Re: [PATCH v7 9/9] PCI: endpoint: pci-ep-msi: Add embedded eDMA doorbell fallback

On Mon, Feb 16, 2026 at 11:24:38AM -0500, Frank Li wrote:
> On Mon, Feb 16, 2026 at 01:38:47AM +0900, Koichiro Den wrote:
> > Some endpoint platforms cannot use platform MSI / GIC ITS to implement
> > EP-side doorbells. In those cases, EPF drivers cannot provide an
> > interrupt-driven doorbell and often fall back to polling.
> >
> > Add an "embedded" doorbell backend that uses a controller-integrated
> > doorbell target (e.g. DesignWare integrated eDMA interrupt-emulation
> > doorbell).
> >
> > The backend locates the doorbell register and a corresponding Linux IRQ
> > via the EPC aux-resource API. If the doorbell register is already
> > exposed via a fixed BAR mapping, provide BAR+offset. Otherwise provide
> > the physical address so EPF drivers can map it into BAR space.
> >
> > When MSI doorbell allocation fails with -ENODEV,
> > pci_epf_alloc_doorbell() falls back to this embedded backend.
> >
> > Signed-off-by: Koichiro Den <[email protected]>
> > ---
> >  drivers/pci/endpoint/pci-ep-msi.c | 90 +++++++++++++++++++++++++++++++
> >  1 file changed, 90 insertions(+)
> >
> > diff --git a/drivers/pci/endpoint/pci-ep-msi.c b/drivers/pci/endpoint/pci-ep-msi.c
> > index a42f69ad24ad..6e1524c2d891 100644
> > --- a/drivers/pci/endpoint/pci-ep-msi.c
> > +++ b/drivers/pci/endpoint/pci-ep-msi.c
> > @@ -6,6 +6,7 @@
> >   * Author: Frank Li <[email protected]>
> >   */
> >
> > +#include <linux/cleanup.h>
> >  #include <linux/device.h>
> >  #include <linux/export.h>
> >  #include <linux/interrupt.h>
> > @@ -36,6 +37,82 @@ static void pci_epf_write_msi_msg(struct msi_desc *desc, struct msi_msg *msg)
> >  	pci_epc_put(epc);
> >  }
> >
> > +static int pci_epf_alloc_doorbell_embedded(struct pci_epf *epf, u16 num_db)
> > +{
> > +	const struct pci_epc_aux_resource *doorbell = NULL;
> > +	struct pci_epf_doorbell_msg *msg;
> > +	struct pci_epc *epc = epf->epc;
> > +	struct device *dev = &epf->dev;
> > +	int count, ret, i;
> > +	u64 addr;
> > +
> > +	count = pci_epc_get_aux_resources(epc, epf->func_no, epf->vfunc_no,
> > +					  NULL, 0);
> > +	if (count == -EOPNOTSUPP || count == 0)
> > +		return -ENODEV;
> > +	if (count < 0)
> > +		return count;
> > +
> > +	struct pci_epc_aux_resource *res __free(kfree) =
> > +				kcalloc(count, sizeof(*res), GFP_KERNEL);
> > +	if (!res)
> > +		return -ENOMEM;
> > +
> > +	ret = pci_epc_get_aux_resources(epc, epf->func_no, epf->vfunc_no,
> > +					res, count);
> > +	if (ret == -EOPNOTSUPP || ret == 0)
> > +		return -ENODEV;
> > +	if (ret < 0)
> > +		return ret;
> > +
> > +	count = ret;
> > +
> > +	for (i = 0; i < count; i++) {
> > +		if (res[i].type == PCI_EPC_AUX_DOORBELL_MMIO) {
> > +			if (doorbell) {
> > +				dev_warn(dev,
> > +					 "Duplicate DOORBELL_MMIO resource found\n");
> > +				continue;
> > +			}
> > +			doorbell = &res[i];
> > +		}
> > +	}
> > +	if (!doorbell)
> > +		return -ENODEV;
> > +
> > +	msg = kcalloc(num_db, sizeof(*msg), GFP_KERNEL);
> > +	if (!msg)
> > +		return -ENOMEM;
> > +
> > +	addr = doorbell->phys_addr;
> > +
> > +	/*
> > +	 * Embedded doorbell backends (e.g. DesignWare eDMA interrupt emulation)
> > +	 * typically provide a single IRQ and do not offer per-doorbell
> > +	 * distinguishable address/data pairs. The EPC aux resource therefore
> > +	 * exposes one DOORBELL_MMIO entry (u.db_mmio.irq).
> > +	 *
> > +	 * Still, pci_epf_alloc_doorbell() allows requesting multiple doorbells.
> > +	 * For such backends we replicate the same address/data for each entry
> > +	 * and mark the IRQ as shared (IRQF_SHARED). Consumers must treat them
> > +	 * as equivalent "kick" doorbells.
> > +	 */
> > +	for (i = 0; i < num_db; i++) {
> > +		msg[i].msg.address_lo = (u32)addr;
> > +		msg[i].msg.address_hi = (u32)(addr >> 32);
> > +		msg[i].msg.data = 0;
> > +		msg[i].virq = doorbell->u.db_mmio.irq;
> > +		msg[i].irq_flags = IRQF_SHARED;
> > +		msg[i].type = PCI_EPF_DOORBELL_EMBEDDED;
> > +		msg[i].bar = doorbell->bar;
> > +		msg[i].offset = (doorbell->bar == NO_BAR) ? 0 : doorbell->bar_offset;
> > +	}
> > +
> > +	epf->num_db = num_db;
> > +	epf->db_msg = msg;
> > +	return 0;
> > +}
> > +
> >  static int pci_epf_alloc_doorbell_msi(struct pci_epf *epf, u16 num_db)
> >  {
> >  	struct pci_epf_doorbell_msg *msg;
> > @@ -110,6 +187,19 @@ int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 num_db)
> >  	if (!ret)
> >  		return 0;
> >
> > +	/*
> > +	 * Fall back to embedded doorbell only when platform MSI is unavailable
> > +	 * for this EPC.
> > +	 */
> > +	if (ret != -ENODEV)
> > +		return ret;
> > +
> > +	ret = pci_epf_alloc_doorbell_embedded(epf, num_db);
> > +	if (!ret) {
> > +		dev_info(dev, "Using embedded (DMA) doorbell fallback\n");
> > +		return 0;
> > +	}
> 
> here, needn't reverise logic, mostly we put err path in if branch.
> 
> 	if (ret) {
> 		dev_err(dev, "Failed to allocate doorbell: %d\n", ret);
> 		return ret;
> 	}
> 
> 	dev_info(dev, "Using embedded (DMA) doorbell fallback\n");
> 	return 0;

Thanks for pointing this out. I'll update it.

Koichiro

> 
> Frank
> > +
> >  	dev_err(dev, "Failed to allocate doorbell: %d\n", ret);
> >  	return ret;
> >  }
> > --
> > 2.51.0
> >