Re: [PATCH V6] PCI: rcar: Add L1 link state fix into data abort hook

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, May 14, 2021 at 10:05:49PM +0200, marek.vasut@gmail.com wrote:
> From: Marek Vasut <marek.vasut+renesas@gmail.com>
> 
> The R-Car PCIe controller is capable of handling L0s/L1 link states.
> While the controller can enter and exit L0s link state, and exit L1
> link state, without any additional action from the driver, to enter
> L1 link state, the driver must complete the link state transition by
> issuing additional commands to the controller.
> 
> The problem is, this transition is not atomic. The controller sets
> PMEL1RX bit in PMSR register upon reception of PM_ENTER_L1 DLLP from
> the PCIe card, but then the controller enters some sort of inbetween
> state. The driver must detect this condition and complete the link
> state transition, by setting L1IATN bit in PMCTLR and waiting for
> the link state transition to complete.
> 
> If a PCIe access happens inside this window, where the controller
> is between L0 and L1 link states, the access generates a fault and
> the ARM 'imprecise external abort' handler is invoked.
> 
> Just like other PCI controller drivers, here we hook the fault handler,
> perform the fixup to help the controller enter L1 link state, and then
> restart the instruction which triggered the fault. Since the controller
> is in L1 link state now, the link can exit from L1 link state to L0 and
> successfully complete the access.
> 
> While it was suggested to disable L1 link state support completely on
> the controller level, this would not prevent the L1 link state entry
> initiated by the link partner. This happens e.g. in case a PCIe card
> enters D3Hot state, which could be initiated from pci_set_power_state()
> if the card indicates D3Hot support, which in turn means link must enter
> L1 state. So instead, fix up the L1 link state after all.
> 
> Note that this fixup is applicable only to Aarch32 R-Car controllers,
> the Aarch64 R-Car perform the same fixup in TFA, see TFA commit [1]
> 0969397f2 ("rcar_gen3: plat: Prevent PCIe hang during L1X config access")
> [1] https://github.com/ARM-software/arm-trusted-firmware/commit/0969397f295621aa26b3d14b76dd397d22be58bf

This patch is horribly ugly but it's working around a horrible
hardware problem, and I don't have any better suggestions, so I guess
we don't really have much choice.

I do think the commit log is a bit glib:

  - "The R-Car PCIe controller is capable of handling L0s/L1 link
    states."  AFAICT every PCIe device is required to handle L0 and L1
    without software assistance.  So saying R-Car is "capable" puts a
    better face on this than seems warranted.

    L0s doesn't seem relevant at all; at least it doesn't seem to play
    a role in the patch.  There's no such thing as "returning to L0s"
    as mentioned in the comment below; L0s is only reachable from L0.
    Returns from L1 only go to L0 (PCIe r5.0, fig 5-1).

  - "The problem is, this transition is not atomic."  I think the
    *problem* is the hardware is broken in the first place.  This
    transition is supposed to be invisible to software.

  - "Just like other PCI controller drivers ..." suggests that this is
    an ordinary situation that we shouldn't be concerned about.  This
    patch may be the best we can do to work around a bad hardware
    defect, but it's definitely not ordinary.

    I think the other hook_fault_code() uses are for reporting
    legitimate PCIe errors, which most controllers log and turn
    into ~0 data responses without generating an abort or machine
    check, not things caused by hardware defects, so they're not
    really comparable.

Has Renesas documented this as an erratum?  Will future devices
require additions to rcar_pcie_abort_handler_of_match[]?

It'd be nice if the commit log mentioned the user-visible effect of
this problem.  I guess it does mention external aborts -- I assume you
see those when downstream devices go to D3hot or when ASPM puts the
link in L1?  And the abort results in a reboot?

To be clear, I'm not objecting to the patch.  It's a hardware problem
and we should work around it as best we can.

> Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
> Cc: Bjorn Helgaas <bhelgaas@google.com>
> Cc: Geert Uytterhoeven <geert+renesas@glider.be>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Cc: Wolfram Sang <wsa@the-dreams.de>
> Cc: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
> Cc: linux-renesas-soc@vger.kernel.org
> ---
> V2: - Update commit message, add link to TFA repository commit
>     - Handle the LPAE case as in ARM fault.c and fsr-{2,3}level.c
>     - Cache clock and check whether they are enabled before register
>       access
> V3: - Fix commit message according to spellchecker
>     - Use of_find_matching_node() to apply hook only on Gen1 and Gen2 RCar
>       (in case the kernel is multiplatform)
> V4: - Mark rcar_pcie_abort_handler_of_match with __initconst
> V5: - Add mutex around rcar_pcie_aarch32_abort_handler()
>     - Update commit message again to point out issues with L1/D3Hot states
> V6: - Return 1 only if condition cannot be fixed
> ---
>  drivers/pci/controller/pcie-rcar-host.c | 84 +++++++++++++++++++++++++
>  drivers/pci/controller/pcie-rcar.h      |  7 +++
>  2 files changed, 91 insertions(+)
> 
> diff --git a/drivers/pci/controller/pcie-rcar-host.c b/drivers/pci/controller/pcie-rcar-host.c
> index 765cf2b45e24..0d3f8dc5ff8a 100644
> --- a/drivers/pci/controller/pcie-rcar-host.c
> +++ b/drivers/pci/controller/pcie-rcar-host.c
> @@ -13,6 +13,7 @@
>  
>  #include <linux/bitops.h>
>  #include <linux/clk.h>
> +#include <linux/clk-provider.h>
>  #include <linux/delay.h>
>  #include <linux/interrupt.h>
>  #include <linux/irq.h>
> @@ -41,6 +42,21 @@ struct rcar_msi {
>  	int irq2;
>  };
>  
> +#ifdef CONFIG_ARM
> +/*
> + * Here we keep a static copy of the remapped PCIe controller address.
> + * This is only used on aarch32 systems, all of which have one single
> + * PCIe controller, to provide quick access to the PCIe controller in
> + * the L1 link state fixup function, called from the ARM fault handler.
> + */
> +static void __iomem *pcie_base;
> +/*
> + * Static copy of bus clock pointer, so we can check whether the clock
> + * is enabled or not.
> + */
> +static struct clk *pcie_bus_clk;
> +#endif
> +
>  /* Structure representing the PCIe interface */
>  struct rcar_pcie_host {
>  	struct rcar_pcie	pcie;
> @@ -776,6 +792,12 @@ static int rcar_pcie_get_resources(struct rcar_pcie_host *host)
>  	}
>  	host->msi.irq2 = i;
>  
> +#ifdef CONFIG_ARM
> +	/* Cache static copy for L1 link state fixup hook on aarch32 */
> +	pcie_base = pcie->base;
> +	pcie_bus_clk = host->bus_clk;
> +#endif
> +
>  	return 0;
>  
>  err_irq2:
> @@ -1031,4 +1053,66 @@ static struct platform_driver rcar_pcie_driver = {
>  	},
>  	.probe = rcar_pcie_probe,
>  };
> +
> +#ifdef CONFIG_ARM
> +static DEFINE_SPINLOCK(pmsr_lock);
> +static int rcar_pcie_aarch32_abort_handler(unsigned long addr,
> +		unsigned int fsr, struct pt_regs *regs)
> +{
> +	unsigned long flags;
> +	int ret = 0;
> +	u32 pmsr;
> +
> +	spin_lock_irqsave(&pmsr_lock, flags);
> +
> +	if (!pcie_base || !__clk_is_enabled(pcie_bus_clk)) {
> +		ret = 1;
> +		goto unlock_exit;
> +	}
> +
> +	pmsr = readl(pcie_base + PMSR);
> +
> +	/*
> +	 * Test if the PCIe controller received PM_ENTER_L1 DLLP and
> +	 * the PCIe controller is not in L1 link state. If true, apply
> +	 * fix, which will put the controller into L1 link state, from
> +	 * which it can return to L0s/L0 on its own.
> +	 */
> +	if ((pmsr & PMEL1RX) && ((pmsr & PMSTATE) != PMSTATE_L1)) {
> +		writel(L1IATN, pcie_base + PMCTLR);
> +		while (!(readl(pcie_base + PMSR) & L1FAEG))
> +			;
> +		writel(L1FAEG | PMEL1RX, pcie_base + PMSR);
> +	}
> +
> +unlock_exit:
> +	spin_unlock_irqrestore(&pmsr_lock, flags);
> +	return ret;
> +}
> +
> +static const struct of_device_id rcar_pcie_abort_handler_of_match[] __initconst = {
> +	{ .compatible = "renesas,pcie-r8a7779" },
> +	{ .compatible = "renesas,pcie-r8a7790" },
> +	{ .compatible = "renesas,pcie-r8a7791" },
> +	{ .compatible = "renesas,pcie-rcar-gen2" },
> +	{},
> +};
> +
> +static int __init rcar_pcie_init(void)
> +{
> +	if (of_find_matching_node(NULL, rcar_pcie_abort_handler_of_match)) {
> +#ifdef CONFIG_ARM_LPAE
> +		hook_fault_code(17, rcar_pcie_aarch32_abort_handler, SIGBUS, 0,
> +				"asynchronous external abort");
> +#else
> +		hook_fault_code(22, rcar_pcie_aarch32_abort_handler, SIGBUS, 0,
> +				"imprecise external abort");
> +#endif
> +	}
> +
> +	return platform_driver_register(&rcar_pcie_driver);
> +}
> +device_initcall(rcar_pcie_init);
> +#else
>  builtin_platform_driver(rcar_pcie_driver);
> +#endif
> diff --git a/drivers/pci/controller/pcie-rcar.h b/drivers/pci/controller/pcie-rcar.h
> index d4c698b5f821..9bb125db85c6 100644
> --- a/drivers/pci/controller/pcie-rcar.h
> +++ b/drivers/pci/controller/pcie-rcar.h
> @@ -85,6 +85,13 @@
>  #define  LTSMDIS		BIT(31)
>  #define  MACCTLR_INIT_VAL	(LTSMDIS | MACCTLR_NFTS_MASK)
>  #define PMSR			0x01105c
> +#define  L1FAEG			BIT(31)
> +#define  PMEL1RX		BIT(23)
> +#define  PMSTATE		GENMASK(18, 16)
> +#define  PMSTATE_L1		(3 << 16)
> +#define PMCTLR			0x011060
> +#define  L1IATN			BIT(31)
> +
>  #define MACS2R			0x011078
>  #define MACCGSPSETR		0x011084
>  #define  SPCNGRSN		BIT(31)
> -- 
> 2.30.2
>