Re: [PATCH 5/6] PCI: mvebu: Add support for sending Set_Slot_Power_Limit message

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Feb 22, 2022 at 05:31:57PM +0100, Pali Rohár wrote:
> This PCIe message is sent automatically by mvebu HW when link changes
> status from down to up.

I *think* the intent of the above is:

  If DT supplies the 'slot-power-limit-milliwatt' property, program
  the value in the Slot Power Limit in the Slot Capabilities register
  and program the Root Port to send a Set_Slot_Power_Limit Message
  when the Link transitions to DL_Up.

PCIe r6.0, sec 2.2.8.5 and 7.5.3.9, also say Set_Slot_Power_Limit must
be sent on a config write to Slot Capabilities.  I don't really
understand that, since AFAICS, everything in that register is
read-only.  But there must be some use case for forcing a message.

> Slot power limit is read from DT property 'slot-power-limit-milliwatt' and
> set to mvebu HW. When this DT property is not specified then driver treat
> it as "Slot Capabilities register has not yet been initialized".

Does this last sentence mean "the Slot Power Limit is set to 0W
(Value = 00h and Scale = 00b = 1.0x) and Auto Slot Power Limit Disable
is set, so Set_Slot_Power_Limit Messages will never be sent"?

> Signed-off-by: Pali Rohár <pali@kernel.org>
> ---
>  drivers/pci/controller/pci-mvebu.c | 85 ++++++++++++++++++++++++++++--
>  1 file changed, 80 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/pci/controller/pci-mvebu.c b/drivers/pci/controller/pci-mvebu.c
> index a75d2b9196f9..c786feec4d17 100644
> --- a/drivers/pci/controller/pci-mvebu.c
> +++ b/drivers/pci/controller/pci-mvebu.c
> @@ -66,6 +66,12 @@
>  #define  PCIE_STAT_BUS                  0xff00
>  #define  PCIE_STAT_DEV                  0x1f0000
>  #define  PCIE_STAT_LINK_DOWN		BIT(0)
> +#define PCIE_SSPL_OFF		0x1a0c
> +#define  PCIE_SSPL_VALUE_SHIFT		0
> +#define  PCIE_SSPL_VALUE_MASK		GENMASK(7, 0)
> +#define  PCIE_SSPL_SCALE_SHIFT		8
> +#define  PCIE_SSPL_SCALE_MASK		GENMASK(9, 8)
> +#define  PCIE_SSPL_ENABLE		BIT(16)
>  #define PCIE_RC_RTSTA		0x1a14
>  #define PCIE_DEBUG_CTRL         0x1a60
>  #define  PCIE_DEBUG_SOFT_RESET		BIT(20)
> @@ -111,6 +117,8 @@ struct mvebu_pcie_port {
>  	struct mvebu_pcie_window iowin;
>  	u32 saved_pcie_stat;
>  	struct resource regs;
> +	u8 slot_power_limit_value;
> +	u8 slot_power_limit_scale;
>  	struct irq_domain *intx_irq_domain;
>  	raw_spinlock_t irq_lock;
>  	int intx_irq;
> @@ -239,7 +247,7 @@ static void mvebu_pcie_setup_wins(struct mvebu_pcie_port *port)
>  
>  static void mvebu_pcie_setup_hw(struct mvebu_pcie_port *port)
>  {
> -	u32 ctrl, lnkcap, cmd, dev_rev, unmask;
> +	u32 ctrl, lnkcap, cmd, dev_rev, unmask, sspl;
>  
>  	/* Setup PCIe controller to Root Complex mode. */
>  	ctrl = mvebu_readl(port, PCIE_CTRL_OFF);
> @@ -292,6 +300,20 @@ static void mvebu_pcie_setup_hw(struct mvebu_pcie_port *port)
>  	/* Point PCIe unit MBUS decode windows to DRAM space. */
>  	mvebu_pcie_setup_wins(port);
>  
> +	/*
> +	 * Program Root Complex to automatically sends Set Slot Power Limit
> +	 * PCIe Message when changing status from Dl-Down to Dl-Up and valid
> +	 * slot power limit was specified.

s/Root Complex/Root Port/, right?  AFAIK the message would be sent by
a Downstream Port, i.e., a Root Port in this case.

s/sends/send/
s/Set Slot Power Limit/Set_Slot_Power_Limit/ to match spec usage (also
below)
s/Dl-Down/DL_Down/ to match spec usage
s/Dl-Up/DL_Up/ ditto

> +	 */
> +	sspl = mvebu_readl(port, PCIE_SSPL_OFF);
> +	sspl &= ~(PCIE_SSPL_VALUE_MASK | PCIE_SSPL_SCALE_MASK | PCIE_SSPL_ENABLE);
> +	if (port->slot_power_limit_value && port->slot_power_limit_scale) {
> +		sspl |= port->slot_power_limit_value << PCIE_SSPL_VALUE_SHIFT;
> +		sspl |= port->slot_power_limit_scale << PCIE_SSPL_SCALE_SHIFT;
> +		sspl |= PCIE_SSPL_ENABLE;
> +	}
> +	mvebu_writel(port, sspl, PCIE_SSPL_OFF);
> +
>  	/* Mask all interrupt sources. */
>  	mvebu_writel(port, ~PCIE_INT_ALL_MASK, PCIE_INT_UNMASK_OFF);
>  
> @@ -628,9 +650,16 @@ mvebu_pci_bridge_emul_pcie_conf_read(struct pci_bridge_emul *bridge,
>  			  (PCI_EXP_LNKSTA_DLLLA << 16) : 0);
>  		break;
>  
> -	case PCI_EXP_SLTCTL:
> -		*value = PCI_EXP_SLTSTA_PDS << 16;
> +	case PCI_EXP_SLTCTL: {
> +		u16 slotsta = le16_to_cpu(bridge->pcie_conf.slotsta);
> +		u32 val = 0;
> +		if (!(mvebu_readl(port, PCIE_SSPL_OFF) & PCIE_SSPL_ENABLE))
> +			val |= PCI_EXP_SLTCTL_ASPL_DISABLE;
> +		/* PCI_EXP_SLTCTL is 32-bit and contains also slot status bits */

This comment is a little bit confusing because PCI_EXP_SLTCTL is not
actually 32 bits; it's 16 bits.  It's just that we "read" 32 bits,
which includes both PCI_EXP_SLTCTL and PCI_EXP_SLTSTA.  If you use
"PCI_EXP_SLTCTL", I think it would be helpful to also say
"PCI_EXP_SLTSTA" instead of "slot status bits".

> +		val |= slotsta << 16;
> +		*value = val;
>  		break;
> +	}
>  
>  	case PCI_EXP_RTSTA:
>  		*value = mvebu_readl(port, PCIE_RC_RTSTA);
> @@ -774,6 +803,19 @@ mvebu_pci_bridge_emul_pcie_conf_write(struct pci_bridge_emul *bridge,
>  		mvebu_writel(port, new, PCIE_CAP_PCIEXP + PCI_EXP_LNKCTL);
>  		break;
>  
> +	case PCI_EXP_SLTCTL:
> +		if ((mask & PCI_EXP_SLTCTL_ASPL_DISABLE) &&
> +		    port->slot_power_limit_value &&
> +		    port->slot_power_limit_scale) {
> +			u32 sspl = mvebu_readl(port, PCIE_SSPL_OFF);
> +			if (new & PCI_EXP_SLTCTL_ASPL_DISABLE)
> +				sspl &= ~PCIE_SSPL_ENABLE;
> +			else
> +				sspl |= PCIE_SSPL_ENABLE;
> +			mvebu_writel(port, sspl, PCIE_SSPL_OFF);

IIUC, the behavior of PCI_EXP_SLTCTL_ASPL_DISABLE as observed by
software that sets it and reads it back will depend on whether the DT
contains "slot-power-limit-milliwatt".

If there is no DT property, port->slot_power_limit_value will be zero
and PCIE_SSPL_ENABLE will never be set.  So if I clear
PCI_EXP_SLTCTL_ASPL_DISABLE, then read it back, it looks like it will
read as being set.  That's not what I would expect from the spec
(PCIe r6.0, sec 7.5.3.10).

> +		}
> +		break;
> +
>  	case PCI_EXP_RTSTA:
>  		/*
>  		 * PME Status bit in Root Status Register (PCIE_RC_RTSTA)
> @@ -868,8 +910,26 @@ static int mvebu_pci_bridge_emul_init(struct mvebu_pcie_port *port)
>  	/*
>  	 * Older mvebu hardware provides PCIe Capability structure only in
>  	 * version 1. New hardware provides it in version 2.
> +	 * Enable slot support which is emulated.
>  	 */
> -	bridge->pcie_conf.cap = cpu_to_le16(pcie_cap_ver);
> +	bridge->pcie_conf.cap = cpu_to_le16(pcie_cap_ver | PCI_EXP_FLAGS_SLOT);
> +
> +	/*
> +	 * Set Presence Detect State bit permanently as there is no support for
> +	 * unplugging PCIe card from the slot. Assume that PCIe card is always
> +	 * connected in slot.
> +	 *
> +	 * Set physical slot number to port+1 as mvebu ports are indexed from
> +	 * zero and zero value is reserved for ports within the same silicon
> +	 * as Root Port which is not mvebu case.
> +	 *
> +	 * Also set correct slot power limit.
> +	 */
> +	bridge->pcie_conf.slotcap = cpu_to_le32(
> +		(port->slot_power_limit_value << PCI_EXP_SLTCAP_SPLV_SHIFT) |
> +		(port->slot_power_limit_scale << PCI_EXP_SLTCAP_SPLS_SHIFT) |
> +		((port->port+1) << PCI_EXP_SLTCAP_PSN_SHIFT));
> +	bridge->pcie_conf.slotsta = cpu_to_le16(PCI_EXP_SLTSTA_PDS);
>  
>  	bridge->subsystem_vendor_id = ssdev_id & 0xffff;
>  	bridge->subsystem_id = ssdev_id >> 16;
> @@ -1191,6 +1251,7 @@ static int mvebu_pcie_parse_port(struct mvebu_pcie *pcie,
>  {
>  	struct device *dev = &pcie->pdev->dev;
>  	enum of_gpio_flags flags;
> +	u32 slot_power_limit;
>  	int reset_gpio, ret;
>  	u32 num_lanes;
>  
> @@ -1291,6 +1352,15 @@ static int mvebu_pcie_parse_port(struct mvebu_pcie *pcie,
>  		port->reset_gpio = gpio_to_desc(reset_gpio);
>  	}
>  
> +	slot_power_limit = of_pci_get_slot_power_limit(child,
> +				&port->slot_power_limit_value,
> +				&port->slot_power_limit_scale);
> +	if (slot_power_limit)
> +		dev_info(dev, "%s: Slot power limit %u.%uW\n",
> +			 port->name,
> +			 slot_power_limit / 1000,
> +			 (slot_power_limit / 100) % 10);
> +
>  	port->clk = of_clk_get_by_name(child, NULL);
>  	if (IS_ERR(port->clk)) {
>  		dev_err(dev, "%s: cannot get clock\n", port->name);
> @@ -1587,7 +1657,7 @@ static int mvebu_pcie_remove(struct platform_device *pdev)
>  {
>  	struct mvebu_pcie *pcie = platform_get_drvdata(pdev);
>  	struct pci_host_bridge *bridge = pci_host_bridge_from_priv(pcie);
> -	u32 cmd;
> +	u32 cmd, sspl;
>  	int i;
>  
>  	/* Remove PCI bus with all devices. */
> @@ -1624,6 +1694,11 @@ static int mvebu_pcie_remove(struct platform_device *pdev)
>  		/* Free config space for emulated root bridge. */
>  		pci_bridge_emul_cleanup(&port->bridge);
>  
> +		/* Disable sending Set Slot Power Limit PCIe Message. */
> +		sspl = mvebu_readl(port, PCIE_SSPL_OFF);
> +		sspl &= ~(PCIE_SSPL_VALUE_MASK | PCIE_SSPL_SCALE_MASK | PCIE_SSPL_ENABLE);
> +		mvebu_writel(port, sspl, PCIE_SSPL_OFF);
> +
>  		/* Disable and clear BARs and windows. */
>  		mvebu_pcie_disable_wins(port);
>  
> -- 
> 2.20.1
>