linux-kernel - Re: [PATCH 0/2] Add framework for user controlled driver probes

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Message-ID: <26fed82b-7c60-4fda-8951-b22654728743@kernel.org>
Date: Tue, 17 Sep 2024 11:03:14 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Nayeemahmed Badebade <nayeemahmed.badebade@...y.com>,
 Greg KH <gregkh@...uxfoundation.org>
Cc: robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org,
 rafael@...nel.org, yoshihiro.toyama@...y.com, linux-kernel@...r.kernel.org,
 devicetree@...r.kernel.org
Subject: Re: [PATCH 0/2] Add framework for user controlled driver probes

On 17/09/2024 11:06, Nayeemahmed Badebade wrote:
> Hi Greg,
> 
> Thank you for taking the time to check our patch and provide
> valuable feedback. We appreciate your comments/suggestions.
> 
> Please find our reply to your comments.
> 
> On Fri, Sep 13, 2024 at 06:36:38AM +0200, Greg KH wrote:
>> On Wed, Sep 11, 2024 at 07:53:17PM +0530, Nayeemahmed Badebade wrote:
>>> Hi,
>>
>> If Rob hadn't responded, I wouldn't have noticed these as they ended up
>> in spam for some reason.  You might want to check your email settings...
>>
> 
> I have ensured standard settings which we have been using are used this
> time, let me know if this email is received properly.
> 
>>> This patch series introduces a new framework in the form of a driver
>>> probe-control, aimed at addressing the need for deferring the probes
>>> from built-in drivers in kernels where modules are not used.
>>
>> Wait, why?
>>
> 
> We have a scenario where a driver cannot be built as a module and ends up
> as a built-in driver. We don't want to probe this driver during boot as its

Fix this instead.

> not required at the time of booting.
> Example: drivers/pci/controller/dwc/pci-imx6.c
> So the intention is to only postpone some driver probes similar to:
> https://elinux.org/Deferred_Initcalls
> But instead of delaying initcall execution(which requires initmem to be kept
> and not freed during boot) we are trying to delay driver probes as this is
> much simpler.
> The proposed driver is a generic solution for managing such driver
> probes.
> 
>>> In such scenario, delaying the initialization of certain devices such
>>> as pcie based devices not needed during boot and giving user the control
>>> on probing these devices post boot, can help reduce overall boot time.
>>> This is achieved without modifying the driver code, simply by configuring
>>> the platform device tree.
>>
>> PCI devices should not be on the platform device tree.
>>
> 
> You are right, we are referring to the pci host controller that will be
> listed in device tree and skipping its probe during boot as an example
> here.
> 
>> And what's wrong with async probing?  That was written for this very
>> issue.
>>
> 
> We have explored async probe as an option, but we noticed below:
> 1) Probe initiated via async
> 2) Boot continues with other setup.
> 3) Boot reaches stage where ip configuration is to be done via
>    ip_auto_config() and 1) is still in progress, then boot waits for all
>    async calls to be completed before proceeding with network setup.
>    ip_auto_config()
>     -> wait_for_devices()
>       -> wait_for_device_probe()
>          -> async_synchronize_full()
> 4) Similar thing happens with rootfs mount step in prepare_namespace()
>    initcall
> 
> So to avoid getting blocked on these probes which are not required
> during boot, we proposed this driver for managing such built-in driver
> probes execution.
> 
>>> This patch series includes 2 patches:
>>>
>>> 1) dt-binding document for the probe-control driver
>>>    This document explains how device tree of a platform can be configured
>>>    to use probe-control devices for deferring the probes of certain
>>>    devices.
>>
>> But what does that have to do with PCI devices?
> 
> As explained before, the driver is generic one and is for managing probes of
> drivers that are built-in.
> 
> To delay such probes, in DT we add dummy devices managed by the proposed
> driver. These dummy devices(probe control devices) will be setup as
> supplier to the device nodes that we want to probe later.

You embedded OS policy into DT. That's not really the way to go. Look
how boot phase does it. First of all - it already solves your problem.
Second - it's property of each device, not some fake provider.

> dt-binding doc patch explains this process with pci controller node as
> an example that needs to be probed later after the boot.
> 
>>


Best regards,
Krzysztof