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

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Message-ID: <20190930165230.GA12568@voidbox>
Date:   Mon, 30 Sep 2019 18:52:30 +0200
From:   Remi Pommarel <repk@...plefau.lt>
To:     Andrew Murray <andrew.murray@....com>
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 Mon, Sep 30, 2019 at 04:40:18PM +0100, Andrew Murray wrote:
> 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.

Actually vendor eventually responded that the bit was reserved, but
during my tests it remains to 0.

So yes I am masking this out mainly for belt-and-braces and legibility.

> > +		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?
> 

Before PCI_EXP_DEVCTL and PCI_EXP_LNKCTL were doing the same thing, but
as now PCI_EXP_LNKCTL does extra things (i.e. wait for link to
successfully retrain), they do have different behaviours.

So this is here so PCI_EXP_DEVCTL keeps its old behaviour and do not
wait for link retrain in case an unrelated (PCI_EXP_LNKCTL_RL) bit is
set.

-- 
Remi

> > +
> >  	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
> >