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Date:   Thu, 25 Apr 2019 16:23:53 +0200
From:   Remi Pommarel <repk@triplefau.lt>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc:     Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Ellie Reeves <ellierevves@gmail.com>,
        linux-pci@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] PCI: aardvark: Use LTSSM state to build link training
 flag
Message-ID: <20190425142353.GO2754@voidbox.localdomain>
References: <20190316161243.29517-1-repk@triplefau.lt>
 <20190425110830.GC10833@e121166-lin.cambridge.arm.com>
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Hi Lorenzo,

On Thu, Apr 25, 2019 at 12:08:30PM +0100, Lorenzo Pieralisi wrote:
> 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 ?

Not exactly, I am saying that PCI_EXP_LNKSTA_LT from PCI_EXP_LNKSTA
register can't be used with this hardware, but can be emulated with
LTSSM flag.

> 
> 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.

The problem here is that the hardware does not change PCI_EXP_LNKSTA_LT
at all. So in order to support link re-training feature we need to
emulate this flag. To do so LTSSM flag can be used.

Indeed we can set the emulated PCI_EXP_LNKSTA_LT as soon as re-training
is asked and wait for LTSSM flag to be back to a configured state
(e.g. L0, L0s) before clearing it.

The problem with that is that LTSSM flag does not change instantly after
link re-training has been asked, and will stay in configured state for a
small amount of time. So the idea is to poll the LTSSM flag and wait for
it to enter a recovery state then waiting for it to be back in
configured state.

The timeout is only here as a fallback in the unlikely event that we
missed the LTSSM flag entering recovery state.

> 
> 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.

Yes, sorry, it is a bit of a hack because I try to workaround a
hardware issue.

Please note that vendor has been contacted about this in the meantime
and answered the following:

"FW can poll LTSSM state equals any of the following values: 0xB or 0xD
or 0xC or 0xE. After that, polls for LTSSM equals 0x10. For your
information, LTSSM will transit from 0x10 -> 0xB -> 0xD -> 0xC or 0xE
........... -> 0x10".

It is basically what this patch does, I've just added a timeout fallback
to not poll LTSSM state forever if its transition to 0xB, 0xD, 0xC or
0xE has been missed.

-- 
Remi

> 
> 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
> >