linux-kernel - [PATCH V4] Revert "PCI: tegra194: Enable support for 256 Byte payload"

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Message-ID: <20230718025221.4001329-1-vidyas@nvidia.com>
Date:   Tue, 18 Jul 2023 08:22:21 +0530
From:   Vidya Sagar <vidyas@...dia.com>
To:     <lpieralisi@...nel.org>, <kw@...ux.com>, <robh@...nel.org>,
        <bhelgaas@...gle.com>, <thierry.reding@...il.com>,
        <jonathanh@...dia.com>, <Sergey.Semin@...kalelectronics.ru>
CC:     <linux-pci@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <kthota@...dia.com>,
        <mmaddireddy@...dia.com>, <vidyas@...dia.com>, <sagar.tv@...il.com>
Subject: [PATCH V4] Revert "PCI: tegra194: Enable support for 256 Byte payload"

After commit 4fb8e46c1bc4 ("PCI: tegra194: Enable support for 256 Byte
payload"), we set MPS=256 for tegra194 Root Ports.

By default (CONFIG_PCIE_BUS_DEFAULT set and no "pci=pcie_bus_*"
parameter), Linux configures the MPS of every device to match the
upstream bridge, which is impossible if the Root Port has MPS=256
and a device only supports MPS=128.

This scenario results in uncorrectable Malformed TLP errors if the
Root Port sends TLPs with payloads larger than 128 bytes.  These
errors can be avoided by using the "pci=pcie_bus_safe" parameter,
but it doesn't seem to be a good idea to always have this parameter
even for basic functionality to work.

Revert commit 4fb8e46c1bc4 ("PCI: tegra194: Enable support for 256 Byte
payload") so the Root Ports default to MPS=128, which all devices
support.

If peer-to-peer DMA is not required, one can use "pci=pcie_bus_perf"
to get the benefit of larger MPS settings.

[ rewrote commit message based on Bjorn's suggestion ]

Fixes: 4fb8e46c1bc4 ("PCI: tegra194: Enable support for 256 Byte payload")
Signed-off-by: Vidya Sagar <vidyas@...dia.com>
---
V4:
* Rewrote commit message based on Bjorn's suggestion

V3:
* Fixed a build issue

V2:
* Addressed review comments from Bjorn

 drivers/pci/controller/dwc/pcie-tegra194.c | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/drivers/pci/controller/dwc/pcie-tegra194.c b/drivers/pci/controller/dwc/pcie-tegra194.c
index 4fdadc7b045f..a772faff14b5 100644
--- a/drivers/pci/controller/dwc/pcie-tegra194.c
+++ b/drivers/pci/controller/dwc/pcie-tegra194.c
@@ -900,11 +900,6 @@ static int tegra_pcie_dw_host_init(struct dw_pcie_rp *pp)
 		pcie->pcie_cap_base = dw_pcie_find_capability(&pcie->pci,
 							      PCI_CAP_ID_EXP);
 
-	val_16 = dw_pcie_readw_dbi(pci, pcie->pcie_cap_base + PCI_EXP_DEVCTL);
-	val_16 &= ~PCI_EXP_DEVCTL_PAYLOAD;
-	val_16 |= PCI_EXP_DEVCTL_PAYLOAD_256B;
-	dw_pcie_writew_dbi(pci, pcie->pcie_cap_base + PCI_EXP_DEVCTL, val_16);
-
 	val = dw_pcie_readl_dbi(pci, PCI_IO_BASE);
 	val &= ~(IO_BASE_IO_DECODE | IO_BASE_IO_DECODE_BIT8);
 	dw_pcie_writel_dbi(pci, PCI_IO_BASE, val);
@@ -1756,7 +1751,6 @@ static void pex_ep_event_pex_rst_deassert(struct tegra_pcie_dw *pcie)
 	struct device *dev = pcie->dev;
 	u32 val;
 	int ret;
-	u16 val_16;
 
 	if (pcie->ep_state == EP_STATE_ENABLED)
 		return;
@@ -1887,20 +1881,16 @@ static void pex_ep_event_pex_rst_deassert(struct tegra_pcie_dw *pcie)
 	pcie->pcie_cap_base = dw_pcie_find_capability(&pcie->pci,
 						      PCI_CAP_ID_EXP);
 
-	val_16 = dw_pcie_readw_dbi(pci, pcie->pcie_cap_base + PCI_EXP_DEVCTL);
-	val_16 &= ~PCI_EXP_DEVCTL_PAYLOAD;
-	val_16 |= PCI_EXP_DEVCTL_PAYLOAD_256B;
-	dw_pcie_writew_dbi(pci, pcie->pcie_cap_base + PCI_EXP_DEVCTL, val_16);
-
 	/* Clear Slot Clock Configuration bit if SRNS configuration */
 	if (pcie->enable_srns) {
+		u16 val_16;
+
 		val_16 = dw_pcie_readw_dbi(pci, pcie->pcie_cap_base +
 					   PCI_EXP_LNKSTA);
 		val_16 &= ~PCI_EXP_LNKSTA_SLC;
 		dw_pcie_writew_dbi(pci, pcie->pcie_cap_base + PCI_EXP_LNKSTA,
 				   val_16);
 	}
-
 	clk_set_rate(pcie->core_clk, GEN4_CORE_CLK_FREQ);
 
 	val = (ep->msi_mem_phys & MSIX_ADDR_MATCH_LOW_OFF_MASK);
-- 
2.25.1