Message-ID: <742cf1ae-1723-4cf5-8931-afd35699cc95@tuxon.dev>
Date: Mon, 8 Sep 2025 16:06:50 +0300
From: Claudiu Beznea <claudiu.beznea@...on.dev>
To: Manivannan Sadhasivam <mani@...nel.org>,
 Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: bhelgaas@...gle.com, lpieralisi@...nel.org, kwilczynski@...nel.org,
 robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org,
 magnus.damm@...il.com, catalin.marinas@....com, will@...nel.org,
 mturquette@...libre.com, sboyd@...nel.org, p.zabel@...gutronix.de,
 lizhi.hou@....com, linux-pci@...r.kernel.org,
 linux-renesas-soc@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-clk@...r.kernel.org, Claudiu Beznea
 <claudiu.beznea.uj@...renesas.com>,
 Wolfram Sang <wsa+renesas@...g-engineering.com>
Subject: Re: [PATCH v3 5/9] PCI: rzg3s-host: Add Initial PCIe Host Driver for
 Renesas RZ/G3S SoC

Hi, Manivannan,

On 9/1/25 18:54, Manivannan Sadhasivam wrote:
> On Mon, Sep 01, 2025 at 04:22:16PM GMT, Geert Uytterhoeven wrote:
>> Hi Mani,
>>
>> On Mon, 1 Sept 2025 at 16:04, Manivannan Sadhasivam <mani@...nel.org> wrote:
>>> On Mon, Sep 01, 2025 at 11:25:30AM GMT, Geert Uytterhoeven wrote:
>>>> On Sun, 31 Aug 2025 at 06:07, Manivannan Sadhasivam <mani@...nel.org> wrote:
>>>>> On Sat, Aug 30, 2025 at 02:22:45PM GMT, Claudiu Beznea wrote:
>>>>>> On 30.08.2025 09:59, Manivannan Sadhasivam wrote:
>>>>>>> On Fri, Jul 04, 2025 at 07:14:05PM GMT, Claudiu wrote:
>>>>>>>> From: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
>>>>>>>>
>>>>>>>> The Renesas RZ/G3S features a PCIe IP that complies with the PCI Express
>>>>>>>> Base Specification 4.0 and supports speeds of up to 5 GT/s. It functions
>>>>>>>> only as a root complex, with a single-lane (x1) configuration. The
>>>>>>>> controller includes Type 1 configuration registers, as well as IP
>>>>>>>> specific registers (called AXI registers) required for various adjustments.
>>>>>>>>
>>>>>>>> Hardware manual can be downloaded from the address in the "Link" section.
>>>>>>>> The following steps should be followed to access the manual:
>>>>>>>> 1/ Click the "User Manual" button
>>>>>>>> 2/ Click "Confirm"; this will start downloading an archive
>>>>>>>> 3/ Open the downloaded archive
>>>>>>>> 4/ Navigate to r01uh1014ej*-rzg3s-users-manual-hardware -> Deliverables
>>>>>>>> 5/ Open the file r01uh1014ej*-rzg3s.pdf
>>>>>>>>
>>>>>>>> Link: https://www.renesas.com/en/products/rz-g3s?queryID=695cc067c2d89e3f271d43656ede4d12
>>>>>>>> Tested-by: Wolfram Sang <wsa+renesas@...g-engineering.com>
>>>>>>>> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
>>>>
>>>>>>>> +  ret = pm_runtime_resume_and_get(dev);
>>>>>>>> +  if (ret)
>>>>>>>> +          return ret;
>>>>>>>> +
>>>>>>>
>>>>>>> Do you really need to do resume_and_get()? If not, you should do:
>>>>>>
>>>>>> It it's needed to enable the clock PM domain the device is part of.
>>>>>>
>>>>>
>>>>> I've replied below.
>>>>>
>>>>>>>
>>>>>>>     pm_runtime_set_active()
>>>>>>>     pm_runtime_no_callbacks()
>>>>>>>     devm_pm_runtime_enable()
>>>>
>>>>>>>> +static int rzg3s_pcie_suspend_noirq(struct device *dev)
>>>>>>>> +{
>>>>>>>> +  struct rzg3s_pcie_host *host = dev_get_drvdata(dev);
>>>>>>>> +  const struct rzg3s_pcie_soc_data *data = host->data;
>>>>>>>> +  struct regmap *sysc = host->sysc;
>>>>>>>> +  int ret;
>>>>>>>> +
>>>>>>>> +  ret = pm_runtime_put_sync(dev);
>>>>>>>> +  if (ret)
>>>>>>>> +          return ret;
>>>>>>>
>>>>>>> Since there are no runtime callbacks present, managing runtime PM in the driver
>>>>>>> makes no sense.
>>>>>>
>>>>>> The PCIe device is part of a clock power domain. Dropping
>>>>>> pm_runtime_enable()/pm_runtime_put_sync() in this driver will lead to this
>>>>>> IP failing to work as its clocks will not be enabled/disabled. If you don't
>>>>>> like the pm_runtime_* approach that could be replaced with:
>>>>>>
>>>>>> devm_clk_get_enabled() in probe and clk_disable()/clk_enable() on
>>>>>> suspend/resume. W/o clocks the IP can't work.
>>>>>
>>>>> Yes, you should explicitly handle clocks in the driver. Runtime PM makes sense
>>>>> if you have a power domain attached to the IP, which you also do as I see now.
>>>>> So to conclude, you should enable/disable the clocks explicitly for managing
>>>>> clocks and use runtime PM APIs for managing the power domain associated with
>>>>> clock controller.
>>>>
>>>> Why? For the past decade, we've been trying to get rid of explicit
>>>> module clock handling for all devices that are always part of a
>>>> clock domain.
>>>>
>>>> The Linux PM Domain abstraction is meant for both power and clock
>>>> domains.  This is especially useful when a device is present on multiple
>>>> SoCs, on some also part of a power domain,  and the number of module
>>>> clocks that needs to be enabled for it to function is not the same on
>>>> all SoCs.  In such cases, the PM Domain abstraction takes care of many
>>>> of the integration-specific differences.
>>>
>>> Hmm, my understanding was that we need to explicitly handle clocks from the
>>> consumer drivers. But that maybe because, the client drivers I've dealt with
>>> requires configuring the clocks (like setting the rate, re-parenting etc...) on
>>> their own. But if there is no such requirement, then I guess it is OK to rely on
>>> the PM core and clock controller drivers.
>>
>> When you need to know the actual clock rate, or change it, you
>> indeed have to handle the clock explicitly.  But it still may be enabled
>> automatically through the clock domain.
>>
> 
> Yeah!
> 
>>>>> But please add a comment above pm_runtime_resume_and_get() to make it clear as
>>>>> most of the controller drivers are calling it for no reason.
>>>>
>>>> Note that any child device that uses Runtime PM depends on all
>>>> its parents in the hierarchy to call pm_runtime_enable() and
>>>> pm_runtime_resume_and_get().
>>>
>>> Two things to note from your statement:
>>>
>>> 1. 'child device that uses runtime PM' - Not all child drivers are doing
>>> runtime PM on their own. So there is no need to do pm_runtime_resume_and_get()
>>> unless they depend on the parent for resource enablement as below.
>>
>> It indeed depends on the child device, and on the bus.  For e.g. an
>> Ethernet controller connected to a simple SoC expansion bus, the bus must
>> be powered and clock, which is what "simple-pm-bus" takes care of
>> ("simple-bus" does not).
>>
> 
> Right. But most of the PCI controller drivers call pm_runtime_resume_and_get()
> for no good reasons. They might have just copied the code from a driver that did
> it on purpose. So I tend to scrutinize these calls whenever they get added for a
> driver.

To be sure I will prepare the next version with something that was
requested: are you OK with keeping pm_runtime_resume_and_get() and add a
comment for it?

Thank you,
Claudiu

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