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* [PATCH v3] PCIe: limit Max Read Request Size on i.MX to 512 bytes
@ 2021-08-17  5:06 Krzysztof Hałasa
  0 siblings, 0 replies; only message in thread
From: Krzysztof Hałasa @ 2021-08-17  5:06 UTC (permalink / raw)
  To: Bjorn Helgaas
  Cc: linux-pci, Artem Lapkin, Neil Armstrong, Huacai Chen,
	Rob Herring, Lorenzo Pieralisi, Krzysztof Wilczyński,
	Richard Zhu, Lucas Stach, linux-kernel

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 <>
While ATM needed only on ARM, this version is compiled in on all

diff --git a/drivers/pci/controller/dwc/pci-imx6.c b/drivers/pci/controller/dwc/pci-imx6.c
index 80fc98acf097..225380e75fff 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..abeb48a64ee3 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -112,6 +112,8 @@ enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_PEER2PEER;
 enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_DEFAULT;
+u16 max_pcie_mrrs = 4096; // no limit
  * 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 +5818,9 @@ int pcie_set_readrq(struct pci_dev *dev, int rq)
 			rq = mps;
+	if (rq > max_pcie_mrrs)
+		rq = max_pcie_mrrs;
 	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 06ff1186c1ef..2b95a8204819 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -996,6 +996,7 @@ 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

Sieć Badawcza Łukasiewicz
Przemysłowy Instytut Automatyki i Pomiarów PIAP
Al. Jerozolimskie 202, 02-486 Warszawa

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