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

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Message-ID: <53d1bf79-4353-49b5-76b6-c2266535c778@nvidia.com>
Date:   Fri, 21 Jul 2023 09:23:01 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Vidya Sagar <vidyas@...dia.com>, lpieralisi@...nel.org,
        kw@...ux.com, robh@...nel.org, bhelgaas@...gle.com,
        thierry.reding@...il.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, sagar.tv@...il.com
Subject: Re: [PATCH V4] Revert "PCI: tegra194: Enable support for 256 Byte
 payload"

Hi Krzysztof, Bjorn,

On 18/07/2023 03:52, Vidya Sagar wrote:
> 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);



I see a version of this patch here ...

https://git.kernel.org/pub/scm/linux/kernel/git/pci/pci.git/log/?h=controller/tegra194

However, I don't see this in -next yet. If you are happy with this 
latest version, could we get this into -next?

FWIW ...

Acked-by: Jon Hunter <jonathanh@...dia.com>

Thanks!
Jon

-- 
nvpublic