Re: [PATCH v2] PCI: aardvark: Use LTSSM state to build link training flag

Lorenzo Pieralisi
2019-04-25T11:08:39Z

On Sat, Mar 16, 2019 at 05:12:43PM +0100, Remi Pommarel wrote:
> The PCI_EXP_LNKSTA_LT flag in the emulated root device's PCI_EXP_LNKSTA
> config register does not reflect the actual link training state and is
> always cleared. The Link Training and Status State Machine (LTSSM) flag
> in LMI config register could be used as a link training indicator.
> Indeed if the LTSSM is in L0 or upper state then link training has
> completed (see [1]).
>
> Unfortunately because setting the PCI_EXP_LINCTL_RL flag does not
> instantly imply a LTSSM state change (e.g. L0s to recovery state
> transition takes some time), LTSSM can be in L0 but link training has
> not finished yet. Thus a lower L0 LTSSM state followed by a L0 or upper
> state sequence has to be seen to be sure that link training has been
> done.

Hi Remi,

I am a bit confused, so you are saying that the LTSSM flag in the
LMI config register can't be used to detect when training is completed ?

Certainly it can't be used by ASPM core that relies on:

PCI_EXP_LNKSTA_LT flag

in the PCI_EXP_LNKSTA register, and that's what you are setting through
this timeout mechanism IIUC.

Please elaborate on that.

I am picking Bjorn's brain on this patch since what you are doing
seems quite arbitrary and honestly it is a bit of a hack.

I wonder whether it is not better to avoid advertising ASPM support.

Lorenzo

> Because one may not call a pcie conf register read on LNKSTA after
> doing a retrain link or may miss the link down state due to timing, a
> 20ms timeout is used. Passing this timeout link is considered retrained.
>
> This fixes boot hang or kernel panic with the following callstack due to
> ASPM setup doing a link re-train and polling for PCI_EXP_LNKSTA_LT flag
> to be cleared before using it.
>
> -------------------- 8< -------------------
> [ 0.915389] dump_backtrace+0x0/0x140
> [ 0.915391] show_stack+0x14/0x20
> [ 0.915393] dump_stack+0x90/0xb4
> [ 0.915394] panic+0x134/0x2c0
> [ 0.915396] nmi_panic+0x6c/0x70
> [ 0.915398] arm64_serror_panic+0x74/0x80
> [ 0.915400] is_valid_bugaddr+0x0/0x8
> [ 0.915402] el1_error+0x7c/0xe4
> [ 0.915404] advk_pcie_rd_conf+0x4c/0x250
> [ 0.915406] pci_bus_read_config_word+0x7c/0xd0
> [ 0.915408] pcie_capability_read_word+0x90/0xc8
> [ 0.915410] pcie_get_aspm_reg+0x68/0x118
> [ 0.915412] pcie_aspm_init_link_state+0x460/0xa98
> [ 0.915414] pci_scan_slot+0xe8/0x100
> [ 0.915416] pci_scan_child_bus_extend+0x50/0x288
> [ 0.915418] pci_scan_bridge_extend+0x348/0x4f0
> [ 0.915420] pci_scan_child_bus_extend+0x1dc/0x288
> [ 0.915423] pci_scan_root_bus_bridge+0xc4/0xe0
> [ 0.915424] pci_host_probe+0x14/0xa8
> [ 0.915426] advk_pcie_probe+0x838/0x910
> [...]
> -------------------- 8< -------------------
>
> [1] "PCI Express Base Specification", REV. 2.1
> PCI Express, March 4 2009, Table 4-7
>
> Signed-off-by: Remi Pommarel <repk@triplefau.lt>
> ---
> Changes since v1:
> - Rename retraining flag field
> - Fix DEVCTL register writing
> ---
> drivers/pci/controller/pci-aardvark.c | 33 ++++++++++++++++++++++++++-
> 1 file changed, 32 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pci/controller/pci-aardvark.c b/drivers/pci/controller/pci-aardvark.c
> index eb58dfdaba1b..47b707b5fc2c 100644
> --- a/drivers/pci/controller/pci-aardvark.c
> +++ b/drivers/pci/controller/pci-aardvark.c
> @@ -180,6 +180,7 @@
> #define LINK_WAIT_MAX_RETRIES 10
> #define LINK_WAIT_USLEEP_MIN 90000
> #define LINK_WAIT_USLEEP_MAX 100000
> +#define LINK_RETRAIN_DELAY_MAX (20 * HZ / 1000) /* 20 ms */
>
> #define MSI_IRQ_NUM 32
>
> @@ -199,6 +200,8 @@ struct advk_pcie {
> u16 msi_msg;
> int root_bus_nr;
> struct pci_bridge_emul bridge;
> + unsigned long rl_deadline; /* Retrain link jiffies deadline */
> + u8 rl_asked; /* Retraining has been asked and is in transition */
> };
>
> static inline void advk_writel(struct advk_pcie *pcie, u32 val, u64 reg)
> @@ -400,6 +403,19 @@ static int advk_pcie_wait_pio(struct advk_pcie *pcie)
> return -ETIMEDOUT;
> }
>
> +static int advk_pcie_link_retraining(struct advk_pcie *pcie)
> +{
> + if (!advk_pcie_link_up(pcie)) {
> + pcie->rl_asked = 0;
> + return 1;
> + }
> +
> + if (pcie->rl_asked && time_before(jiffies, pcie->rl_deadline))
> + return 1;
> +
> + pcie->rl_asked = 0;
> + return 0;
> +}
>
> static pci_bridge_emul_read_status_t
> advk_pci_bridge_emul_pcie_conf_read(struct pci_bridge_emul *bridge,
> @@ -426,11 +442,19 @@ advk_pci_bridge_emul_pcie_conf_read(struct pci_bridge_emul *bridge,
> return PCI_BRIDGE_EMUL_HANDLED;
> }
>
> + case PCI_EXP_LNKCTL: {
> + u32 val = advk_readl(pcie, PCIE_CORE_PCIEXP_CAP + reg) &
> + ~(PCI_EXP_LNKSTA_LT << 16);
> + if (advk_pcie_link_retraining(pcie))
> + val |= (PCI_EXP_LNKSTA_LT << 16);
> + *value = val;
> + return PCI_BRIDGE_EMUL_HANDLED;
> + }
> +
> case PCI_CAP_LIST_ID:
> case PCI_EXP_DEVCAP:
> case PCI_EXP_DEVCTL:
> case PCI_EXP_LNKCAP:
> - case PCI_EXP_LNKCTL:
> *value = advk_readl(pcie, PCIE_CORE_PCIEXP_CAP + reg);
> return PCI_BRIDGE_EMUL_HANDLED;
> default:
> @@ -447,8 +471,15 @@ advk_pci_bridge_emul_pcie_conf_write(struct pci_bridge_emul *bridge,
>
> switch (reg) {
> case PCI_EXP_DEVCTL:
> + advk_writel(pcie, new, PCIE_CORE_PCIEXP_CAP + reg);
> + break;
> +
> case PCI_EXP_LNKCTL:
> advk_writel(pcie, new, PCIE_CORE_PCIEXP_CAP + reg);
> + if (new & PCI_EXP_LNKCTL_RL) {
> + pcie->rl_asked = 1;
> + pcie->rl_deadline = jiffies + LINK_RETRAIN_DELAY_MAX;
> + }
> break;
>
> case PCI_EXP_RTCTL:
> --
> 2.20.1