Re: [PATCH] cxl/region: Fix logic for finding a free cxl decoder

[Alison Schofield <alison.schofield@intel.com>]

On Tue, Sep 03, 2024 at 08:41:44PM +0800, Zijun Hu wrote:
> From: Zijun Hu <quic_zijuhu@quicinc.com>
> 
> match_free_decoder()'s logic for finding a free cxl decoder depends on
> a prerequisite that all child decoders are sorted by ID in ascending order
> but the prerequisite may not be guaranteed, fix by finding a free cxl
> decoder with minimal ID.

After reading the 'Closes' tag below I have a better understanding of
why you may be doing this, but I don't want to have to jump to that
Link. Can you describe here examples of when the ordered allocation
may not be guaranteed, and the impact when that happens.

This includes a change to device_for_each_child() which I see mentioned
in the Closes tag discussion too. Is that required for this fix?

It's feeling like the fix and api update are comingled. Please clarify.

Thanks,
Alison

> 
> Fixes: 384e624bb211 ("cxl/region: Attach endpoint decoders")
> Closes: https://lore.kernel.org/all/cdfc6f98-1aa0-4cb5-bd7d-93256552c39b@icloud.com/
> Cc: stable@vger.kernel.org
> Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
> ---
>  drivers/cxl/core/region.c | 27 ++++++++++++++++-----------
>  1 file changed, 16 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 21ad5f242875..b9607b4fc40b 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -797,21 +797,26 @@ static size_t show_targetN(struct cxl_region *cxlr, char *buf, int pos)
>  static int match_free_decoder(struct device *dev, void *data)
>  {
>  	struct cxl_decoder *cxld;
> -	int *id = data;
> +	struct cxl_decoder *target_cxld;
> +	struct device **target_device = data;
>  
>  	if (!is_switch_decoder(dev))
>  		return 0;
>  
>  	cxld = to_cxl_decoder(dev);
> -
> -	/* enforce ordered allocation */
> -	if (cxld->id != *id)
> +	if (cxld->region)
>  		return 0;
>  
> -	if (!cxld->region)
> -		return 1;
> -
> -	(*id)++;
> +	if (!*target_device) {
> +		*target_device = get_device(dev);
> +		return 0;
> +	}
> +	/* enforce ordered allocation */
> +	target_cxld = to_cxl_decoder(*target_device);
> +	if (cxld->id < target_cxld->id) {
> +		put_device(*target_device);
> +		*target_device = get_device(dev);
> +	}
>  
>  	return 0;
>  }
> @@ -839,8 +844,7 @@ cxl_region_find_decoder(struct cxl_port *port,
>  			struct cxl_endpoint_decoder *cxled,
>  			struct cxl_region *cxlr)
>  {
> -	struct device *dev;
> -	int id = 0;
> +	struct device *dev = NULL;
>  
>  	if (port == cxled_to_port(cxled))
>  		return &cxled->cxld;
> @@ -849,7 +853,8 @@ 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);
> +		/* Need to put_device(@dev) after use */
> +		device_for_each_child(&port->dev, &dev, match_free_decoder);
>  	if (!dev)
>  		return NULL;
>  	/*
> 
> ---
> base-commit: 67784a74e258a467225f0e68335df77acd67b7ab
> change-id: 20240903-fix_cxld-4f6575a90619
> 
> Best regards,
> -- 
> Zijun Hu <quic_zijuhu@quicinc.com>
>