Subject: Re: [PATCH 09/10] PCI: tegra: Add Tegra194 PCIe support

From: Thierry Reding <[email-masked]>
Date: Tue, 2 Apr 2019 16:14:24 +0200

On Tue, Apr 02, 2019 at 12:47:48PM +0530, Vidya Sagar wrote:
> On 3/30/2019 2:22 AM, Bjorn Helgaas wrote:
[...]
> > > +static int tegra_pcie_dw_host_init(struct pcie_port *pp)
> > > +{
[...]
> > > +	val_w = dw_pcie_readw_dbi(pci, CFG_LINK_STATUS);
> > > +	while (!(val_w & PCI_EXP_LNKSTA_DLLLA)) {
> > > +		if (!count) {
> > > +			val = readl(pcie->appl_base + APPL_DEBUG);
> > > +			val &= APPL_DEBUG_LTSSM_STATE_MASK;
> > > +			val >>= APPL_DEBUG_LTSSM_STATE_SHIFT;
> > > +			tmp = readl(pcie->appl_base + APPL_LINK_STATUS);
> > > +			tmp &= APPL_LINK_STATUS_RDLH_LINK_UP;
> > > +			if (val == 0x11 && !tmp) {
> > > +				dev_info(pci->dev, "link is down in DLL");
> > > +				dev_info(pci->dev,
> > > +					 "trying again with DLFE disabled\n");
> > > +				/* disable LTSSM */
> > > +				val = readl(pcie->appl_base + APPL_CTRL);
> > > +				val &= ~APPL_CTRL_LTSSM_EN;
> > > +				writel(val, pcie->appl_base + APPL_CTRL);
> > > +
> > > +				reset_control_assert(pcie->core_rst);
> > > +				reset_control_deassert(pcie->core_rst);
> > > +
> > > +				offset =
> > > +				dw_pcie_find_ext_capability(pci,
> > > +							    PCI_EXT_CAP_ID_DLF)
> > > +				+ PCI_DLF_CAP;
> > 
> > This capability offset doesn't change, does it?  Could it be computed
> > outside the loop?
> This is the only place where DLF offset is needed and gets calculated and this
> scenario is very rare as so far only a legacy ASMedia USB3.0 card requires DLF
> to be disabled to get PCIe link up. So, I thought of calculating the offset
> here itself instead of using a separate variable.
> 
> > 
> > > +				val = dw_pcie_readl_dbi(pci, offset);
> > > +				val &= ~DL_FEATURE_EXCHANGE_EN;
> > > +				dw_pcie_writel_dbi(pci, offset, val);
> > > +
> > > +				tegra_pcie_dw_host_init(&pcie->pci.pp);
> > 
> > This looks like some sort of "wait for link up" retry loop, but a
> > recursive call seems a little unusual.  My 5 second analysis is that
> > the loop could run this 200 times, and you sure don't want the
> > possibility of a 200-deep call chain.  Is there way to split out the
> > host init from the link-up polling?
> Again, this recursive calling comes into picture only for a legacy ASMedia
> USB3.0 card and it is going to be a 1-deep call chain as the recursion takes
> place only once depending on the condition. Apart from the legacy ASMedia card,
> there is no other card at this point in time out of a huge number of cards that we have
> tested.

A more idiomatic way would be to add a "retry:" label somewhere and goto
that after disabling DLFE. That way you achieve the same effect, but you
can avoid the recursion, even if it is harmless in practice.

> > > +static int tegra_pcie_dw_probe(struct platform_device *pdev)
> > > +{
> > > +	struct tegra_pcie_dw *pcie;
> > > +	struct pcie_port *pp;
> > > +	struct dw_pcie *pci;
> > > +	struct phy **phy;
> > > +	struct resource	*dbi_res;
> > > +	struct resource	*atu_dma_res;
> > > +	const struct of_device_id *match;
> > > +	const struct tegra_pcie_of_data *data;
> > > +	char *name;
> > > +	int ret, i;
> > > +
> > > +	pcie = devm_kzalloc(&pdev->dev, sizeof(*pcie), GFP_KERNEL);
> > > +	if (!pcie)
> > > +		return -ENOMEM;
> > > +
> > > +	pci = &pcie->pci;
> > > +	pci->dev = &pdev->dev;
> > > +	pci->ops = &tegra_dw_pcie_ops;
> > > +	pp = &pci->pp;
> > > +	pcie->dev = &pdev->dev;
> > > +
> > > +	match = of_match_device(of_match_ptr(tegra_pcie_dw_of_match),
> > > +				&pdev->dev);
> > > +	if (!match)
> > > +		return -EINVAL;
> > 
> > Logically could be the first thing in the function since it doesn't
> > depend on anything.
> Done
> 
> > 
> > > +	data = (struct tegra_pcie_of_data *)match->data;

of_device_get_match_data() can help remove some of the above
boilerplate. Also, there's no reason to check for a failure with these
functions. The driver is OF-only and can only ever be probed if the
device exists, in which case match (or data for that matter) will never
be NULL.

> > I see that an earlier patch added "bus" to struct pcie_port.  I think
> > it would be better to somehow connect to the pci_host_bridge struct.
> > Several other drivers already do this; see uses of
> > pci_host_bridge_from_priv().
> All non-DesignWare based implementations save their private data structure
> in 'private' pointer of struct pci_host_bridge and use pci_host_bridge_from_priv()
> to get it back. But, DesignWare based implementations save pcie_port in 'sysdata'
> and nothing in 'private' pointer. So,  I'm not sure if pci_host_bridge_from_priv()
> can be used in this case. Please do let me know if you think otherwise.

If nothing is currently stored in the private pointer, why not do like
the other drivers and store the struct pci_host_bridge pointer there?

Thierry
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