Re: [PATCH v2] PCI: dw-rockchip: Delay link training after hot reset in EP mode

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jun 13, 2025 at 12:19:09PM +0200, Niklas Cassel wrote:
> From: Wilfred Mallawa <wilfred.mallawa@wdc.com>
> 
> RK3588 TRM, section "11.6.1.3.3 Hot Reset and Link-Down Reset" states that:
> """
> If you want to delay link re-establishment (after reset) so that you can
> reprogram some registers through DBI, you must set app_ltssm_enable =0
> immediately after core_rst_n as shown in above. This can be achieved by
> enable the app_dly2_en, and end-up the delay by assert app_dly2_done.
> """

Ugh.  Is """ some sort of markup?  There's a nice English convention
of indenting block quotes a couple spaces with no quote marks at all
that would work nicely here.

> I.e. setting app_dly2_en will automatically deassert app_ltssm_enable on
> a hot reset, and setting app_dly2_done will re-assert app_ltssm_enable,
> re-enabling link training.
> 
> When receiving a hot reset/link-down IRQ when running in EP mode, we will
> call dw_pcie_ep_linkdown(), which will call the .link_down() callback in
> the currently bound endpoint function (EPF) drivers.
> 
> The callback in an EPF driver can theoretically take a long time to
> complete, so make sure that the link is not re-established until after
> dw_pcie_ep_linkdown() (which calls the .link_down() callback(s)
> synchronously).

I don't know why we care *how long* EPF callbacks might take.  

From the TRM quote, it sounds like the important thing is that you
don't want the link to train before dw_pcie_ep_linkdown() calls
dw_pcie_ep_init_non_sticky_registers(), which looks like it programs
registers through DBI.

Maybe you also want to allow the EFP ->link_down() callbacks to also
program things via DBI before link training?  But I don't think the
amount of time they take is relevant.  If you need to do *anything*
via DBI before the link trains, you have to prevent training until
you're finished with DBI.

> Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
> Co-developed-by: Niklas Cassel <cassel@kernel.org>
> Signed-off-by: Niklas Cassel <cassel@kernel.org>
> ---
> Changes since v1:
> -Rebased on v6.16-rc1
> 
>  drivers/pci/controller/dwc/pcie-dw-rockchip.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-dw-rockchip.c b/drivers/pci/controller/dwc/pcie-dw-rockchip.c
> index 93171a392879..cd1e9352b21f 100644
> --- a/drivers/pci/controller/dwc/pcie-dw-rockchip.c
> +++ b/drivers/pci/controller/dwc/pcie-dw-rockchip.c
> @@ -58,6 +58,8 @@
>  
>  /* Hot Reset Control Register */
>  #define PCIE_CLIENT_HOT_RESET_CTRL	0x180
> +#define  PCIE_LTSSM_APP_DLY2_EN		BIT(1)
> +#define  PCIE_LTSSM_APP_DLY2_DONE	BIT(3)
>  #define  PCIE_LTSSM_ENABLE_ENHANCE	BIT(4)
>  
>  /* LTSSM Status Register */
> @@ -474,7 +476,7 @@ static irqreturn_t rockchip_pcie_ep_sys_irq_thread(int irq, void *arg)
>  	struct rockchip_pcie *rockchip = arg;
>  	struct dw_pcie *pci = &rockchip->pci;
>  	struct device *dev = pci->dev;
> -	u32 reg;
> +	u32 reg, val;
>  
>  	reg = rockchip_pcie_readl_apb(rockchip, PCIE_CLIENT_INTR_STATUS_MISC);
>  	rockchip_pcie_writel_apb(rockchip, reg, PCIE_CLIENT_INTR_STATUS_MISC);
> @@ -485,6 +487,10 @@ static irqreturn_t rockchip_pcie_ep_sys_irq_thread(int irq, void *arg)
>  	if (reg & PCIE_LINK_REQ_RST_NOT_INT) {
>  		dev_dbg(dev, "hot reset or link-down reset\n");
>  		dw_pcie_ep_linkdown(&pci->ep);
> +		/* Stop delaying link training. */
> +		val = HIWORD_UPDATE_BIT(PCIE_LTSSM_APP_DLY2_DONE);
> +		rockchip_pcie_writel_apb(rockchip, val,
> +					 PCIE_CLIENT_HOT_RESET_CTRL);
>  	}
>  
>  	if (reg & PCIE_RDLH_LINK_UP_CHGED) {
> @@ -566,8 +572,11 @@ static int rockchip_pcie_configure_ep(struct platform_device *pdev,
>  		return ret;
>  	}
>  
> -	/* LTSSM enable control mode */
> -	val = HIWORD_UPDATE_BIT(PCIE_LTSSM_ENABLE_ENHANCE);
> +	/*
> +	 * LTSSM enable control mode, and automatically delay link training on
> +	 * hot reset/link-down reset.
> +	 */
> +	val = HIWORD_UPDATE_BIT(PCIE_LTSSM_ENABLE_ENHANCE | PCIE_LTSSM_APP_DLY2_EN);
>  	rockchip_pcie_writel_apb(rockchip, val, PCIE_CLIENT_HOT_RESET_CTRL);
>  
>  	rockchip_pcie_writel_apb(rockchip, PCIE_CLIENT_EP_MODE,
> -- 
> 2.49.0
>