linux-kernel - Re: [PATCH v3] PCI: aardvark: Use LTSSM state to build link training flag

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Message-ID: <20190930154017.GF42880@e119886-lin.cambridge.arm.com>
Date:   Mon, 30 Sep 2019 16:40:18 +0100
From:   Andrew Murray <andrew.murray@....com>
To:     Remi Pommarel <repk@...plefau.lt>
Cc:     Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ellie Reeves <ellierevves@...il.com>,
        linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] PCI: aardvark: Use LTSSM state to build link training
 flag

On Wed, May 22, 2019 at 11:33:51PM +0200, Remi Pommarel wrote:
> Aardvark's PCI_EXP_LNKSTA_LT flag in its link status register is not
> implemented and does not reflect the actual link training state (the
> flag is always set to 0). In order to support link re-training feature
> this flag has to be emulated. The Link Training and Status State
> Machine (LTSSM) flag in Aardvark 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 after asking a link retraining it takes a while
> for the LTSSM state to become less than 0x10 (due to L0s to recovery
> state transition delays), LTSSM can still be in L0 while link training
> has not finished yet. So this waits for link to be in recovery or lesser
> state before returning after asking for a link retrain.
> 
> [1] "PCI Express Base Specification", REV. 4.0
>     PCI Express, February 19 2014, Table 4-14
> 
> Signed-off-by: Remi Pommarel <repk@...plefau.lt>
> ---
> Changes since v1:
>   - Rename retraining flag field
>   - Fix DEVCTL register writing
> 
> Changes since v2:
>   - Rewrite patch logic so it is more legible
> 
> Please note that I will unlikely be able to answer any comments from May
> 24th to June 10th.
> ---
>  drivers/pci/controller/pci-aardvark.c | 29 ++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/pci-aardvark.c b/drivers/pci/controller/pci-aardvark.c
> index 134e0306ff00..8803083b2174 100644
> --- a/drivers/pci/controller/pci-aardvark.c
> +++ b/drivers/pci/controller/pci-aardvark.c
> @@ -180,6 +180,8 @@
>  #define LINK_WAIT_MAX_RETRIES		10
>  #define LINK_WAIT_USLEEP_MIN		90000
>  #define LINK_WAIT_USLEEP_MAX		100000
> +#define RETRAIN_WAIT_MAX_RETRIES	10
> +#define RETRAIN_WAIT_USLEEP_US		2000
>  
>  #define MSI_IRQ_NUM			32
>  
> @@ -239,6 +241,17 @@ static int advk_pcie_wait_for_link(struct advk_pcie *pcie)
>  	return -ETIMEDOUT;
>  }
>  
> +static void advk_pcie_wait_for_retrain(struct advk_pcie *pcie)
> +{
> +	size_t retries;
> +
> +	for (retries = 0; retries < RETRAIN_WAIT_MAX_RETRIES; ++retries) {
> +		if (!advk_pcie_link_up(pcie))
> +			break;
> +		udelay(RETRAIN_WAIT_USLEEP_US);
> +	}
> +}
> +
>  static void advk_pcie_setup_hw(struct advk_pcie *pcie)
>  {
>  	u32 reg;
> @@ -426,11 +439,20 @@ advk_pci_bridge_emul_pcie_conf_read(struct pci_bridge_emul *bridge,
>  		return PCI_BRIDGE_EMUL_HANDLED;
>  	}
>  
> +	case PCI_EXP_LNKCTL: {
> +		/* u32 contains both PCI_EXP_LNKCTL and PCI_EXP_LNKSTA */
> +		u32 val = advk_readl(pcie, PCIE_CORE_PCIEXP_CAP + reg) &
> +			~(PCI_EXP_LNKSTA_LT << 16);

The commit message says "the flag is always set to 0" - therefore I guess
you don't *need* to mask out the LT bit here? I assume this is just
belt-and-braces but thought I'd check incase I've misunderstood or if your
commit message is inaccurate.

In any case masking out the bit (or adding a comment) makes this code more
readable as the reader doesn't need to know what the hardware does with this
bit.


> +		if (!advk_pcie_link_up(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 +469,13 @@ 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;

Why is this here?

Thanks,

Andrew Murray

> +
>  	case PCI_EXP_LNKCTL:
>  		advk_writel(pcie, new, PCIE_CORE_PCIEXP_CAP + reg);
> +		if (new & PCI_EXP_LNKCTL_RL)
> +			advk_pcie_wait_for_retrain(pcie);
>  		break;
>  
>  	case PCI_EXP_RTCTL:
> -- 
> 2.20.1
>