linux-kernel - Re: [PATCH 1/2] driver core:add device's platform

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Message-ID: <0ef733c0-a945-4eae-8af2-2fe7bb60ed92@163.com>
Date: Fri, 9 May 2025 10:41:11 +0800
From: Zongmin Zhou <min_halo@....com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: rafael@...nel.org, dakr@...nel.org, markgross@...nel.org, arnd@...db.de,
 eric.piel@...mplin-utc.net, valentina.manea.m@...il.com, shuah@...nel.org,
 i@...ithal.me, linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
 Zongmin Zhou <zhouzongmin@...inos.cn>
Subject: Re: [PATCH 1/2] driver core:add device's platform_data set for faux
 device

On 2025/5/8 17:45, Greg KH wrote:
> On Thu, May 08, 2025 at 05:11:47PM +0800, Zongmin Zhou wrote:
>> From: Zongmin Zhou <zhouzongmin@...inos.cn>
>>
>> Most drivers based on platform bus may have specific data
>> for the device.And will get this specific data to use
>> after device added.
>> So keep the setting for device's platform_data is necessary
>> for converting platform device to faux device.
> I do not understand, why not just use the platform_data field directly
> in the faux device structure?  Why change all callers to now have to
> keep track of an additional pointer in these create functions?  That
> just adds complexity for everyone when almost no one will need it.
In fact, I have tried other approaches.
However, I found that it must be set after creating faux_dev and before 
calling the device_add() function.

Because the execution of the driver init and the device probe function 
is asynchronous,
and the actual test shows that the probe function is executed
before faux_device_create_with_groups () returns faux_device for the caller.
But the probe and related functions may need to get plat_data.If 
plat_data is set after
faux_device_create_with_groups() is completed and fdev is returned, the 
probe function will get NULL.

Take vhci-hcd as an example:
vhci_hcd_init() calls faux_device_create_with_groups(),
Once device_add() is called, vhci_hcd_probe() will be executed immediately.
Therefore, the probe function will attempt to obtain plat_data
before vhci_hcd_init() receives the return value of faux_device.
It's too late to set plat_data after get the return value of faux_device.

If there is anything not clearly or other good ways to handle this, 
please let me know.

Thanks very much.

>
> thanks,
>
> greg k-h