Re: [PATCH v4 13/18] PCI: dwc: Let glue drivers hide the Root Port MSI capabilities

[Koichiro Den]

On Thu, Oct 15, 2026 at 12:11:45PM +0800, Shawn Lin wrote:
> Hi Koichiro
> 
> 在 2026/10/15 星期四 11:30, Koichiro Den 写道:
> > Commit f5cd8a929c82 ("PCI: dwc: Remove MSI/MSIX capability for Root Port
> > if iMSI-RX is used as MSI controller") hides the Root Port MSI/MSI-X
> > capabilities when iMSI-RX is the MSI controller, as it cannot receive
> > the Root Port's own MSIs.
> > 
> > Some platforms cannot deliver them with an external MSI controller
> > either. The DesignWare core signals Root Port interrupts (AER, PME,
> > bandwidth, hotplug) as sideband signals on its SII interface and only
> > turns them into MSI writes with the optional SII2MSI feature. Otherwise
> > it is up to the vendor wrapper. The controller on R-Car Gen4 SoCs
> > (DWC 5.20a) appears to lack SII2MSI, and its APP block merely latches
> > the signals into status registers wired to platform IRQs, so an MSI
> > targeted at the GIC ITS is never sent.
> > 
> > Add a no_rp_msi flag so glue drivers can request the same INTx fallback
> > regardless of the MSI controller in use. It is the counterpart of
> > 'keep_rp_msi_en' from commit 3a4e8302e72f ("PCI: imx6: Keep Root Port
> > MSI capability with iMSI-RX to work around hardware bug"), which keeps
> > precedence: that one is needed for endpoint MSIs to work at all, whereas
> > hiding the capability only moves the Root Port services to INTx. The
> > removal stays in dw_pcie_setup_rc(), which also runs after a Root Port
> > reset.
> > 
> 
> With it, dw_pcie_rp will carry three flags with overlapping and
> priority-dependent semantics:
> 
>   use_imsi_rx     - the MSI controller is iMSI-RX
>   keep_rp_msi_en  - do NOT remove the RP MSI caps (i.MX6 workaround)
>   no_rp_msi       - the platform never gets the RP's own MSIs
> 
> and the resulting condition:
> 
>   if ((pp->use_imsi_rx || pp->no_rp_msi) && !pp->keep_rp_msi_en)
> 
> is getting hard to reason about. Adding no_rp_msi for a single platform
> whose controller is not even iMSI-RX makes the combination worse.
> 
> Since only R-Car Gen4 needs this, and it has its own host init, would it
> be fine for the glue driver to call dw_pcie_remove_capability()
> directly? I'd like to avoid growing the shared struct if a local
> solution is possible.

That makes sense. Let me respin and add dw_pcie_remove_capability() calls
to pcie-rcar-gen4.c's host_setup(), with a short comment.
Then I'll drop this patch ([PATCH v4 13/18]).

Thanks for the review!

Best regards,
Koichiro Den

> 
> > Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
> > ---
> > Changes since v3:
> >    - No changes.
> > 
> >   drivers/pci/controller/dwc/pcie-designware-host.c | 6 ++++--
> >   drivers/pci/controller/dwc/pcie-designware.h      | 1 +
> >   2 files changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> > index 774e1936d999..32e179603cf4 100644
> > --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > @@ -1191,9 +1191,11 @@ int dw_pcie_setup_rc(struct dw_pcie_rp *pp)
> >   	 * The iMSI-RX module does not support receiving MSI or MSI-X generated
> >   	 * by the Root Port. If iMSI-RX is used as the MSI controller, remove
> >   	 * the MSI and MSI-X capabilities of the Root Port to allow the drivers
> > -	 * to fall back to INTx instead.
> > +	 * to fall back to INTx instead. Glue drivers set no_rp_msi when the
> > +	 * platform never delivers the Root Port's own MSIs, whatever the MSI
> > +	 * controller in use.
> >   	 */
> > -	if (pp->use_imsi_rx && !pp->keep_rp_msi_en) {
> > +	if ((pp->use_imsi_rx || pp->no_rp_msi) && !pp->keep_rp_msi_en) {
> >   		dw_pcie_remove_capability(pci, PCI_CAP_ID_MSI);
> >   		dw_pcie_remove_capability(pci, PCI_CAP_ID_MSIX);
> >   	}
> > diff --git a/drivers/pci/controller/dwc/pcie-designware.h b/drivers/pci/controller/dwc/pcie-designware.h
> > index 2ce61709b58c..d94116402b74 100644
> > --- a/drivers/pci/controller/dwc/pcie-designware.h
> > +++ b/drivers/pci/controller/dwc/pcie-designware.h
> > @@ -445,6 +445,7 @@ struct dw_pcie_host_ops {
> >   struct dw_pcie_rp {
> >   	bool			use_imsi_rx:1;
> >   	bool			keep_rp_msi_en:1;
> > +	bool			no_rp_msi:1;
> >   	bool			cfg0_io_shared:1;
> >   	u64			cfg0_base;
> >   	void __iomem		*va_cfg0_base;
>