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From:   Marc Zyngier <maz@kernel.org>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Bjorn Helgaas <bhelgaas@google.com>
Cc:     Frank Wunderlich <frank-w@public-files.de>,
        Thierry Reding <treding@nvidia.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>,
        Thierry Reding <thierry.reding@gmail.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        Ryder Lee <ryder.lee@mediatek.com>,
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        Michal Simek <michal.simek@xilinx.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
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Subject: [PATCH 12/13] PCI: MSI: Document the various ways of ending up with NO_MSI
Date:   Thu, 25 Feb 2021 15:10:22 +0000
Message-Id: <20210225151023.3642391-13-maz@kernel.org>
In-Reply-To: <20210225151023.3642391-1-maz@kernel.org>
References: <20210225151023.3642391-1-maz@kernel.org>
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We have now 4 ways of ending up with NO_MSI being set.
Document them.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 drivers/pci/msi.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
index d9c73c173c14..425abcdfcaca 100644
--- a/drivers/pci/msi.c
+++ b/drivers/pci/msi.c
@@ -871,8 +871,16 @@ 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:
+	 * - unconditionally advertising the 'no_msi' property
+	 * - 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