linux-kernel - Re: [PATCH 3/5] PCI: dwc: Remove MSI/MSIX capability if iMSI-RX is used as MSI controller

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Message-ID: <cpv4b2d2uxdefxjhequj77y4bizsghmmcmkseksitzrwhr3a5b@3f77unlrpwcr>
Date: Sat, 27 Dec 2025 10:28:21 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Shawn Lin <shawn.lin@...k-chips.com>, 
	Qiang Yu <qiang.yu@....qualcomm.com>, Lorenzo Pieralisi <lpieralisi@...nel.org>, 
	Krzysztof Wilczyński <kwilczynski@...nel.org>, Rob Herring <robh@...nel.org>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Jingoo Han <jingoohan1@...il.com>, linux-pci@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH 3/5] PCI: dwc: Remove MSI/MSIX capability if iMSI-RX is
 used as MSI controller

On Fri, Dec 26, 2025 at 03:25:29PM -0600, Bjorn Helgaas wrote:
> On Thu, Nov 20, 2025 at 10:30:41PM +0530, Manivannan Sadhasivam wrote:
> > On Thu, Nov 20, 2025 at 10:06:03PM +0800, Shawn Lin wrote:
> > > 在 2025/11/10 星期一 14:59, Qiang Yu 写道:
> > > > Some platforms may not support ITS (Interrupt Translation Service) and
> > > > MBI (Message Based Interrupt), or there are not enough available empty SPI
> > > > lines for MBI, in which case the msi-map and msi-parent property will not
> > > > be provided in device tree node. For those cases, the DWC PCIe driver
> > > > defaults to using the iMSI-RX module as MSI controller. However, due to
> > > > DWC IP design, iMSI-RX cannot generate MSI interrupts for Root Ports even
> > > > when MSI is properly configured and supported as iMSI-RX will only monitor
> > > > and intercept incoming MSI TLPs from PCIe link, but the memory write
> > > > generated by Root Port are internal system bus transactions instead of
> > > > PCIe TLPs, so they are ignored.
> 
> > > > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > @@ -1083,6 +1083,16 @@ int dw_pcie_setup_rc(struct dw_pcie_rp *pp)
> > > >   	dw_pcie_dbi_ro_wr_dis(pci);
> > > > +	/*
> > > > +	 * If iMSI-RX module is used as the MSI controller, remove MSI and
> > > > +	 * MSI-X capabilities from PCIe Root Ports to ensure fallback to INTx
> > > > +	 * interrupt handling.
> > > > +	 */
> > > > +	if (pp->has_msi_ctrl) {
> > > 
> > > Isn't has_msi_ctrl means you have something like GIC-ITS
> > > support instead of iMSI module? Am I missing anything?
> > 
> > It is the other way around. Presence of this flag means, iMSI-RX is
> > used. But I think the driver should clear the CAPs irrespective of
> > this flag.
> 
> I didn't see any response to this.

Shawn did respond to my comment:
https://lore.kernel.org/linux-pci/3ac0d6c5-0c49-45fd-b855-d9b040249096@rock-chips.com/

>  Is there a case where we want to
> preserve these capabilities?  To ask another way, is there some
> platform where DWC-based Root Ports can generate MSIs themselves?
> 

Unless the DWC controller implements a custom internal MSI controller (if it
does then dw_pcie_host_ops::msi_init() callback will be populated), Root Port
MSIs cannot be received. This series did exclude only one of those platforms,
pci-keystone by skipping capability removal if 'dw_pcie_rp::has_msi_ctrl' is
set.

- Mani

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