Re: [PATCH v6 8/8] PCI: endpoint: pci-ep-msi: Add embedded doorbell fallback

On Tue, Feb 10, 2026 at 11:10:52AM +0900, Koichiro Den wrote:
> On Mon, Feb 09, 2026 at 10:51:56AM -0500, Frank Li wrote:
> > On Mon, Feb 09, 2026 at 09:53:16PM +0900, Koichiro Den wrote:
> > > pci_epf_alloc_doorbell() currently allocates MSI-backed doorbells using
> > > the MSI domain returned by of_msi_map_get_device_domain(...,
> > > DOMAIN_BUS_PLATFORM_MSI). On platforms where such an MSI irq domain is
> > > not available, EPF drivers cannot provide doorbells to the RC. Even if
> > > it's available and MSI device domain successfully created, the write
> > > into the message address via BAR inbound mapping might not work for
> > > platform-specific reasons (e.g. write into GITS_TRANSLATOR via iATU IB
> > > mapping does not reach ITS at least on R-Car Gen4 Spider).
> > >
> > > Add an "embedded (DMA) doorbell" fallback path that uses EPC-provided
> > > auxiliary resources to build doorbell address/data pairs backed by a
> > > platform device MMIO region (e.g. dw-edma) and a shared platform IRQ.
> > >
> > > To let EPF drivers request interrupts correctly, extend struct
> > > pci_epf_doorbell_msg with the doorbell type and required IRQ request
> > > flags. Update pci-epf-test to handle shared IRQ constraints by using a
> > > trivial primary handler to wake the threaded handler, and update
> > > pci-epf-vntb to use the required irq_flags.
> > >
> > > Signed-off-by: Koichiro Den <[email protected]>
> > > ---
> > >  drivers/pci/endpoint/functions/pci-epf-test.c |  29 +++-
> > >  drivers/pci/endpoint/functions/pci-epf-vntb.c |   3 +-
> > >  drivers/pci/endpoint/pci-ep-msi.c             | 129 ++++++++++++++++--
> > >  include/linux/pci-epf.h                       |  17 ++-
> > >  4 files changed, 160 insertions(+), 18 deletions(-)
> > >
> > > diff --git a/drivers/pci/endpoint/functions/pci-epf-test.c b/drivers/pci/endpoint/functions/pci-epf-test.c
> > > index 23034f548c90..2f3b2e6a3e29 100644
> > > --- a/drivers/pci/endpoint/functions/pci-epf-test.c
> > > +++ b/drivers/pci/endpoint/functions/pci-epf-test.c
> > > @@ -711,6 +711,26 @@ static irqreturn_t pci_epf_test_doorbell_handler(int irq, void *data)
> > >  	return IRQ_HANDLED;
> > >  }
> > >
> > > +/*
> > > + * Embedded doorbell fallback uses a platform IRQ which is already owned by a
> > > + * platform driver (e.g. dw-edma) and therefore must be requested IRQF_SHARED.
> > > + * We cannot add IRQF_ONESHOT here because shared IRQ handlers must agree on
> > > + * IRQF_ONESHOT.
> > > + *
> > > + * request_threaded_irq() with handler == NULL would be rejected for !ONESHOT
> > > + * because the default primary handler only wakes the thread and does not
> > > + * mask/ack the interrupt, which can livelock on level-triggered IRQs.
> > > + *
> > > + * In the embedded doorbell fallback, the IRQ owner is responsible for
> > > + * acknowledging/deasserting the interrupt source in hardirq context before the
> > > + * IRQ line is unmasked. Therefore this driver only needs a trivial primary
> > > + * handler to wake the threaded handler.
> > > + */
> > > +static irqreturn_t pci_epf_test_doorbell_primary(int irq, void *data)
> > > +{
> > > +	return IRQ_WAKE_THREAD;
> > > +}
> > > +
> > >  static void pci_epf_test_doorbell_cleanup(struct pci_epf_test *epf_test)
> > >  {
> > >  	struct pci_epf_test_reg *reg = epf_test->reg[epf_test->test_reg_bar];
> > > @@ -731,6 +751,7 @@ static void pci_epf_test_enable_doorbell(struct pci_epf_test *epf_test,
> > >  	u32 status = le32_to_cpu(reg->status);
> > >  	struct pci_epf *epf = epf_test->epf;
> > >  	struct pci_epc *epc = epf->epc;
> > > +	unsigned long irq_flags;
> > >  	struct msi_msg *msg;
> > >  	enum pci_barno bar;
> > >  	size_t offset;
> > > @@ -745,10 +766,14 @@ static void pci_epf_test_enable_doorbell(struct pci_epf_test *epf_test,
> > >  	if (bar < BAR_0)
> > >  		goto err_doorbell_cleanup;
> > >
> > > +	irq_flags = epf->db_msg[0].irq_flags;
> > > +	if (!(irq_flags & IRQF_SHARED))
> > > +		irq_flags |= IRQF_ONESHOT;
> > >  	epf_test->db_irq_requested = false;
> > >
> > > -	ret = request_threaded_irq(epf->db_msg[0].virq, NULL,
> > > -				   pci_epf_test_doorbell_handler, IRQF_ONESHOT,
> > > +	ret = request_threaded_irq(epf->db_msg[0].virq,
> > > +				   pci_epf_test_doorbell_primary,
> > > +				   pci_epf_test_doorbell_handler, irq_flags,
> > >  				   "pci-ep-test-doorbell", epf_test);
> > >  	if (ret) {
> > >  		dev_err(&epf->dev,
> > > diff --git a/drivers/pci/endpoint/functions/pci-epf-vntb.c b/drivers/pci/endpoint/functions/pci-epf-vntb.c
> > > index 3ecc5059f92b..d2fd1e194088 100644
> > > --- a/drivers/pci/endpoint/functions/pci-epf-vntb.c
> > > +++ b/drivers/pci/endpoint/functions/pci-epf-vntb.c
> > > @@ -535,7 +535,8 @@ static int epf_ntb_db_bar_init_msi_doorbell(struct epf_ntb *ntb,
> > >
> > >  	for (i = 0; i < ntb->db_count; i++) {
> > >  		ret = request_irq(epf->db_msg[i].virq, epf_ntb_doorbell_handler,
> > > -				  0, "pci_epf_vntb_db", ntb);
> > > +				  epf->db_msg[i].irq_flags, "pci_epf_vntb_db",
> > > +				  ntb);
> > >
> > >  		if (ret) {
> > >  			dev_err(&epf->dev,
> > > diff --git a/drivers/pci/endpoint/pci-ep-msi.c b/drivers/pci/endpoint/pci-ep-msi.c
> > > index ad8a81d6ad77..0e93d4789abd 100644
> > > --- a/drivers/pci/endpoint/pci-ep-msi.c
> > > +++ b/drivers/pci/endpoint/pci-ep-msi.c
> > > @@ -8,6 +8,7 @@
> > >
> > >  #include <linux/device.h>
> > >  #include <linux/export.h>
> > > +#include <linux/interrupt.h>
> > >  #include <linux/irqdomain.h>
> > >  #include <linux/module.h>
> > >  #include <linux/msi.h>
> > > @@ -35,23 +36,84 @@ static void pci_epf_write_msi_msg(struct msi_desc *desc, struct msi_msg *msg)
> > >  	pci_epc_put(epc);
> > >  }
> > >
> > > -int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 num_db)
> > > +static int pci_epf_alloc_doorbell_embedded(struct pci_epf *epf, u16 num_db)
> > >  {
> > >  	struct pci_epc *epc = epf->epc;
> > > -	struct device *dev = &epf->dev;
> > > -	struct irq_domain *domain;
> > > -	void *msg;
> > > -	int ret;
> > > -	int i;
> > > +	const struct pci_epc_aux_resource *dma_ctrl = NULL;
> > > +	struct pci_epf_doorbell_msg *msg;
> > > +	int count, ret, i, db;
> > >
> > > -	/* TODO: Multi-EPF support */
> > > -	if (list_first_entry_or_null(&epc->pci_epf, struct pci_epf, list) != epf) {
> > > -		dev_err(dev, "MSI doorbell doesn't support multiple EPF\n");
> > > -		return -EINVAL;
> > > +	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_DMA_CTRL_MMIO) {
> >
> > I suggest use PCI_EPC_EMBEDED_DOORBELL_MMIO directly because some vendor
> > have really doorbell register space.
> >
> > below addr is that phys_addr + res[i].u.dma_chan_desc.db_offset. So vendor
> > can easily change to their doorbell register space in future.
>
> That makes sense. I can extend enum pci_epc_aux_resource_type with a
> dedicated doorbell MMIO resource type and use it, instead of assuming the
> embedded doorbell lives inside the DMA control MMIO window
> (PCI_EPC_AUX_DMA_CTRL_MMIO).
>
> With that approach, I plan to drop u.dma_chan_desc.db_offset for now, since
> it would always be zero in the current dw-edma v0 case (the INT_STATUS
> offset is not channel-specific). If another vendor implementation needs a
> channel-specific offset in the future, we can just extend this again when
> it's actually needed.
>
> In short, after your feedback, I'm thinking of the following change (diff
> against v6):
>
>       enum pci_epc_aux_resource_type {
>       	PCI_EPC_AUX_DMA_CTRL_MMIO,
>       	PCI_EPC_AUX_DMA_CHAN_DESC,
>     + 	PCI_EPC_AUX_DOORBELL_MMIO,

Yes, thanks

Frank

>       };
>
>       struct pci_epc_aux_resource {
>       	enum pci_epc_aux_resource_type type;
>       	phys_addr_t phys_addr;
>       	resource_size_t size;
>
>       	union {
>       		/* PCI_EPC_AUX_DMA_CHAN_DESC */
>       		struct {
>       			int irq;
>     - 			resource_size_t db_offset;
>       		} dma_chan_desc;
>       	} u;
>       };
>
> Thanks for the review,
> Koichiro
>
> >
> > Frank
> >
> > > +			dma_ctrl = &res[i];
> > > +			break;
> > > +		}
> > > +	}
> > > +	if (!dma_ctrl)
> > > +		return -ENODEV;
> > > +
> > > +	msg = kcalloc(num_db, sizeof(*msg), GFP_KERNEL);
> > > +	if (!msg)
> > > +		return -ENOMEM;
> > > +
> > > +	for (i = 0, db = 0; i < count && db < num_db; i++) {
> > > +		u64 addr;
> > > +
> > > +		if (res[i].type != PCI_EPC_AUX_DMA_CHAN_DESC)
> > > +			continue;
> > > +
> > > +		if (res[i].u.dma_chan_desc.db_offset >= dma_ctrl->size)
> > > +			continue;
> > > +
> > > +		addr = (u64)dma_ctrl->phys_addr + res[i].u.dma_chan_desc.db_offset;
> > > +
> > > +		msg[db].msg.address_lo = (u32)addr;
> > > +		msg[db].msg.address_hi = (u32)(addr >> 32);
> > > +		msg[db].msg.data = 0;
> > > +		msg[db].virq = res[i].u.dma_chan_desc.irq;
> > > +		msg[db].irq_flags = IRQF_SHARED;
> > > +		msg[db].type = PCI_EPF_DOORBELL_EMBEDDED;
> > > +		db++;
> > >  	}
> > >
> > > -	if (epf->db_msg)
> > > -		return -EBUSY;
> > > +	if (db != num_db) {
> > > +		kfree(msg);
> > > +		return -ENOSPC;
> > > +	}
> > > +
> > > +	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)
> > > +{
> >
> > suggest create patch, which only add helper function
> > pci_epf_alloc_doorbell_msi().
> >
> > Then add pci_epf_alloc_doorbell_embedded.
> >
> > Frank
> > > +	struct pci_epf_doorbell_msg *msg;
> > > +	struct device *dev = &epf->dev;
> > > +	struct pci_epc *epc = epf->epc;
> > > +	struct irq_domain *domain;
> > > +	int ret, i;
> > >
> > >  	domain = of_msi_map_get_device_domain(epc->dev.parent, 0,
> > >  					      DOMAIN_BUS_PLATFORM_MSI);
> > > @@ -74,6 +136,11 @@ int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 num_db)
> > >  	if (!msg)
> > >  		return -ENOMEM;
> > >
> > > +	for (i = 0; i < num_db; i++) {
> > > +		msg[i].irq_flags = 0;
> > > +		msg[i].type = PCI_EPF_DOORBELL_MSI;
> > > +	}
> > > +
> > >  	epf->num_db = num_db;
> > >  	epf->db_msg = msg;
> > >
> > > @@ -90,13 +157,49 @@ int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 num_db)
> > >  	for (i = 0; i < num_db; i++)
> > >  		epf->db_msg[i].virq = msi_get_virq(epc->dev.parent, i);
> > >
> > > +	return 0;
> > > +}
> > > +
> > > +int pci_epf_alloc_doorbell(struct pci_epf *epf, u16 num_db)
> > > +{
> > > +	struct pci_epc *epc = epf->epc;
> > > +	struct device *dev = &epf->dev;
> > > +	int ret;
> > > +
> > > +	/* TODO: Multi-EPF support */
> > > +	if (list_first_entry_or_null(&epc->pci_epf, struct pci_epf, list) != epf) {
> > > +		dev_err(dev, "Doorbell doesn't support multiple EPF\n");
> > > +		return -EINVAL;
> > > +	}
> > > +
> > > +	if (epf->db_msg)
> > > +		return -EBUSY;
> > > +
> > > +	ret = pci_epf_alloc_doorbell_msi(epf, num_db);
> > > +	if (!ret)
> > > +		return 0;
> > > +
> > > +	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;
> > > +	}
> > > +
> > > +	dev_err(dev, "Failed to allocate doorbell: %d\n", ret);
> > >  	return ret;
> > >  }
> > >  EXPORT_SYMBOL_GPL(pci_epf_alloc_doorbell);
> > >
> > >  void pci_epf_free_doorbell(struct pci_epf *epf)
> > >  {
> > > -	platform_device_msi_free_irqs_all(epf->epc->dev.parent);
> > > +	if (!epf->db_msg)
> > > +		return;
> > > +
> > > +	if (epf->db_msg[0].type == PCI_EPF_DOORBELL_MSI)
> > > +		platform_device_msi_free_irqs_all(epf->epc->dev.parent);
> > >
> > >  	kfree(epf->db_msg);
> > >  	epf->db_msg = NULL;
> > > diff --git a/include/linux/pci-epf.h b/include/linux/pci-epf.h
> > > index 7737a7c03260..e6625198f401 100644
> > > --- a/include/linux/pci-epf.h
> > > +++ b/include/linux/pci-epf.h
> > > @@ -152,14 +152,27 @@ struct pci_epf_bar {
> > >  	struct pci_epf_bar_submap	*submap;
> > >  };
> > >
> > > +enum pci_epf_doorbell_type {
> > > +	PCI_EPF_DOORBELL_MSI = 0,
> > > +	PCI_EPF_DOORBELL_EMBEDDED,
> > > +};
> > > +
> > >  /**
> > >   * struct pci_epf_doorbell_msg - represents doorbell message
> > > - * @msg: MSI message
> > > - * @virq: IRQ number of this doorbell MSI message
> > > + * @msg: Doorbell address/data pair to be mapped into BAR space.
> > > + *       For MSI-backed doorbells this is the MSI message, while for
> > > + *       "embedded" doorbells this represents an MMIO write that asserts
> > > + *       an interrupt on the EP side.
> > > + * @virq: IRQ number of this doorbell message
> > > + * @irq_flags: Required flags for request_irq()/request_threaded_irq().
> > > + *             Callers may OR-in additional flags (e.g. IRQF_ONESHOT).
> > > + * @type: Doorbell type.
> > >   */
> > >  struct pci_epf_doorbell_msg {
> > >  	struct msi_msg msg;
> > >  	int virq;
> > > +	unsigned long irq_flags;
> > > +	enum pci_epf_doorbell_type type;
> > >  };
> > >
> > >  /**
> > > --
> > > 2.51.0
> > >