Re: [PATCH v4 1/2] cxl/region: Find free cxl decoder by device_for_each_child()

[Greg Kroah-Hartman]

On Thu, Sep 05, 2024 at 08:36:09AM +0800, Zijun Hu wrote:
> From: Zijun Hu <quic_zijuhu@xxxxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Signed-off-by: Zijun Hu <quic_zijuhu@xxxxxxxxxxx>
> ---
>  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?

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

>  		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.

thanks,

greg k-h