Re: [PATCH v4 4/4] PCI: mediatek-gen3: Add Airoha EN7581 support

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Jim, Krishna, Vidya, Shashank]

On Wed, Jul 03, 2024 at 06:12:44PM +0200, Lorenzo Bianconi wrote:
> Introduce support for Airoha EN7581 PCIe controller to mediatek-gen3
> PCIe controller driver.

> +++ b/drivers/pci/controller/pcie-mediatek-gen3.c

> +#define PCIE_EQ_PRESET_01_REG		0x100
> +#define PCIE_VAL_LN0_DOWNSTREAM		GENMASK(6, 0)
> +#define PCIE_VAL_LN0_UPSTREAM		GENMASK(14, 8)
> +#define PCIE_VAL_LN1_DOWNSTREAM		GENMASK(22, 16)
> +#define PCIE_VAL_LN1_UPSTREAM		GENMASK(30, 24)
> ...

> +static int mtk_pcie_en7581_power_up(struct mtk_gen3_pcie *pcie)
> +{
> ...

> +	val = FIELD_PREP(PCIE_VAL_LN0_DOWNSTREAM, 0x47) |
> +	      FIELD_PREP(PCIE_VAL_LN1_DOWNSTREAM, 0x47) |
> +	      FIELD_PREP(PCIE_VAL_LN0_UPSTREAM, 0x41) |
> +	      FIELD_PREP(PCIE_VAL_LN1_UPSTREAM, 0x41);
> +	writel_relaxed(val, pcie->base + PCIE_EQ_PRESET_01_REG);

This looks like it might be for the Lane Equalization Control
registers (PCIe r6.0, sec 7.7.3.4)?

I would expect those values (0x47, 0x41) to be related to the platform
design, so maybe not completely determined by the SoC itself?  Jim and
Krishna have been working on DT schema for the equalization values,
which seems like the right place for them:

https://lore.kernel.org/linux-pci/20241018182247.41130-2-james.quinlan@broadcom.com/
https://lore.kernel.org/r/77d3a1a9-c22d-0fd3-5942-91b9a3d74a43@quicinc.com

Maybe that would be applicable here as well?  It would at least be
nice to use a common #define for the Lane Equalization Control
register offset from the capability base.

Although I see that no such #define exists in pci_regs.h, so I guess
there's nothing to do here yet.

The only users of equalization settings I could find so far are:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/controller/dwc/pcie-tegra194.c?id=v6.11#n832
  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/controller/dwc/pcie-qcom-common.c?id=v6.12-rc1#n11
  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/controller/pcie-mediatek-gen3.c?id=v6.12-rc1#n909

Bjorn