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

[Vidya Sagar <vidyas@nvidia.com>]

On 8/2/2023 2:10 AM, Bjorn Helgaas wrote:
> External email: Use caution opening links or attachments
> 
> 
> On Tue, Jul 18, 2023 at 08:22:21AM +0530, 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.
> 
> Thanks for pointing out that I broke this log by omitting the mention
> of a switch.  Is the rewording below better?  If so, Krzysztof can
> amend the commit.
Yes. The below rewording looks good.

Thanks,
Vidya Sagar

> 
>    After commit 4fb8e46c1bc4 ("PCI: tegra194: Enable support for 256 Byte
>    payload"), we initialize MPS=256 for tegra194 Root Ports before enumerating
>    the hierarchy.
> 
>    Consider an Endpoint that supports only MPS=128.  In the default situation
>    (CONFIG_PCIE_BUS_DEFAULT set and no "pci=pcie_bus_*" parameter), Linux
>    tries to configure the MPS of every device to match the upstream bridge.
>    If the Endpoint is directly below the Root Port, Linux can reduce the Root
>    Port MPS to 128 to match the Endpoint.  But if there's a switch in the
>    middle, Linux doesn't reduce the Root Port MPS because other devices below
>    the switch may already be configured with MPS larger than 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")
> 
> 4fb8e46c1bc4 appeared in v6.0-rc1, so this wouldn't be a candidate for
> v6.5, but it does sound like it should be tagged for stable?  If so,
> Krzysztof can probably add that as well.
> 
>> Signed-off-by: Vidya Sagar <vidyas@nvidia.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
>>