linux-kernel - Re: [PATCH v9 08/10] PCI: dwc: ep: Add a generic dw_pcie_ep_linkdown() API to handle LINK

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Date: Fri, 8 Mar 2024 15:16:06 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Niklas Cassel <cassel@...nel.org>
Cc: Jingoo Han <jingoohan1@...il.com>,
	Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Marek Vasut <marek.vasut+renesas@...il.com>,
	Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Jonathan Hunter <jonathanh@...dia.com>,
	Kishon Vijay Abraham I <kishon@...com>,
	Vidya Sagar <vidyas@...dia.com>,
	Vignesh Raghavendra <vigneshr@...com>,
	Richard Zhu <hongxing.zhu@....com>,
	Lucas Stach <l.stach@...gutronix.de>,
	Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Pengutronix Kernel Team <kernel@...gutronix.de>,
	Fabio Estevam <festevam@...il.com>,
	NXP Linux Team <linux-imx@....com>,
	Minghuan Lian <minghuan.Lian@....com>,
	Mingkai Hu <mingkai.hu@....com>, Roy Zang <roy.zang@....com>,
	Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	Jesper Nilsson <jesper.nilsson@...s.com>,
	Srikanth Thokala <srikanth.thokala@...el.com>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-renesas-soc@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-tegra@...r.kernel.org, linux-omap@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
	linux-arm-kernel@...s.com
Subject: Re: [PATCH v9 08/10] PCI: dwc: ep: Add a generic
 dw_pcie_ep_linkdown() API to handle LINK_DOWN event

On Fri, Mar 08, 2024 at 09:56:33AM +0100, Niklas Cassel wrote:
> On Fri, Mar 08, 2024 at 11:11:52AM +0530, Manivannan Sadhasivam wrote:
> > On Thu, Mar 07, 2024 at 10:43:19PM +0100, Niklas Cassel wrote:
> > > On Mon, Mar 04, 2024 at 02:52:20PM +0530, Manivannan Sadhasivam wrote:
> > > > The PCIe link can go to LINK_DOWN state in one of the following scenarios:
> > > > 
> > > > 1. Fundamental (PERST#)/hot/warm reset
> > > > 2. Link transition from L2/L3 to L0
> > > > 
> > > > In those cases, LINK_DOWN causes some non-sticky DWC registers to loose the
> > > > state (like REBAR, PTM_CAP etc...). So the drivers need to reinitialize
> > > > them to function properly once the link comes back again.
> > > > 
> > > > This is not a problem for drivers supporting PERST# IRQ, since they can
> > > > reinitialize the registers in the PERST# IRQ callback. But for the drivers
> > > > not supporting PERST#, there is no way they can reinitialize the registers
> > > > other than relying on LINK_DOWN IRQ received when the link goes down. So
> > > > let's add a DWC generic API dw_pcie_ep_linkdown() that reinitializes the
> > > > non-sticky registers and also notifies the EPF drivers about link going
> > > > down.
> > > > 
> > > > This API can also be used by the drivers supporting PERST# to handle the
> > > > scenario (2) mentioned above.
> > > > 
> > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > > > ---
> > > >  drivers/pci/controller/dwc/pcie-designware-ep.c | 111 ++++++++++++++----------
> > > >  drivers/pci/controller/dwc/pcie-designware.h    |   5 ++
> > > >  2 files changed, 72 insertions(+), 44 deletions(-)
> > > > 
> > > > diff --git a/drivers/pci/controller/dwc/pcie-designware-ep.c b/drivers/pci/controller/dwc/pcie-designware-ep.c
> > > > index 278bdc9b2269..fed4c2936c78 100644
> > > > --- a/drivers/pci/controller/dwc/pcie-designware-ep.c
> > > > +++ b/drivers/pci/controller/dwc/pcie-designware-ep.c
> > > > @@ -14,14 +14,6 @@
> > > >  #include <linux/pci-epc.h>
> > > >  #include <linux/pci-epf.h>
> > > >  
> > > > -void dw_pcie_ep_linkup(struct dw_pcie_ep *ep)
> > > > -{
> > > > -	struct pci_epc *epc = ep->epc;
> > > > -
> > > > -	pci_epc_linkup(epc);
> > > > -}
> > > > -EXPORT_SYMBOL_GPL(dw_pcie_ep_linkup);
> > > > -
> > > >  void dw_pcie_ep_init_notify(struct dw_pcie_ep *ep)
> > > >  {
> > > >  	struct pci_epc *epc = ep->epc;
> > > > @@ -603,19 +595,56 @@ static unsigned int dw_pcie_ep_find_ext_capability(struct dw_pcie *pci, int cap)
> > > >  	return 0;
> > > >  }
> > > >  
> > > > +static void dw_pcie_ep_init_non_sticky_registers(struct dw_pcie *pci)
> > > > +{
> > > > +	unsigned int offset, ptm_cap_base;
> > > > +	unsigned int nbars;
> > > > +	u32 reg, i;
> > > > +
> > > > +	offset = dw_pcie_ep_find_ext_capability(pci, PCI_EXT_CAP_ID_REBAR);
> > > > +	ptm_cap_base = dw_pcie_ep_find_ext_capability(pci, PCI_EXT_CAP_ID_PTM);
> > > > +
> > > > +	dw_pcie_dbi_ro_wr_en(pci);
> > > > +
> > > > +	if (offset) {
> > > > +		reg = dw_pcie_readl_dbi(pci, offset + PCI_REBAR_CTRL);
> > > > +		nbars = (reg & PCI_REBAR_CTRL_NBAR_MASK) >>
> > > > +			PCI_REBAR_CTRL_NBAR_SHIFT;
> > > > +
> > > > +		for (i = 0; i < nbars; i++, offset += PCI_REBAR_CTRL)
> > > > +			dw_pcie_writel_dbi(pci, offset + PCI_REBAR_CAP, 0x0);
> > > 
> > > If you look at PCI_REBAR_CAP, you will see that it is sticky,
> > > but you have to actually read the databook to see that:
> > > 
> > > "The RESBAR_CTRL_REG_BAR_SIZE field is automatically updated
> > > when you write to RESBAR_CAP_REG_0_REG through the DBI."
> > > 
> > > So the reason why we need to write this register, even though
> > > it is sticky, is to update the RESBAR_CTRL_REG_BAR_SIZE register,
> > > which is not sticky :)
> > > 
> > > (Perhaps we should add that as a comment?)
> > > 
> > 
> > Yeah, makes sense.
> 
> Note that I add a (unrelated) comment related to REBAR_CAP in this patch:
> https://lore.kernel.org/linux-pci/20240307111520.3303774-1-cassel@kernel.org/T/#u
> 
> But once we move/add code to dw_pcie_ep_init_non_sticky_registers(), I think
> that it might be a good "rule" to have a small comment for each write in
> dw_pcie_ep_init_non_sticky_registers() which explains why the code should be
> in dw_pcie_ep_init_non_sticky_registers() instead of dw_pcie_ep_init_registers(),
> even if it just a small:
> 
> /* Field PCI_XXX_YYY.ZZZ is non-sticky */
> writel_dbi(pci, offset + PCI_XXX_YYY, 0);
> 

Why? The function name itself suggests that we are reinitializing non-sticky
registers. So a comment for each write is overkill.

- Mani

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