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Date:   Tue, 25 Jul 2023 13:21:59 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
To:     Vidya Sagar <vidyas@nvidia.com>
Cc:     lpieralisi@kernel.org, kw@linux.com, robh@kernel.org,
        bhelgaas@google.com, thierry.reding@gmail.com,
        jonathanh@nvidia.com, Sergey.Semin@baikalelectronics.ru,
        linux-pci@vger.kernel.org, linux-tegra@vger.kernel.org,
        linux-kernel@vger.kernel.org, kthota@nvidia.com,
        mmaddireddy@nvidia.com, sagar.tv@gmail.com
Subject: Re: [PATCH V1] Revert "PCI: tegra194: Enable support for 256 Byte
 payload"
Message-ID: <20230725075159.GB22139@thinkpad>
References: <20230608093652.1409485-1-vidyas@nvidia.com>
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On Thu, Jun 08, 2023 at 03:06:52PM +0530, Vidya Sagar wrote:
> This reverts commit 4fb8e46c1bc4 ("PCI: tegra194: Enable
> support for 256 Byte payload")
> 
> Consider a PCIe hierarchy with a PCIe switch and a device connected
> downstream of the switch that has support for MPS which is the minimum
> in the hierarchy, and root port programmed with an MPS in its DevCtl
> register that is greater than the minimum. In this scenario, the default
> bus configuration of the kernel i.e. "PCIE_BUS_DEFAULT" doesn't
> configure the MPS settings in the hierarchy correctly resulting in the
> device with support for minimum MPS in the hierarchy receiving the TLPs
> of size more than that. Although this can be addresed by appending
> "pci=pcie_bus_safe" to the kernel command line, it doesn't seem to be a
> good idea to always have this commandline argument even for the basic
> functionality to work.
> Reverting commit 4fb8e46c1bc4 ("PCI: tegra194: Enable support for 256
> Byte payload") avoids this requirement and ensures that the basic
> functionality of the devices irrespective of the hierarchy and the MPS of
> the devices in the hierarchy.
> To reap the benefits of having support for higher MPS, optionally, one can
> always append the kernel command line with "pci=pcie_bus_perf".
> 
> Fixes: 4fb8e46c1bc4 ("PCI: tegra194: Enable support for 256 Byte payload")
> Signed-off-by: Vidya Sagar <vidyas@nvidia.com>

I know that this patch is merged. But I happen to test a similar change on Qcom
platform during a patch review and found that the PCI core changes MPS to 128
when a 128byte supported device is found:

[    3.174290] pci 0000:01:00.0: Upstream bridge's Max Payload Size set to 128 (was 256, max 128)
[    3.186538] pci 0000:01:00.0: Max Payload Size set to 128 (was 128, max 128)

This was just randomly tested on a platform whose Root port DEVCAP was 128, but
it shouldn't matter. And I didn't change the default bus configuration.

Wondering how you ended up facing issues with it.

- Mani

> ---
>  drivers/pci/controller/dwc/pcie-tegra194.c | 13 -------------
>  1 file changed, 13 deletions(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-tegra194.c b/drivers/pci/controller/dwc/pcie-tegra194.c
> index 4fdadc7b045f..877d81b13334 100644
> --- a/drivers/pci/controller/dwc/pcie-tegra194.c
> +++ b/drivers/pci/controller/dwc/pcie-tegra194.c
> @@ -892,7 +892,6 @@ static int tegra_pcie_dw_host_init(struct dw_pcie_rp *pp)
>  	struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
>  	struct tegra_pcie_dw *pcie = to_tegra_pcie(pci);
>  	u32 val;
> -	u16 val_16;
>  
>  	pp->bridge->ops = &tegra_pci_ops;
>  
> @@ -900,11 +899,6 @@ 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 +1750,6 @@ 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;
> @@ -1887,11 +1880,6 @@ 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) {
>  		val_16 = dw_pcie_readw_dbi(pci, pcie->pcie_cap_base +
> @@ -1900,7 +1888,6 @@ static void pex_ep_event_pex_rst_deassert(struct tegra_pcie_dw *pcie)
>  		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
> 

-- 
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