linux-kernel - Re: [PATCH v2 2/5] PCI: stm32: Add PCIe host support for STM32MP25

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Message-ID: <20241129205822.GA2772018@bhelgaas>
Date: Fri, 29 Nov 2024 14:58:22 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Christian Bruel <christian.bruel@...s.st.com>,
	Rob Herring <robh+dt@...nel.org>
Cc: lpieralisi@...nel.org, kw@...ux.com, manivannan.sadhasivam@...aro.org,
	robh@...nel.org, bhelgaas@...gle.com, krzk+dt@...nel.org,
	conor+dt@...nel.org, mcoquelin.stm32@...il.com,
	alexandre.torgue@...s.st.com, p.zabel@...gutronix.de,
	cassel@...nel.org, quic_schintav@...cinc.com,
	fabrice.gasnier@...s.st.com, linux-pci@...r.kernel.org,
	devicetree@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/5] PCI: stm32: Add PCIe host support for STM32MP25

[+to Rob, DMA mask question]

On Tue, Nov 26, 2024 at 04:51:16PM +0100, Christian Bruel wrote:
> Add driver for the STM32MP25 SoC PCIe Gen2 controller based on the
> DesignWare PCIe core.

Can you include the numeric rate, not just "gen2", so we don't have to
search for it?

> +static int stm32_pcie_resume_noirq(struct device *dev)
> +{
> +	struct stm32_pcie *stm32_pcie = dev_get_drvdata(dev);
> +	struct dw_pcie *pci = stm32_pcie->pci;
> +	struct dw_pcie_rp *pp = &pci->pp;
> +	int ret;
> +
> +	/* init_state must be called first to force clk_req# gpio when no
> +	 * device is plugged.
> +	 */

Use drivers/pci/ conventional comment style:

  /*
   * text ...
   */

> +static bool is_stm32_pcie_driver(struct device *dev)
> +{
> +	/* PCI bridge */
> +	dev = get_device(dev);
> +
> +	/* Platform driver */
> +	dev = get_device(dev->parent);
> +
> +	return (dev->driver == &stm32_pcie_driver.driver);
> +}
> +
> +/*
> + * DMA masters can only access the first 4GB of memory space,
> + * so we setup the bus DMA limit accordingly.
> + */
> +static int stm32_dma_limit(struct pci_dev *pdev, void *data)
> +{
> +	dev_dbg(&pdev->dev, "disabling DMA DAC for device");
> +
> +	pdev->dev.bus_dma_limit = DMA_BIT_MASK(32);

I don't think this is the right way to do this.  Surely there's a way
to describe the DMA capability of the bridge once instead of iterating
over all the downstream devices?  This quirk can't work for hot-added
devices anyway.

> +	return 0;
> +}
> +
> +static void quirk_stm32_dma_mask(struct pci_dev *pci)
> +{
> +	struct pci_dev *root_port;
> +
> +	root_port = pcie_find_root_port(pci);
> +
> +	if (root_port && is_stm32_pcie_driver(root_port->dev.parent))
> +		pci_walk_bus(pci->bus, stm32_dma_limit, NULL);
> +}
> +DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_SYNOPSYS, 0x0550, quirk_stm32_dma_mask);