Re: [PATCH v3 02/11] PCI/bwctrl: Add support to scale bandwidth before & after link re-training

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

On Mon, 19 May 2025, Krishna Chaitanya Chundru wrote:

> If the driver wants to move to higher data rate/speed than the current data
> rate then the controller driver may need to change certain votes so that
> link may come up at requested data rate/speed like QCOM PCIe controllers
> need to change their RPMh (Resource Power Manager-hardened) state. Once
> link retraining is done controller drivers needs to adjust their votes
> based on the final data rate.
> 
> Some controllers also may need to update their bandwidth voting like
> ICC bw votings etc.
> 
> So, add pre_scale_bus_bw() & post_scale_bus_bw() op to call before & after
> the link re-train. There is no explicit locking mechanisms as these are
> called by a single client endpoint driver.
> 
> In case of PCIe switch, if there is a request to change target speed for a
> downstream port then no need to call these function ops as these are
> outside the scope of the controller drivers.
> 
> Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
> ---
>  drivers/pci/pcie/bwctrl.c | 15 +++++++++++++++
>  include/linux/pci.h       | 14 ++++++++++++++
>  2 files changed, 29 insertions(+)
> 
> diff --git a/drivers/pci/pcie/bwctrl.c b/drivers/pci/pcie/bwctrl.c
> index d8d2aa85a22928b99c5bba1d2bcc5647c0edeeb6..3525bc0cd10f1dd7794abbe84ccb10e2c53a10af 100644
> --- a/drivers/pci/pcie/bwctrl.c
> +++ b/drivers/pci/pcie/bwctrl.c
> @@ -161,6 +161,8 @@ static int pcie_bwctrl_change_speed(struct pci_dev *port, u16 target_speed, bool
>  int pcie_set_target_speed(struct pci_dev *port, enum pci_bus_speed speed_req,
>  			  bool use_lt)
>  {
> +	struct pci_host_bridge *host = pci_find_host_bridge(port->bus);
> +	bool is_rootbus = pci_is_root_bus(port->bus);
>  	struct pci_bus *bus = port->subordinate;
>  	u16 target_speed;
>  	int ret;
> @@ -173,6 +175,16 @@ int pcie_set_target_speed(struct pci_dev *port, enum pci_bus_speed speed_req,
>  
>  	target_speed = pcie_bwctrl_select_speed(port, speed_req);
>  
> +	/*
> +	 * The host bridge driver may need to be scaled for targeted speed
> +	 * otherwise link might not come up at requested speed.
> +	 */
> +	if (is_rootbus && host->pre_scale_bus_bw) {
> +		ret = host->pre_scale_bus_bw(host, port, target_speed);
> +		if (ret)
> +			return ret;
> +	}
> +
>  	scoped_guard(rwsem_read, &pcie_bwctrl_setspeed_rwsem) {
>  		struct pcie_bwctrl_data *data = port->link_bwctrl;
>  
> @@ -197,6 +209,9 @@ int pcie_set_target_speed(struct pci_dev *port, enum pci_bus_speed speed_req,
>  	    !list_empty(&bus->devices))
>  		ret = -EAGAIN;
>  
> +	if (bus && is_rootbus && host->post_scale_bus_bw)
> +		host->post_scale_bus_bw(host, port, pci_bus_speed2lnkctl2(bus->cur_bus_speed));
> +
>  	return ret;
>  }
>  
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 51e2bd6405cda5acc33d268bbe1d491b145e083f..7eb0856ba0ed20bd1336683b68add124c7483902 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -601,6 +601,20 @@ struct pci_host_bridge {
>  	void (*release_fn)(struct pci_host_bridge *);
>  	int (*enable_device)(struct pci_host_bridge *bridge, struct pci_dev *dev);
>  	void (*disable_device)(struct pci_host_bridge *bridge, struct pci_dev *dev);
> +	/*
> +	 * Callback to the host bridge drivers to update ICC bw votes, clock frequencies etc

BW

> +	 * for the link re-train to come up in targeted speed. These are intended to be
> +	 * called by devices directly attached to the root port. These are called by a single

Root Port

> +	 * client endpoint driver, so there is no need for explicit locking mechanisms.

Endpoint

> +	 */
> +	int (*pre_scale_bus_bw)(struct pci_host_bridge *bridge, struct pci_dev *dev, int speed);
> +	/*
> +	 * Callback to the host bridge drivers to adjust ICC bw votes, clock frequencies etc
> +	 * to the updated speed after link re-train. These are intended to be called by
> +	 * devices directly attached to the root port. These are called by a single client
> +	 * endpoint driver, so there is no need for explicit locking mechanisms.
> +	 */

Please fold comments to 80 characters.

> +	void (*post_scale_bus_bw)(struct pci_host_bridge *bridge, struct pci_dev *dev, int speed);

I still don't like the names. Maybe simply pre/post_link_speed_change 
would sound more generic.

Not a show-stopper but the current name sounds pretty esoteric.

>  	void		*release_data;
>  	unsigned int	ignore_reset_delay:1;	/* For entire hierarchy */
>  	unsigned int	no_ext_tags:1;		/* No Extended Tags */
> 
> 

-- 
 i.