netdev - Re: [PATCH v4 1/2] cxl/region: Find free cxl decoder by device_for_each

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Message-ID: <c95932bf-4d11-4952-8835-b212fdb490a7@quicinc.com>
Date: Thu, 5 Sep 2024 16:48:47 +0800
From: quic_zijuhu <quic_zijuhu@...cinc.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Zijun Hu
	<zijun_hu@...oud.com>
CC: Davidlohr Bueso <dave@...olabs.net>,
        Jonathan Cameron
	<jonathan.cameron@...wei.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Alison
 Schofield <alison.schofield@...el.com>,
        Vishal Verma
	<vishal.l.verma@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Dan Williams
	<dan.j.williams@...el.com>,
        Timur Tabi <timur@...nel.org>,
        "David S. Miller"
	<davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, <linux-cxl@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 1/2] cxl/region: Find free cxl decoder by
 device_for_each_child()

On 9/5/2024 1:32 PM, Greg Kroah-Hartman wrote:
> On Thu, Sep 05, 2024 at 08:36:09AM +0800, Zijun Hu wrote:
>> From: Zijun Hu <quic_zijuhu@...cinc.com>
>>
>> To prepare for constifying the following old driver core API:
>>
>> struct device *device_find_child(struct device *dev, void *data,
>> 		int (*match)(struct device *dev, void *data));
>> to new:
>> struct device *device_find_child(struct device *dev, const void *data,
>> 		int (*match)(struct device *dev, const void *data));
>>
>> The new API does not allow its match function (*match)() to modify
>> caller's match data @*data, but match_free_decoder() as the old API's
>> match function indeed modifies relevant match data, so it is not suitable
>> for the new API any more, solved by using device_for_each_child() to
>> implement relevant finding free cxl decoder function.
>>
>> By the way, this commit does not change any existing logic.
>>
>> Suggested-by: Ira Weiny <ira.weiny@...el.com>
>> Signed-off-by: Zijun Hu <quic_zijuhu@...cinc.com>
>> ---
>>  drivers/cxl/core/region.c | 30 ++++++++++++++++++++++++------
>>  1 file changed, 24 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
>> index 21ad5f242875..c2068e90bf2f 100644
>> --- a/drivers/cxl/core/region.c
>> +++ b/drivers/cxl/core/region.c
>> @@ -794,10 +794,15 @@ static size_t show_targetN(struct cxl_region *cxlr, char *buf, int pos)
>>  	return rc;
>>  }
>>  
>> +struct cxld_match_data {
>> +	int id;
>> +	struct device *target_device;
>> +};
>> +
>>  static int match_free_decoder(struct device *dev, void *data)
>>  {
>> +	struct cxld_match_data *match_data = data;
>>  	struct cxl_decoder *cxld;
>> -	int *id = data;
>>  
>>  	if (!is_switch_decoder(dev))
>>  		return 0;
>> @@ -805,17 +810,31 @@ static int match_free_decoder(struct device *dev, void *data)
>>  	cxld = to_cxl_decoder(dev);
>>  
>>  	/* enforce ordered allocation */
>> -	if (cxld->id != *id)
>> +	if (cxld->id != match_data->id)
>>  		return 0;
>>  
>> -	if (!cxld->region)
>> +	if (!cxld->region) {
>> +		match_data->target_device = get_device(dev);
> 
> Where is put_device() called?
>

it is called within cxl_region_find_decoder()

> Ah, it's on the drop later on after find_free_decoder(), right?

yes, it shares the same put_device() which is used for original
device_find_child().

> 
>>  		return 1;
>> +	}
>>  
>> -	(*id)++;
>> +	match_data->id++;
>>  
>>  	return 0;
>>  }
>>  
>> +/* NOTE: need to drop the reference with put_device() after use. */
>> +static struct device *find_free_decoder(struct device *parent)
>> +{
>> +	struct cxld_match_data match_data = {
>> +		.id = 0,
>> +		.target_device = NULL,
>> +	};
>> +
>> +	device_for_each_child(parent, &match_data, match_free_decoder);
>> +	return match_data.target_device;
>> +}
>> +
>>  static int match_auto_decoder(struct device *dev, void *data)
>>  {
>>  	struct cxl_region_params *p = data;
>> @@ -840,7 +859,6 @@ cxl_region_find_decoder(struct cxl_port *port,
>>  			struct cxl_region *cxlr)
>>  {
>>  	struct device *dev;
>> -	int id = 0;
>>  
>>  	if (port == cxled_to_port(cxled))
>>  		return &cxled->cxld;
>> @@ -849,7 +867,7 @@ cxl_region_find_decoder(struct cxl_port *port,
>>  		dev = device_find_child(&port->dev, &cxlr->params,
>>  					match_auto_decoder);
>>  	else
>> -		dev = device_find_child(&port->dev, &id, match_free_decoder);
>> +		dev = find_free_decoder(&port->dev);
> 
> This still feels more complex that I think it should be.  Why not just
> modify the needed device information after the device is found?  What
> exactly is being changed in the match_free_decoder that needs to keep
> "state"?  This feels odd.
> 

for match_auto_decoder() original logic, nothing of aim device is
modified, it just need to modifies state or @id to find the aim device.


> thanks,
> 
> greg k-h