linux-kernel - [PATCH v3 13/14] PCI/MSI: Document the various ways of ending up with NO

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Date:   Tue, 30 Mar 2021 16:11:44 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     Frank Wunderlich <frank-w@...lic-files.de>,
        Thierry Reding <treding@...dia.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Rob Herring <robh@...nel.org>, Will Deacon <will@...nel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Ryder Lee <ryder.lee@...iatek.com>,
        Marek Vasut <marek.vasut+renesas@...il.com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Michal Simek <michal.simek@...inx.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Bharat Kumar Gogada <bharatku@...inx.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-hyperv@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        linux-renesas-soc@...r.kernel.org, kernel-team@...roid.com
Subject: [PATCH v3 13/14] PCI/MSI: Document the various ways of ending up with NO_MSI

We have now three ways of ending up with NO_MSI being set.
Document them.

Acked-by: Bjorn Helgaas <bhelgaas@...gle.com>
Signed-off-by: Marc Zyngier <maz@...nel.org>
---
 drivers/pci/msi.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
index d9c73c173c14..217dc9f0231f 100644
--- a/drivers/pci/msi.c
+++ b/drivers/pci/msi.c
@@ -871,8 +871,15 @@ static int pci_msi_supported(struct pci_dev *dev, int nvec)
 	 * Any bridge which does NOT route MSI transactions from its
 	 * secondary bus to its primary bus must set NO_MSI flag on
 	 * the secondary pci_bus.
-	 * We expect only arch-specific PCI host bus controller driver
-	 * or quirks for specific PCI bridges to be setting NO_MSI.
+	 *
+	 * The NO_MSI flag can either be set directly by:
+	 * - arch-specific PCI host bus controller drivers (deprecated)
+	 * - quirks for specific PCI bridges
+	 *
+	 * or indirectly by platform-specific PCI host bridge drivers by
+	 * advertising the 'msi_domain' property, which results in
+	 * the NO_MSI flag when no MSI domain is found for this bridge
+	 * at probe time.
 	 */
 	for (bus = dev->bus; bus; bus = bus->parent)
 		if (bus->bus_flags & PCI_BUS_FLAGS_NO_MSI)
-- 
2.29.2