linux-kernel - RE: [PATCH V3 04/16] PCI: dwc: Perform dbi regs write lock towards the end

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Message-ID: <305100E33629484CBB767107E4246BBB0A22FA36@de02wembxa.internal.synopsys.com>
Date:   Tue, 23 Apr 2019 09:40:40 +0000
From:   Gustavo Pimentel <Gustavo.Pimentel@...opsys.com>
To:     Jisheng Zhang <Jisheng.Zhang@...aptics.com>,
        Gustavo Pimentel <Gustavo.Pimentel@...opsys.com>,
        Hou Zhiqiang <Zhiqiang.Hou@....com>
CC:     Vidya Sagar <vidyas@...dia.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "jonathanh@...dia.com" <jonathanh@...dia.com>,
        "kishon@...com" <kishon@...com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will.deacon@....com" <will.deacon@....com>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "jingoohan1@...il.com" <jingoohan1@...il.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "mmaddireddy@...dia.com" <mmaddireddy@...dia.com>,
        "kthota@...dia.com" <kthota@...dia.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mperttunen@...dia.com" <mperttunen@...dia.com>,
        "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "sagar.tv@...il.com" <sagar.tv@...il.com>
Subject: RE: [PATCH V3 04/16] PCI: dwc: Perform dbi regs write lock towards
 the end

On Mon, Apr 22, 2019 at 8:54:32, Jisheng Zhang 
<Jisheng.Zhang@...aptics.com> wrote:

> On Wed, 17 Apr 2019 09:56:33 +0000 Gustavo Pimentel wrote:
> 
> > 
> > On Tue, Apr 16, 2019 at 20:27:18, Vidya Sagar <vidyas@...dia.com> wrote:
> > 
> > > Remove multiple write enable and disable sequences of dbi registers as
> > > Tegra194 implements writes to BAR-0 register (offset: 0x10) controlled by
> > > DBI write-lock enable bit thereby not allowing any further writes to BAR-0
> > > register in config space to take place. Hence disabling write permission
> > > only towards the end.
> > >
> > > Signed-off-by: Vidya Sagar <vidyas@...dia.com>
> > > ---
> > > Changes since [v2]:
> > > * None
> > >
> > > Changes since [v1]:
> > > * None
> > >
> > >  drivers/pci/controller/dwc/pcie-designware-host.c | 3 ---
> > >  1 file changed, 3 deletions(-)
> > >
> > > diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > index 2a5332e5ccfa..c0334c92c1a6 100644
> > > --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> > > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > @@ -683,7 +683,6 @@ void dw_pcie_setup_rc(struct pcie_port *pp)
> > >       val &= 0xffff00ff;
> > >       val |= 0x00000100;
> > >       dw_pcie_writel_dbi(pci, PCI_INTERRUPT_LINE, val);
> > > -     dw_pcie_dbi_ro_wr_dis(pci);
> > >
> > >       /* Setup bus numbers */
> > >       val = dw_pcie_readl_dbi(pci, PCI_PRIMARY_BUS);
> > > @@ -723,8 +722,6 @@ void dw_pcie_setup_rc(struct pcie_port *pp)
> > >
> > >       dw_pcie_wr_own_conf(pp, PCI_BASE_ADDRESS_0, 4, 0);
> > >
> > > -     /* Enable write permission for the DBI read-only register */
> > > -     dw_pcie_dbi_ro_wr_en(pci);
> > >       /* Program correct class for RC */
> > >       dw_pcie_wr_own_conf(pp, PCI_CLASS_DEVICE, 2, PCI_CLASS_BRIDGE_PCI);
> > >       /* Better disable write permission right after the update */
> > > --
> > > 2.17.1  
> > 
> > This setup sequence was written by Jingoo Han, let's check if he did this
> > by some particular reason.
> > Jingoo do you remember why you wrote the code like this?
> 
> FWICT, enabling RO writeable in the setup sequence is introduced in
> commit d91dfe5054d4 ("PCI: dwc: Enable write permission for Class Code, 
> Interrupt Pin updates"). The Reason why not towards the end maybe
> only enable the RO writeable when necessary.

I also share that belief, in any case, I just want to confirm with the 
original developer if this was coded like this for some good reason, 
maybe to force some additional protection. Otherwise, I think the 
presented patch is harmless.

Gustavo

> 
> thanks