Message-ID: <d7bf10d1-9294-44b0-b9f4-193d1a4f26a0@ideasonboard.com>
Date: Fri, 10 May 2024 13:19:43 +0300
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Ulf Hansson <ulf.hansson@...aro.org>, Abel Vesa <abel.vesa@...aro.org>,
 Saravana Kannan <saravanak@...gle.com>, Stephen Boyd <sboyd@...nel.org>
Cc: Nishanth Menon <nm@...com>, Tero Kristo <kristo@...nel.org>,
 Santosh Shilimkar <ssantosh@...nel.org>, Dave Gerlach <d-gerlach@...com>,
 J Keerthy <j-keerthy@...com>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Santosh Shilimkar <santosh.shilimkar@...cle.com>,
 linux-arm-kernel@...ts.infradead.org, linux-pm@...r.kernel.org,
 linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, Devarsh Thakkar <devarsht@...com>
Subject: Re: [PATCH RFC 2/2] pmdomain: ti-sci: Support retaining PD boot time
 state

Hi,

On 03/05/2024 16:45, Ulf Hansson wrote:
> + Abel, Saravanna, Stephen
> 
> On Mon, 15 Apr 2024 at 19:17, Tomi Valkeinen
> <tomi.valkeinen@...asonboard.com> wrote:
>>
>> On 15/04/2024 19:00, Tomi Valkeinen wrote:
>>> Add a new flag, TI_SCI_PD_KEEP_BOOT_STATE, which can be set in the dts
>>> when referring to power domains. When this flag is set, the ti-sci
>>> driver will check if the PD is currently enabled in the HW, and if so,
>>> set the GENPD_FLAG_ALWAYS_ON flag so that the PD will stay enabled.
>>>
>>> The main issue I'm trying to solve here is this:
>>>
>>> If the Display Subsystem (DSS) has been enabled by the bootloader, the
>>> related PD has also been enabled in the HW. When the tidss driver
>>> probes, the driver framework will automatically enable the PD. While
>>> executing the probe function it is very common for the probe to return
>>> EPROBE_DEFER, and, in rarer cases, an actual error. When this happens
>>> (probe() returns an error), the driver framework will automatically
>>> disable the related PD.
>>>
>>> Powering off the PD while the DSS is enabled and displaying a picture
>>> will cause the DSS HW to enter a bad state, from which (afaik) it can't
>>> be woken up except with full power-cycle. Trying to access the DSS in
>>> this state (e.g. when retrying the probe) will usually cause the board
>>> to hang sooner or later.
>>>
>>> Even if we wouldn't have this board-hangs issue, it's nice to be able to
>>> keep the DSS PD enabled: we want to keep the DSS enabled when the
>>> bootloader has enabled the screen. If, instead, we disable the PD at the
>>> first EPROBE_DEFER, the screen will (probably) go black.
>>
>> A few things occurred to me. The driver is supposed to clear the
>> GENPD_FLAG_ALWAYS_ON when the driver has probed successfully. There are
>> two possible issues with that:
>>
>> - Afaics, there's no API to do that, and currently I just clear the bit
>> in genpd->flags. There's a clear race there, so some locking would be
>> required.
>>
>> - This uses the GENPD_FLAG_ALWAYS_ON flag to say "PD is always on, until
>> the driver has started". If the PD would have GENPD_FLAG_ALWAYS_ON set
>> for other reasons, the driver would still go and clear the flag, which
>> might break things.
>>
>> Also, unrelated to the above and not a problem in practice at the very
>> moment, but I think clocks should also be dealt with somehow. Something,
>> at early-ish boot stage, should mark the relevant clocks as in use, so
>> that there's no chance they would be turned off when the main kernel has
>> started (the main display driver is often a module).
>>
>> It would be nice to deal with all the above in a single place. I wonder
>> if the tidss driver itself could somehow be split into two parts, an
>> early part that would probe with minimal dependencies, mainly to reserve
>> the core resources without doing any kind of DRM init. And a main part
>> which would (somehow) finish the initialization at a later point, when
>> we have the filesystem (for firmware) and the other bridge/panel drivers
>> have probed.
>>
>> That can be somewhat achieved with simplefb or simpledrm, though, but we
>> can't do any TI DSS specific things there, and it also creates a
>> requirement to have either of those drivers built-in, and the related DT
>> nodes to be added.
> 
> Without going into too much detail, this and similar problems have
> been discussed in the past. With the fw_devlink and the ->sync_state()
> callback we are getting closer to a solution, but for genpd a solution
> is still pending.
> 
> If you want to read up on earlier discussions and join us moving
> forward, that would be great. The last attempt for genpd to move this
> forward was posted by Abel Vesa:
> https://lore.kernel.org/linux-pm/20230621144019.3219858-1-abel.vesa@linaro.org/
> 
> Beyond that, we have also discussed various solutions at the last LPC
> in Richmond. I think the consensus at that point was that Saravana
> targeted to post something for clocks - and when that was done, we
> should do the similar thing for genpd. Anyway, I have looped them into
> this thread, so they can share any updates on their side of the
> matter.

If I understand the series correctly, it has an issue at least for this 
case/platform.

The devlinks are between the consumer devices and the PD provider 
device. TI SCI PD provider has quite a lot of PDs, and all the consumers 
would have to be probed before any of the PDs could be disabled. So, to 
get the display PD disabled, I would have to load, e.g., the GPU driver 
(which I don't even have).

I believe this is the case for the clocks also.

Perhaps that can be considered a feature, but I fear that in practice it 
would mean that most of the time for most users all the boot-time 
enabled powerdomains would be always on.

Nevertheless, I believe the series would fix the issue mentioned in this 
patch, so I'll see if I can get the series working on the TI platform to 
get a bit more experience on this whole issue.

  Tomi

> 
>>
>>    Tomi
> 
> Kind regards
> Uffe
> 
>>
>>> Another option here would perhaps be to change the driver framework
>>> (drivers/base/platform.c) which attaches and detaches the PD, and make
>>> it somehow optional, allowing the driver the manage the PD. That option
>>> has two downsides: 1) the driver _has_ to manage the PD, which would
>>> rule out the use of simplefb and simpledrm, and 2) it would leave the PD
>>> in off state from Linux's perspective until a driver enables the PD, and
>>> that might mean that the PD gets actually disabled as part of normal
>>> system wide power management (disabling unused resources).
>>>
>>> Yet another option would be to do this outside the ti_sci_pm_domains
>>> driver: a piece of code that would somehow be ran after the
>>> ti_sci_pm_domains driver has probed (so that we have the PDs), but
>>> before tidss/simplefb/simpledrm probes. The problem here is the
>>> "somehow" part. Also, this would partly have the same issue 2) as
>>> mentioned above.
>>>
>>> TODO: If this approach is ok, sci-pm-domain.yaml needs to be extended.
>>> Also, it sounds a bit like the cell value is not a bit-mask, so maybe
>>> adding TI_SCI_PD_KEEP_BOOT_STATE flag this way is not fine.
>>>
>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
>>> ---
>>>    drivers/pmdomain/ti/ti_sci_pm_domains.c    | 27 +++++++++++++++++++++++++--
>>>    include/dt-bindings/soc/ti,sci_pm_domain.h |  1 +
>>>    2 files changed, 26 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/pmdomain/ti/ti_sci_pm_domains.c b/drivers/pmdomain/ti/ti_sci_pm_domains.c
>>> index 1510d5ddae3d..b71b390aaa39 100644
>>> --- a/drivers/pmdomain/ti/ti_sci_pm_domains.c
>>> +++ b/drivers/pmdomain/ti/ti_sci_pm_domains.c
>>> @@ -103,7 +103,7 @@ static struct generic_pm_domain *ti_sci_pd_xlate(
>>>                return ERR_PTR(-ENOENT);
>>>
>>>        genpd_to_ti_sci_pd(genpd_data->domains[idx])->exclusive =
>>> -             genpdspec->args[1];
>>> +             genpdspec->args[1] & TI_SCI_PD_EXCLUSIVE;
>>>
>>>        return genpd_data->domains[idx];
>>>    }
>>> @@ -161,6 +161,8 @@ static int ti_sci_pm_domain_probe(struct platform_device *pdev)
>>>                                break;
>>>
>>>                        if (args.args_count >= 1 && args.np == dev->of_node) {
>>> +                             bool is_on = false;
>>> +
>>>                                if (args.args[0] > max_id) {
>>>                                        max_id = args.args[0];
>>>                                } else {
>>> @@ -189,7 +191,28 @@ static int ti_sci_pm_domain_probe(struct platform_device *pdev)
>>>                                pd->idx = args.args[0];
>>>                                pd->parent = pd_provider;
>>>
>>> -                             pm_genpd_init(&pd->pd, NULL, true);
>>> +                             /*
>>> +                              * If TI_SCI_PD_KEEP_BOOT_STATE is set and the
>>> +                              * PD has been enabled by the bootloader, set
>>> +                              * the PD to GENPD_FLAG_ALWAYS_ON. This will
>>> +                              * make sure the PD stays enabled until a driver
>>> +                              * takes over and clears the GENPD_FLAG_ALWAYS_ON
>>> +                              * flag.
>>> +                              */
>>> +                             if (args.args_count > 1 &&
>>> +                                 args.args[1] & TI_SCI_PD_KEEP_BOOT_STATE) {
>>> +                                     /*
>>> +                                      * We ignore any error here, and in case
>>> +                                      * of error just assume the PD is off.
>>> +                                      */
>>> +                                     pd_provider->ti_sci->ops.dev_ops.is_on(pd_provider->ti_sci,
>>> +                                             pd->idx, NULL, &is_on);
>>> +
>>> +                                     if (is_on)
>>> +                                             pd->pd.flags |= GENPD_FLAG_ALWAYS_ON;
>>> +                             }
>>> +
>>> +                             pm_genpd_init(&pd->pd, NULL, !is_on);
>>>
>>>                                list_add(&pd->node, &pd_provider->pd_list);
>>>                        }
>>> diff --git a/include/dt-bindings/soc/ti,sci_pm_domain.h b/include/dt-bindings/soc/ti,sci_pm_domain.h
>>> index 8f2a7360b65e..af610208e3a3 100644
>>> --- a/include/dt-bindings/soc/ti,sci_pm_domain.h
>>> +++ b/include/dt-bindings/soc/ti,sci_pm_domain.h
>>> @@ -3,6 +3,7 @@
>>>    #ifndef __DT_BINDINGS_TI_SCI_PM_DOMAIN_H
>>>    #define __DT_BINDINGS_TI_SCI_PM_DOMAIN_H
>>>
>>> +#define TI_SCI_PD_KEEP_BOOT_STATE 2
>>>    #define TI_SCI_PD_EXCLUSIVE 1
>>>    #define TI_SCI_PD_SHARED    0
>>>
>>>
>>

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