Re: [PATCH 01/14] PCI/P2PDMA: Add constants for not-supported result upstream_bridge_distance()

[Koenig, Christian]

Am 23.07.19 um 01:08 schrieb Logan Gunthorpe:
> Add constant flags to indicate two devices are not supported or whether
> the data path goes through the host bridge instead of using the negative
> values -1 and -2.
>
> This helps annotate the code better, but the main reason is so we
> can cache the result in an xarray which does not allow us to store
> negative values.
>
> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>

Reviewed-by: Christian KÃnig <christian.koenig@xxxxxxx>

> ---
>   drivers/pci/p2pdma.c | 52 ++++++++++++++++++++++++++------------------
>   1 file changed, 31 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 234476226529..e8ec86e1dd00 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -273,6 +273,20 @@ static bool root_complex_whitelist(struct pci_dev *dev)
>   	return false;
>   }
>   
> +enum {
> +	/*
> +	 * Thes arbitrary offset are or'd onto the upstream distance
> +	 * calculation for the following conditions:
> +	 */
> +
> +	/* The data path includes the host-bridge */
> +	P2PDMA_THRU_HOST_BRIDGE		= 0x02000000,
> +	/* The data path is forced through the host-bridge due to ACS */
> +	P2PDMA_ACS_FORCES_UPSTREAM	= 0x04000000,
> +	/* The data path is not supported by P2PDMA */
> +	P2PDMA_NOT_SUPPORTED		= 0x08000000,
> +};
> +
>   /*
>    * Find the distance through the nearest common upstream bridge between
>    * two PCI devices.
> @@ -297,22 +311,17 @@ static bool root_complex_whitelist(struct pci_dev *dev)
>    * port of the switch, to the common upstream port, back up to the second
>    * downstream port and then to Device B.
>    *
> - * Any two devices that don't have a common upstream bridge will return -1.
> - * In this way devices on separate PCIe root ports will be rejected, which
> - * is what we want for peer-to-peer seeing each PCIe root port defines a
> - * separate hierarchy domain and there's no way to determine whether the root
> - * complex supports forwarding between them.
> + * Any two devices that cannot communicate using p2pdma will return the distance
> + * with the flag P2PDMA_NOT_SUPPORTED.
>    *
> - * In the case where two devices are connected to different PCIe switches,
> - * this function will still return a positive distance as long as both
> - * switches eventually have a common upstream bridge. Note this covers
> - * the case of using multiple PCIe switches to achieve a desired level of
> - * fan-out from a root port. The exact distance will be a function of the
> - * number of switches between Device A and Device B.
> + * Any two devices that have a data path that goes through the host bridge
> + * will consult a whitelist. If the host bridges are on the whitelist,
> + * then the distance will be returned with the flag P2PDMA_THRU_HOST_BRIDGE set.
> + * If either bridge is not on the whitelist, the flag P2PDMA_NOT_SUPPORTED will
> + * be set.
>    *
>    * If a bridge which has any ACS redirection bits set is in the path
> - * then this functions will return -2. This is so we reject any
> - * cases where the TLPs are forwarded up into the root complex.
> + * then this functions will flag the result with P2PDMA_ACS_FORCES_UPSTREAM.
>    * In this case, a list of all infringing bridge addresses will be
>    * populated in acs_list (assuming it's non-null) for printk purposes.
>    */
> @@ -359,9 +368,9 @@ static int upstream_bridge_distance(struct pci_dev *provider,
>   	 */
>   	if (root_complex_whitelist(provider) &&
>   	    root_complex_whitelist(client))
> -		return 0x1000 + dist_a + dist_b;
> +		return (dist_a + dist_b) | P2PDMA_THRU_HOST_BRIDGE;
>   
> -	return -1;
> +	return (dist_a + dist_b) | P2PDMA_NOT_SUPPORTED;
>   
>   check_b_path_acs:
>   	bb = b;
> @@ -379,7 +388,7 @@ static int upstream_bridge_distance(struct pci_dev *provider,
>   	}
>   
>   	if (acs_cnt)
> -		return -2;
> +		return P2PDMA_NOT_SUPPORTED | P2PDMA_ACS_FORCES_UPSTREAM;
>   
>   	return dist_a + dist_b;
>   }
> @@ -395,16 +404,17 @@ static int upstream_bridge_distance_warn(struct pci_dev *provider,
>   		return -ENOMEM;
>   
>   	ret = upstream_bridge_distance(provider, client, &acs_list);
> -	if (ret == -2) {
> -		pci_warn(client, "cannot be used for peer-to-peer DMA as ACS redirect is set between the client and provider (%s)\n",
> +	if (ret & P2PDMA_ACS_FORCES_UPSTREAM) {
> +		pci_warn(client, "ACS redirect is set between the client and provider (%s)\n",
>   			 pci_name(provider));
>   		/* Drop final semicolon */
>   		acs_list.buffer[acs_list.len-1] = 0;
>   		pci_warn(client, "to disable ACS redirect for this path, add the kernel parameter: pci=disable_acs_redir=%s\n",
>   			 acs_list.buffer);
> +	}
>   
> -	} else if (ret < 0) {
> -		pci_warn(client, "cannot be used for peer-to-peer DMA as the client and provider (%s) do not share an upstream bridge\n",
> +	if (ret & P2PDMA_NOT_SUPPORTED) {
> +		pci_warn(client, "cannot be used for peer-to-peer DMA as the client and provider (%s) do not share an upstream bridge or whitelisted host bridge\n",
>   			 pci_name(provider));
>   	}
>   
> @@ -468,7 +478,7 @@ int pci_p2pdma_distance_many(struct pci_dev *provider, struct device **clients,
>   
>   		pci_dev_put(pci_client);
>   
> -		if (ret < 0)
> +		if (ret & P2PDMA_NOT_SUPPORTED)
>   			not_supported = true;
>   
>   		if (not_supported && !verbose)