linux-kernel - Re: [PATCH 2/2] driver: core: add probe control driver

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Message-ID: <20240912204634.GA738361-robh@kernel.org>
Date: Thu, 12 Sep 2024 15:46:34 -0500
From: Rob Herring <robh@...nel.org>
To: Nayeemahmed Badebade <nayeemahmed.badebade@...y.com>
Cc: krzk+dt@...nel.org, conor+dt@...nel.org, gregkh@...uxfoundation.org,
	rafael@...nel.org, yoshihiro.toyama@...y.com,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 2/2] driver: core: add probe control driver

On Wed, Sep 11, 2024 at 07:53:19PM +0530, Nayeemahmed Badebade wrote:
> Probe control driver framework allows deferring the probes of a group of
> devices to an arbitrary time, giving the user control to trigger the probes
> after boot. This is useful for deferring probes from builtin drivers that
> are not required during boot and probe when user wants after boot.

This seems like the wrong way around to me. Why not define what you want 
to probe first or some priority order? I could see use for kernel to 
probe whatever is the console device first. Or the rootfs device... You 
don't need anything added to DT for those.

Of course, there's the issue that Linux probes are triggered bottom-up 
rather than top-down.


> This is achieved by adding a dummy device aka probe control device node
> as provider to a group of devices(consumer nodes) in platform's device
> tree. Consumers are the devices we want to probe after boot.

There's the obvious question of then why not make those devices modules 
instead of built-in?

> 
> To establish control over consumer device probes, each consumer device node
> need to refer the probe control provider node by the phandle.
> 'probe-control-supply' property is used for this.
> 
> Example:
>     // The node below defines a probe control device/provider node
>     prb_ctrl_dev_0: prb_ctrl_dev_0 {
>         compatible = "linux,probe-control";
>     };
> 
>     // The node below is the consumer device node that refers to provider
>     // node by its phandle and a result will not be probed until provider
>     // node is probed.
>     pcie@...c000 {
>         reg = <0x01ffc000 0x04000>, <0x01f00000 0x80000>;
>         #address-cells = <3>;
>         #size-cells = <2>;
>         device_type = "pci";
>         ranges = <0x81000000 0 0          0x01f80000 0 0x00010000>,
>                  <0x82000000 0 0x01000000 0x01000000 0 0x00f00000>;
> 
>         probe-control-supply = <&prb_ctrl_dev_0>;
>     };

Sorry, but this isn't going to happen in DT.

> 
> fw_devlink ensures consumers are not probed until provider is probed
> successfully. The provider probe during boot returns -ENXIO and is not
> re-probed again.
> 
> The driver provides debug interface /sys/kernel/debug/probe_control_status
> for checking probe control status of registered probe control devices.
>  # cat /sys/kernel/debug/probe_control_status
>  prb_ctrl_dev_0: [not triggered]
>   Consumers: 1ffc000.pcie
> 
> Interface /sys/kernel/probe_control/trigger is provided for triggering
> probes of the probe control devices. User can write to this interface to
> trigger specific or all device probes managed by this driver.
> Once the probe is triggered by user, provider probe control device is added
> to deferred_probe_pending_list and driver_deferred_probe_trigger() is
> triggered. This time the probe of probe control device will be
> successful and its consumers will then be probed.
> 
> To trigger specific provider probe:
>   # echo prb_ctrl_dev_0 > /sys/kernel/probe_control/trigger
> 
> To trigger all registered provider probes
>   # echo all > /sys/kernel/probe_control/trigger
> 
> Signed-off-by: Toyama Yoshihiro <yoshihiro.toyama@...y.com>
> Signed-off-by: Nayeemahmed Badebade <nayeemahmed.badebade@...y.com>

This is wrong. Either Toyama is the author and you need to fix the git 
author, or you both are authors and you need a Co-developed-by tag for 
Toyama.