Re: [PATCH v1 4/4] clk: thead: Add GPU clock gate control with CLKGEN reset support

[Michal Wilczynski <m.wilczynski@samsung.com>]

On 3/26/25 12:24, Ulf Hansson wrote:
> On Tue, 25 Mar 2025 at 23:40, Stephen Boyd <sboyd@kernel.org> wrote:
>>
>> Quoting Michal Wilczynski (2025-03-19 02:22:11)
>>>
>>>
>>> On 3/13/25 10:25, Philipp Zabel wrote:
>>>> On Do, 2025-03-06 at 17:43 +0100, Michal Wilczynski wrote:
>>>>>
>>>>> On 3/6/25 00:47, Stephen Boyd wrote:
>>>>>> Quoting Michal Wilczynski (2025-03-03 06:36:29)
>>>>>>> The T-HEAD TH1520 has three GPU clocks: core, cfg, and mem. The mem
>>>>>>> clock gate is marked as "Reserved" in hardware, while core and cfg are
>>>>>>> configurable. In order for these clock gates to work properly, the
>>>>>>> CLKGEN reset must be managed in a specific sequence.
>>>>>>>
>>>>>>> Move the CLKGEN reset handling to the clock driver since it's
>>>>>>> fundamentally a clock-related workaround [1]. This ensures that clk_enabled
>>>>>>> GPU clocks stay physically enabled without external interference from
>>>>>>> the reset driver.  The reset is now deasserted only when both core and
>>>>>>> cfg clocks are enabled, and asserted when either of them is disabled.
>>>>>>>
>>>>>>> The mem clock is configured to use nop operations since it cannot be
>>>>>>> controlled.
>>>>>>>
>>>>>>> Link: https://lore.kernel.org/all/945fb7e913a9c3dcb40697328b7e9842b75fea5c.camel@pengutronix.de [1]
>>>>>>>
>>>>>>> Signed-off-by: Michal Wilczynski <m.wilczynski@samsung.com>
>>>>>> [...]
>>>>>>> diff --git a/drivers/clk/thead/clk-th1520-ap.c b/drivers/clk/thead/clk-th1520-ap.c
>>>>>>> index ea96d007aecd..1dfcde867233 100644
>>>>>>> --- a/drivers/clk/thead/clk-th1520-ap.c
>>>>>>> +++ b/drivers/clk/thead/clk-th1520-ap.c
>>>>>>> @@ -862,17 +863,70 @@ static CCU_GATE(CLK_SRAM1, sram1_clk, "sram1", axi_aclk_pd, 0x20c, BIT(3), 0);
>>>>>> [...]
>>>>>>>
>>>>>>>  static CCU_GATE_CLK_OPS(CLK_GPU_MEM, gpu_mem_clk, "gpu-mem-clk",
>>>>>>>                         video_pll_clk_pd, 0x0, BIT(2), 0, clk_nop_ops);
>>>>>>> +static CCU_GATE_CLK_OPS(CLK_GPU_CORE, gpu_core_clk, "gpu-core-clk",
>>>>>>> +                       video_pll_clk_pd, 0x0, BIT(3), 0, ccu_gate_gpu_ops);
>>>>>>> +static CCU_GATE_CLK_OPS(CLK_GPU_CFG_ACLK, gpu_cfg_aclk, "gpu-cfg-aclk",
>>>>>>> +                       video_pll_clk_pd, 0x0, BIT(4), 0, ccu_gate_gpu_ops);
>>>>>>> +
>>>>>>> +static void ccu_gpu_clk_disable(struct clk_hw *hw)
>>>>>>> +{
>>>>>>> +       struct ccu_gate *cg = hw_to_ccu_gate(hw);
>>>>>>> +       unsigned long flags;
>>>>>>> +
>>>>>>> +       spin_lock_irqsave(&gpu_reset_lock, flags);
>>>>>>> +
>>>>>>> +       ccu_disable_helper(&cg->common, cg->enable);
>>>>>>> +
>>>>>>> +       if ((cg == &gpu_core_clk &&
>>>>>>> +            !clk_hw_is_enabled(&gpu_cfg_aclk.common.hw)) ||
>>>>>>> +           (cg == &gpu_cfg_aclk &&
>>>>>>> +            !clk_hw_is_enabled(&gpu_core_clk.common.hw)))
>>>>>>> +               reset_control_assert(gpu_reset);
>>>>>>
>>>>>> Why can't the clk consumer control the reset itself? Doing this here is
>>>>>> not ideal because we hold the clk lock when we try to grab the reset
>>>>>> lock. These are all spinlocks that should be small in lines of code
>>>>>> where the lock is held, but we're calling into an entire other framework
>>>>>> under a spinlock. If an (unrelated) reset driver tries to grab the clk
>>>>>> lock it will deadlock.
>>>>>
>>>>> So in our case the clk consumer is the drm/imagination driver. Here is
>>>>> the comment from the maintainer for my previous attempt to use a reset
>>>>> driver to abstract the GPU init sequence [1]:
>>>>>
>>>>> "Do you know what this resets? From our side, the GPU only has a single
>>>>> reset line (which I assume to be GPU_RESET)."
>>>>>
>>>>> "I don't love that this procedure appears in the platform reset driver.
>>>>> I appreciate it may not be clear from the SoC TRM, but this is the
>>>>> standard reset procedure for all IMG Rogue GPUs. The currently
>>>>> supported TI SoC handles this in silicon, when power up/down requests
>>>>> are sent so we never needed to encode it in the driver before.
>>>>>
>>>>> Strictly speaking, the 32 cycle delay is required between power and
>>>>> clocks being enabled and the reset line being deasserted. If nothing
>>>>> here touches power or clocks (which I don't think it should), the delay
>>>>> could potentially be lifted to the GPU driver."
>>>>>
>>>>> From the drm/imagination maintainers point of view their hardware has
>>>>> only one reset, the extra CLKGEN reset is SoC specific.
>>>>
>>>> If I am understanding correctly, the CLKGEN reset doesn't reset
>>>> anything in the GPU itself, but holds the GPU clock generator block in
>>>> reset, effectively disabling the three GPU clocks as a workaround for
>>>> the always-ungated GPU_MEM clock.
>>>>
>>>>> Also the reset driver maintainer didn't like my way of abstracting two
>>>>> resets ("GPU" and and SoC specific"CLKGEN") into one reset
>>>>
>>>> That is one part of it. The other is that (according to my
>>>> understanding as laid out above), the combined GPU+CLKGEN reset would
>>>> effectively disable all three GPU clocks for a while, after the GPU
>>>> driver has already requested them to be enabled.
>>>
>>> Thank you for your comments Philipp, it seems like we're on the same
>>> page here. I was wondering whether there is anything I can do to move the
>>> patches forward.
>>>
>>> Stephen, if the current patch is a no go from your perspective could you
>>> please advise whether there is a way to solve this in a clock that would
>>> be acceptable to you.
>>
>> It looks like the SoC glue makes the interactions between the clk and
>> reset frameworks complicated because GPU clks don't work if a reset is
>> asserted. You're trying to find a place to coordinate the clk and reset.
>> Am I right?
>>
>> I'd advise managing the clks and resets in a generic power domain that
>> is attached to the GPU device. In that power domain, coordinate the clk
>> and reset sequencing so that the reset is deasserted before the clks are
>> enabled (or whatever the actual requirement is). If the GPU driver
>> _must_ have a clk and reset pointer to use, implement one that either
>> does nothing or flag to the GPU driver that the power domain is managing
>> all this for it so it should just use runtime PM and system PM hooks to
>> turn on the clks and take the GPU out of reset.
>>
>> From what I can tell, the GPU driver maintainer doesn't want to think
>> about the wrapper that likely got placed around the hardware block
>> shipped by IMG. This wrapper is the SoC glue that needs to go into a
>> generic power domain so that the different PM resources, reset, clk,
>> etc. can be coordinated based on the GPU device's power state. It's
>> either that, or go the dwc3 route and have SoC glue platform drivers
>> that manage this stuff and create a child device to represent the hard
>> macro shipped by the vendor like Synopsys/Imagination. Doing the parent
>> device design isn't as flexible as PM domains because you can only have
>> one parent device and the child device state can be ignored vs. many PM
>> domains attached in a graph to a device that are more directly
>> influenced by the device using runtime PM.
>>
>> Maybe you'll be heartened to know this problem isn't unique and has
>> existed for decades :) I don't know what state the graphics driver is in
>> but they'll likely be interested in solving this problem in a way that
>> doesn't "pollute" their driver with SoC specific details. It's all a
>> question of where you put the code. The reset framework wants to focus
>> on resets, the clk framework wants to focus on clks, and the graphics
>> driver wants to focus on graphics. BTW, we went through a similar
>> discussion with regulators and clks years ago and ended up handling that
>> with OPPs and power domains.
> 
> Right, power-domain providers are mostly implementing SoC specific code.
> 
> In some cases, power-domain providers also handle per device SoC
> specific constraints/sequences, which seems what you are discussing
> here. For that, genpd has a couple of callbacks that could be
> interesting to have a look at, such as:
> 
> genpd->attach|detach_dev() - for probe/remove
> genpd.dev_ops->start|stop() - for runtime/system PM
> 
> That said, maybe just using the regular genpd->power_on|off() callback
> is sufficient here, depending on how you decide to model things.


Thanks Stephen, Ulf !

So the way forward I see:

1) The reset driver can be merged as-is, if Philipp is fine with this
code [2].
2) I will cook up the update to the thead power-domain driver which will
handle reset and clock management.
3) I think it would be best to convince the drm/imagination maintainers to
make the clock management in their consumer driver optional. This way if
there is a SoC specific sequence the clocks/resets will be managed from
generic PM driver which is SoC specific. Will talk to them.
4) Will remove the reset management from this series, and re-send.

[2] - https://lore.kernel.org/all/4205b786-fb65-468c-a3d8-bce807dd829a@samsung.com/
> 
>>
>> I believe a PM domain is the right place for this kind of stuff, and I
>> actually presented on this topic at OSSEU[1], but I don't maintain that
>> code. Ulf does.
>>
>> [1] https://osseu2024.sched.com/event/1ej38/the-case-for-an-soc-power-management-driver-stephen-boyd-google

Thanks ! Watched the presentation, very interesting. Hopefully I'll be
able to attend in person in Amsterdam this year if you're presenting :-)

> 
> Kind regards
> Uffe
>