero > due to static initializers. If a driver calls clk_enable() before > setting a rate or parent, it will set the parent to '0' which is > (almost?) always XO, and may not reflect the parent at registration. In > the worst case, this switches the RCG from sourcing a fast PLL to the > slow crystal speed. > > The second problem is that the force enable bit isn't cleared. The force > enable bit is only used during parking and unparking of shared RCGs. > Otherwise it shouldn't be set because it keeps the RCG enabled even when > all the branches on the output of the RCG are disabled (the hardware has > a feedback mechanism so that any child branches keep the RCG enabled > when the branch enable bit is set). This problem wastes power if the clk > is unused, and is harmful in the case that the clk framework disables > the parent of the force enabled RCG. In the latter case, the GDSC the > shared RCG is associated with will get wedged if the RCG's source clk is > disabled and the GDSC tries to enable the RCG to do "housekeeping" while > powering on. > > Both of these problems combined with incorrect runtime PM usage in the > display driver lead to a black screen on Qualcomm sc7180 Trogdor > chromebooks. What happens is that the bootloader leaves the > 'disp_cc_mdss_rot_clk' enabled and the 'disp_cc_mdss_rot_clk_src' force > enabled and parented to 'disp_cc_pll0'. The mdss driver probes and > runtime suspends, disabling the mdss_gdsc which uses the > 'disp_cc_mdss_rot_clk_src' for "housekeeping". The > 'disp_cc_mdss_rot_clk' is disabled during late init because the clk is > unused, but the parent 'disp_cc_mdss_rot_clk_src' is still force enabled > because the force enable bit was never cleared. Then 'disp_cc_pll0' is > disabled because it is also unused. That's because the clk framework > believes the parent of the RCG is XO when it isn't. A child device of > the mdss device (e.g. DSI) runtime resumes mdss which powers on the > mdss_gdsc. This wedges the GDSC because 'disp_cc_mdss_rot_clk_src' is > parented to 'disp_cc_pll0' and that PLL is off. With the GDSC wedged, > mdss_runtime_resume() tries to enable 'disp_cc_mdss_mdp_clk' but it > can't because the GDSC has wedged all the clks associated with the GDSC > causing clks to stay stuck off. > > This leads to the following warning seen at boot and a black screen > because the display driver fails to probe. > > disp_cc_mdss_mdp_clk status stuck at 'off' > WARNING: CPU: 1 PID: 81 at drivers/clk/qcom/clk-branch.c:87 clk_branch_toggle+0x114/0x168 > Modules linked in: > CPU: 1 PID: 81 Comm: kworker/u16:4 Not tainted 6.7.0-g0dd3ee311255 #1 f5757d475795053fd2ad52247a070cd50dd046f2 > Hardware name: Google Lazor (rev1 - 2) with LTE (DT) > Workqueue: events_unbound deferred_probe_work_func > pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) > pc : clk_branch_toggle+0x114/0x168 > lr : clk_branch_toggle+0x110/0x168 > sp : ffffffc08084b670 > pmr_save: 00000060 > x29: ffffffc08084b680 x28: ffffff808006de00 x27: 0000000000000001 > x26: ffffff8080dbd4f4 x25: 0000000000000000 x24: 0000000000000000 > x23: 0000000000000000 x22: ffffffd838461198 x21: ffffffd838007997 > x20: ffffffd837541d5c x19: 0000000000000001 x18: 0000000000000004 > x17: 0000000000000000 x16: 0000000000000010 x15: ffffffd837070fac > x14: 0000000000000003 x13: 0000000000000004 x12: 0000000000000001 > x11: c0000000ffffdfff x10: ffffffd838347aa0 x9 : 08dadf92e516c000 > x8 : 08dadf92e516c000 x7 : 0000000000000000 x6 : 0000000000000027 > x5 : ffffffd8385a61f2 x4 : 0000000000000000 x3 : ffffffc08084b398 > x2 : ffffffc08084b3a0 x1 : 00000000ffffdfff x0 : 00000000fffffff0 > Call trace: > clk_branch_toggle+0x114/0x168 > clk_branch2_enable+0x24/0x30 > clk_core_enable+0x5c/0x1c8 > clk_enable+0x38/0x58 > clk_bulk_enable+0x40/0xb0 > mdss_runtime_resume+0x68/0x258 > pm_generic_runtime_resume+0x30/0x44 > __genpd_runtime_resume+0x30/0x80 > genpd_runtime_resume+0x124/0x214 > __rpm_callback+0x7c/0x15c > rpm_callback+0x30/0x88 > rpm_resume+0x390/0x4d8 > rpm_resume+0x43c/0x4d8 > __pm_runtime_resume+0x54/0x98 > __device_attach+0xe0/0x170 > device_initial_probe+0x1c/0x28 > bus_probe_device+0x48/0xa4 > device_add+0x52c/0x6fc > mipi_dsi_device_register_full+0x104/0x1a8 > devm_mipi_dsi_device_register_full+0x28/0x78 > ti_sn_bridge_probe+0x1dc/0x2bc > auxiliary_bus_probe+0x4c/0x94 > really_probe+0xf8/0x270 > __driver_probe_device+0xa8/0x130 > driver_probe_device+0x44/0x104 > __device_attach_driver+0xa4/0xcc > bus_for_each_drv+0x94/0xe8 > __device_attach+0xf8/0x170 > device_initial_probe+0x1c/0x28 > bus_probe_device+0x48/0xa4 > deferred_probe_work_func+0x9c/0xd8 > > Fix these problems by parking shared RCGs at boot. This will properly > initialize the parked_cfg struct member so that the parent is reported > properly and ensure that the clk won't get stuck on or off because the > RCG is parented to the safe source (XO). > > Fixes: 703db1f5da1e ("clk: qcom: rcg2: Cache CFG register updates for parked RCGs") > Reported-by: Stephen Boyd <sboyd@xxxxxxxxxx> > Closes: https://lore.kernel.org/r/1290a5a0f7f584fcce722eeb2a1fd898.sboyd@xxxxxxxxxx > Closes: https://issuetracker.google.com/319956935 > Reported-by: Laura Nao <laura.nao@xxxxxxxxxxxxx> > Closes: https://lore.kernel.org/r/20231218091806.7155-1-laura.nao@xxxxxxxxxxxxx > Cc: Bjorn Andersson <andersson@xxxxxxxxxx> > Cc: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx> > Cc: Douglas Anderson <dianders@xxxxxxxxxxxx> > Cc: Taniya Das <quic_tdas@xxxxxxxxxxx> > Signed-off-by: Stephen Boyd <swboyd@xxxxxxxxxxxx> > --- > > This is an alternative approach to [1] which doesn't require any hacks > to disable clks during driver probe. > > [1] https://lore.kernel.org/r/20240327202740.3075378-1-swboyd@xxxxxxxxxxxx > > drivers/clk/qcom/clk-rcg2.c | 32 ++++++++++++++++++++++++++++++++ > 1 file changed, 32 insertions(+) I'm not even close to an expert on the Qualcomm clock stuff, but the description of what's going on here seems reasonable to me. I agree that getting this fixed is important, so I'd be OK with a somewhat weak: Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx> ...hopefully others more in the know will also review and make sure it's not going to break anyone's use cases.