linux-kernel - Re: [Patch v4 10/10] PCI: tegra194: add interconnect support in Tegra234

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Message-ID: <20230329191703.GA3076491@bhelgaas>
Date:   Wed, 29 Mar 2023 14:17:03 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Sumit Gupta <sumitg@...dia.com>
Cc:     treding@...dia.com, krzysztof.kozlowski@...aro.org,
        dmitry.osipenko@...labora.com, viresh.kumar@...aro.org,
        rafael@...nel.org, jonathanh@...dia.com, robh+dt@...nel.org,
        lpieralisi@...nel.org, linux-kernel@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-pci@...r.kernel.org,
        mmaddireddy@...dia.com, kw@...ux.com, bhelgaas@...gle.com,
        vidyas@...dia.com, sanjayc@...dia.com, ksitaraman@...dia.com,
        ishah@...dia.com, bbasu@...dia.com
Subject: Re: [Patch v4 10/10] PCI: tegra194: add interconnect support in
 Tegra234

On Wed, Mar 29, 2023 at 11:28:40PM +0530, Sumit Gupta wrote:
> On 29/03/23 22:29, Bjorn Helgaas wrote:
> > On Wed, Mar 29, 2023 at 02:44:34PM +0530, Sumit Gupta wrote:
> > > On 28/03/23 23:23, Bjorn Helgaas wrote:
> > > > > +static void tegra_pcie_icc_set(struct tegra_pcie_dw *pcie)
> > > > > +{
> > > > > +     struct dw_pcie *pci = &pcie->pci;
> > > > > +     u32 val, speed, width;
> > > > > +
> > > > > +     val = dw_pcie_readw_dbi(pci, pcie->pcie_cap_base + PCI_EXP_LNKSTA);
> > > > > +
> > > > > +     speed = FIELD_GET(PCI_EXP_LNKSTA_CLS, val);
> > > > > +     width = FIELD_GET(PCI_EXP_LNKSTA_NLW, val);
> > > > > +
> > > > > +     val = width * (PCIE_SPEED2MBS_ENC(pcie_link_speed[speed]) / BITS_PER_BYTE);
> > > > > +
> > > > > +     if (icc_set_bw(pcie->icc_path, MBps_to_icc(val), 0))
> > > > > +             dev_err(pcie->dev, "can't set bw[%u]\n", val);
> > > > > +
> > > > > +     clk_set_rate(pcie->core_clk, pcie_gen_freq[speed - 1]);
> > > > 
> > > > Array bounds violation; PCI_EXP_LNKSTA_CLS is 0x000f, so possible
> > > > speed (CLS) values are 0..0xf and "speed - 1" values are -1..0xe.
> > > > 
> > > > pcie_gen_freq[] is of size 4 (valid indices 0..3).
> > > > 
> > > > I see that you're just *moving* this code, but might as well fix it.
> > > > 
> > > Thank you for the review.
> > > Will include the below change in the same patch. Please let me know if any
> > > issue.
> > > 
> > >   -       clk_set_rate(pcie->core_clk, pcie_gen_freq[speed - 1]);
> > >   +       if (speed && (speed <= ARRAY_SIZE(pcie_gen_freq)))
> > >   +               clk_set_rate(pcie->core_clk, pcie_gen_freq[speed - 1]);
> > >   +       else
> > >   +               clk_set_rate(pcie->core_clk, pcie_gen_freq[0]);
> > 
> > I didn't notice that speed is a u32, so -1 is not a possible value.
> > Also, it's used earlier for PCIE_SPEED2MBS_ENC(), so you could do
> > something like this:
> > 
> >    speed = FIELD_GET(PCI_EXP_LNKSTA_CLS, val) - 1;
> >    if (speed >= ARRAY_SIZE(pcie_gen_freq))
> >      speed = 0;
> > 
> >    val = width * (PCIE_SPEED2MBS_ENC(pcie_link_speed[speed]) /
> >          BITS_PER_BYTE);
> >    ...
> >    clk_set_rate(pcie->core_clk, pcie_gen_freq[speed]);
> 
> I tried this change but PCIE_SPEED2MBS_ENC gives zero when speed value is
> one. The speed value ranges from "1 to 4" and for value "1",
> pcie_link_speed[speed] gives '0xff'.

Oh, my fault, sorry!  I thought both places indexed the same array,
but the first is pcie_link_speed[] (where all the possible values
(0..0xf) are valid indices) and the second is pcie_gen_freq[] (where
only 0..3 are valid).

> The below change works fine. Please share if its OK to add it in patch.
> 
>   speed = FIELD_GET(PCI_EXP_LNKSTA_CLS, val);
>   if (!speed || speed >= ARRAY_SIZE(pcie_gen_freq))
>           speed = 1;
> 
>   val = width * (PCIE_SPEED2MBS_ENC(pcie_link_speed[speed]) /
> BITS_PER_BYTE);

So I don't think you need to clamp "speed" for indexing
pcie_link_speed[] at all.

>   if (icc_set_bw(pcie->icc_path, MBps_to_icc(val), 0))
>           dev_err(pcie->dev, "can't set bw[%u]\n", val);
> 
>   clk_set_rate(pcie->core_clk, pcie_gen_freq[speed - 1]);

What if you added a 0th entry to pcie_gen_freq[] so you can index it
directly with the PCI_EXP_LNKSTA_CLS value the same way as
pcie_link_speed[]?  Then you wouldn't need the "- 1" and only have to
worry about going off the end:

  static const unsigned int pcie_gen_freq[] = {
    GEN1_CORE_CLK_FREQ,	  /* PCI_EXP_LNKSTA_CLS == 0; undefined */
    GEN1_CORE_CLK_FREQ,
    GEN2_CORE_CLK_FREQ,
    GEN3_CORE_CLK_FREQ,
    GEN4_CORE_CLK_FREQ,
  };

  speed = FIELD_GET(PCI_EXP_LNKSTA_CLS, val);

  val = width * (PCIE_SPEED2MBS_ENC(pcie_link_speed[speed]) /
        BITS_PER_BYTE);

  if (speed >= ARRAY_SIZE(pcie_gen_freq))
    speed = 0;
  clk_set_rate(pcie->core_clk, pcie_gen_freq[speed]);

Bjorn