Re: [PATCH v4 2/2] PCI: mediatek-gen3: Add support for restricting link width

["Jianjun Wang (王建军)" <Jianjun.Wang@mediatek.com>]

On Tue, 2024-11-05 at 19:24 -0600, Bjorn Helgaas wrote:
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> 
> On Mon, Nov 04, 2024 at 12:49:35PM +0100, AngeloGioacchino Del Regno
> wrote:
> > Add support for restricting the port's link width by specifying
> > the num-lanes devicetree property in the PCIe node.
> > 
> > The setting is done in the GEN_SETTINGS register (in the driver
> > named as PCIE_SETTING_REG), where each set bit in [11:8] activates
> > a set of lanes (from bits 11 to 8 respectively, x16/x8/x4/x2).
> 
> I guess GEN_SETTINGS doesn't correspond to a register defined by the
> PCIe spec, right?  The only thing in the spec that looks similar is
> the Target Link Width in the Device Control 3 register, but the bit
> position doesn't look like this PCIE_SETTING_LINK_WIDTH mask:

Hi Bjorn,

The "GEN_SETTINGS" and "LINK_WIDTH" settings will be reflected in the
Max Link Speed and Maximum Link Width fields in the PCIe Link
Capabilities Register.

However, these registers have slight differences in hardware design:
The Max Link Speed shown in the Link Capabilities Register can be
directly changed by the "GEN_SETTINGS" register, even if the hardware
cannot support such a high speed. On the other hand, when we set the
"LINK_WIDTH", it will compare with the maximum width that the hardware
actually supports and choose the smaller one.

Thanks.

> 
> >  #define PCIE_SETTING_REG             0x80
> > +#define PCIE_SETTING_LINK_WIDTH              GENMASK(11, 8)
> >  #define PCIE_SETTING_GEN_SUPPORT     GENMASK(14, 12)
> >  #define PCIE_PCI_IDS_1                       0x9c
> >  #define PCI_CLASS(class)             (class << 8)
> > @@ -168,6 +169,7 @@ struct mtk_msi_set {
> >   * @clks: PCIe clocks
> >   * @num_clks: PCIe clocks count for this port
> >   * @max_link_speed: Maximum link speed (PCIe Gen) for this port
> > + * @num_lanes: Number of PCIe lanes for this port
> >   * @irq: PCIe controller interrupt number
> >   * @saved_irq_state: IRQ enable state saved at suspend time
> >   * @irq_lock: lock protecting IRQ register access
> > @@ -189,6 +191,7 @@ struct mtk_gen3_pcie {
> >       struct clk_bulk_data *clks;
> >       int num_clks;
> >       u8 max_link_speed;
> > +     u8 num_lanes;
> > 
> >       int irq;
> >       u32 saved_irq_state;
> > @@ -401,6 +404,14 @@ static int mtk_pcie_startup_port(struct
> > mtk_gen3_pcie *pcie)
> >                       val |= FIELD_PREP(PCIE_SETTING_GEN_SUPPORT,
> >                                         GENMASK(pcie-
> > >max_link_speed - 2, 0));
> >       }
> > +     if (pcie->num_lanes) {
> > +             val &= ~PCIE_SETTING_LINK_WIDTH;
> > +
> > +             /* Zero means one lane, each bit activates
> > x2/x4/x8/x16 */
> > +             if (pcie->num_lanes > 1)
> > +                     val |= FIELD_PREP(PCIE_SETTING_LINK_WIDTH,
> > +                                       GENMASK(fls(pcie->num_lanes 
> > >> 2), 0));
> > +     };
> >       writel_relaxed(val, pcie->base + PCIE_SETTING_REG);
> > 
> >       /* Set Link Control 2 (LNKCTL2) speed restriction, if any */
> > @@ -838,6 +849,7 @@ static int mtk_pcie_parse_port(struct
> > mtk_gen3_pcie *pcie)
> >       struct device *dev = pcie->dev;
> >       struct platform_device *pdev = to_platform_device(dev);
> >       struct resource *regs;
> > +     u32 num_lanes;
> > 
> >       regs = platform_get_resource_byname(pdev, IORESOURCE_MEM,
> > "pcie-mac");
> >       if (!regs)
> > @@ -883,6 +895,14 @@ static int mtk_pcie_parse_port(struct
> > mtk_gen3_pcie *pcie)
> >               return pcie->num_clks;
> >       }
> > 
> > +     ret = of_property_read_u32(dev->of_node, "num-lanes",
> > &num_lanes);
> > +     if (ret == 0) {
> > +             if (num_lanes == 0 || num_lanes > 16 || (num_lanes !=
> > 1 && num_lanes % 2))
> > +                     dev_warn(dev, "Invalid num-lanes, using
> > controller defaults\n");
> > +             else
> > +                     pcie->num_lanes = num_lanes;
> > +     }
> > +
> >       return 0;
> >  }
> > 
> > --
> > 2.46.1
> >