linux-kernel - Re: [PATCH 2/4] x86/amd_nb: add support for newer PCI topologies

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Message-ID: <20181105193840.GA26868@zn.tnic>
Date:   Mon, 5 Nov 2018 20:38:40 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Woods, Brian" <Brian.Woods@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Clemens Ladisch <clemens@...isch.de>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Bjorn Helgaas <bhelgaas@...gle.com>, Pu Wen <puwen@...on.cn>,
        Jia Zhang <qianyue.zj@...baba-inc.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH 2/4] x86/amd_nb: add support for newer PCI topologies

On Fri, Nov 02, 2018 at 06:11:07PM +0000, Woods, Brian wrote:
> Add support for new processors which have multiple PCI root complexes
> per data fabric/SMN interface.

Please write out abbreviations. I believe it is only you and I who know
what SMN means. :)

> The interfaces per root complex are redundant and should be skipped.

And I believe it is only you who understands that sentence. :)

Please elaborate why interfaces need to be skipped, *which* interfaces
need to be skipped and which is the correct interface to access DF/SMN
through?

> This makes sure the DF/SMN interfaces get accessed via the correct
> root complex.
>
> Ex:
> DF/SMN 0 -> 60
> 	    40
> 	    20
> 	    00
> DF/SMN 1 -> e0
> 	    c0
> 	    a0
> 	    80
> 
> Signed-off-by: Brian Woods <brian.woods@....com>
> ---
>  arch/x86/kernel/amd_nb.c | 41 +++++++++++++++++++++++++++++++++++------
>  1 file changed, 35 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/amd_nb.c b/arch/x86/kernel/amd_nb.c
> index 19d489ee2b1e..c0bf26aeb7c3 100644
> --- a/arch/x86/kernel/amd_nb.c
> +++ b/arch/x86/kernel/amd_nb.c
> @@ -213,7 +213,10 @@ int amd_cache_northbridges(void)
>  	const struct pci_device_id *root_ids = amd_root_ids;
>  	struct pci_dev *root, *misc, *link;
>  	struct amd_northbridge *nb;
> -	u16 i = 0;
> +	u16 roots_per_misc = 0;
> +	u16 misc_count = 0;
> +	u16 root_count = 0;
> +	u16 i, j;
>  
>  	if (amd_northbridges.num)
>  		return 0;
> @@ -226,26 +229,52 @@ int amd_cache_northbridges(void)
>  
>  	misc = NULL;
>  	while ((misc = next_northbridge(misc, misc_ids)) != NULL)
> -		i++;
> +		misc_count++;
>  
> -	if (!i)
> +	root = NULL;
> +	while ((root = next_northbridge(root, root_ids)) != NULL)
> +		root_count++;
> +
> +	if (!misc_count)
>  		return -ENODEV;

So you're doing the root_count above but returning in the !misc_count
case. So that root_count iteration was unnecessary work. IOW, you should
keep the misc_count check after its loop.

>  
> -	nb = kcalloc(i, sizeof(struct amd_northbridge), GFP_KERNEL);
> +	if (root_count) {
> +		roots_per_misc = root_count / misc_count;
> +
> +		/*
> +		 * There should be _exactly_ N roots for each DF/SMN
> +		 * interface.
> +		 */
> +		if (!roots_per_misc || (root_count % roots_per_misc)) {
> +			pr_info("Unsupported AMD DF/PCI configuration found\n");
> +			return -ENODEV;
> +		}
> +	}
> +
> +	nb = kcalloc(misc_count, sizeof(struct amd_northbridge), GFP_KERNEL);
>  	if (!nb)
>  		return -ENOMEM;
>  
>  	amd_northbridges.nb = nb;
> -	amd_northbridges.num = i;
> +	amd_northbridges.num = misc_count;
>  
>  	link = misc = root = NULL;
> -	for (i = 0; i != amd_northbridges.num; i++) {
> +	for (i = 0; i < amd_northbridges.num; i++) {
>  		node_to_amd_nb(i)->root = root =
>  			next_northbridge(root, root_ids);
>  		node_to_amd_nb(i)->misc = misc =
>  			next_northbridge(misc, misc_ids);
>  		node_to_amd_nb(i)->link = link =
>  			next_northbridge(link, link_ids);
> +
> +		/*
> +		 * If there are more root devices than data fabric/SMN,
> +		 * interfaces, then the root devices per DF/SMN
> +		 * interface are redundant and N-1 should be skipped so
> +		 * they aren't mapped incorrectly.
> +		 */

This text is trying to explain it a bit better but you still still need
to specify which are the redundant ones. All N-1 or is there a special
root device through which the DF/SMN gets accessed or?

Thx.

-- 
Regards/Gruss,
    Boris.

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