Quoting Stephen Boyd (2024-05-02 15:47:02)
> 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>
> ---
Applied to clk-next
