[PATCH] PCIe: limit Max Read Request Size on i.MX to 512 bytes

["Krzysztof Hałasa" <khalasa@piap.pl>]

DWC PCIe controller imposes limits on the Read Request Size that it can
handle. For i.MX6 family it's fixed at 512 bytes by default.

If a memory read larger than the limit is requested, the CPU responds
with Completer Abort (CA) (on i.MX6 Unsupported Request (UR) is returned
instead due to a design error).

The i.MX6 documentation states that the limit can be changed by writing
to the PCIE_PL_MRCCR0 register, however there is a fixed (and
undocumented) maximum (CX_REMOTE_RD_REQ_SIZE constant). Tests indicate
that values larger than 512 bytes don't work, though.

This patch makes the RTL8111 work on i.MX6.

Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>

diff --git a/drivers/pci/Kconfig b/drivers/pci/Kconfig
index 0c473d75e625..a11ec93a8cd0 100644
--- a/drivers/pci/Kconfig
+++ b/drivers/pci/Kconfig
@@ -34,6 +34,9 @@ config PCI_DOMAINS_GENERIC
 config PCI_SYSCALL
 	bool
 
+config NEED_PCIE_MAX_MRRS
+	bool
+
 source "drivers/pci/pcie/Kconfig"
 
 config PCI_MSI
diff --git a/drivers/pci/controller/dwc/Kconfig b/drivers/pci/controller/dwc/Kconfig
index 423d35872ce4..b59a4c91cb3b 100644
--- a/drivers/pci/controller/dwc/Kconfig
+++ b/drivers/pci/controller/dwc/Kconfig
@@ -98,6 +98,7 @@ config PCI_IMX6
 	depends on ARCH_MXC || COMPILE_TEST
 	depends on PCI_MSI_IRQ_DOMAIN
 	select PCIE_DW_HOST
+	select NEED_PCIE_MAX_MRRS
 
 config PCIE_SPEAR13XX
 	bool "STMicroelectronics SPEAr PCIe controller"
diff --git a/drivers/pci/controller/dwc/pci-imx6.c b/drivers/pci/controller/dwc/pci-imx6.c
index 80fc98acf097..7a83f677a632 100644
--- a/drivers/pci/controller/dwc/pci-imx6.c
+++ b/drivers/pci/controller/dwc/pci-imx6.c
@@ -1148,6 +1148,7 @@ static int imx6_pcie_probe(struct platform_device *pdev)
 		imx6_pcie->vph = NULL;
 	}
 
+	max_pcie_mrrs = 512;
 	platform_set_drvdata(pdev, imx6_pcie);
 
 	ret = imx6_pcie_attach_pd(dev);
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index aacf575c15cf..8ed8d2e75f4c 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -112,6 +112,10 @@ enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_PEER2PEER;
 enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_DEFAULT;
 #endif
 
+#ifdef CONFIG_NEED_PCIE_MAX_MRRS
+u16 max_pcie_mrrs = 4096; // no limit
+#endif
+
 /*
  * The default CLS is used if arch didn't set CLS explicitly and not
  * all pci devices agree on the same value.  Arch can override either
@@ -5816,6 +5820,11 @@ int pcie_set_readrq(struct pci_dev *dev, int rq)
 			rq = mps;
 	}
 
+#ifdef CONFIG_NEED_PCIE_MAX_MRRS
+	if (rq > max_pcie_mrrs)
+		rq = max_pcie_mrrs;
+#endif
+
 	v = (ffs(rq) - 8) << 12;
 
 	ret = pcie_capability_clear_and_set_word(dev, PCI_EXP_DEVCTL,
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 540b377ca8f6..7368be024c31 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -994,6 +994,7 @@ enum pcie_bus_config_types {
 };
 
 extern enum pcie_bus_config_types pcie_bus_config;
+extern u16 max_pcie_mrrs;
 
 extern struct bus_type pci_bus_type;
 

-- 
Krzysztof "Chris" Hałasa

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