Re: [PATCH V3] Revert "PCI: tegra194: Enable support for 256 Byte payload"

[Vidya Sagar <vidyas@nvidia.com>]

On 7/14/2023 3:09 AM, Bjorn Helgaas wrote:
> External email: Use caution opening links or attachments
> 
> 
> On Mon, Jun 19, 2023 at 03:56:04PM +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 addressed 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.
> 
> I think this has some irrelevant detail (IIUC the problem should
> happen even without a switch) and could be more specific (I think the
> problem case is RP MPS=256, EP only supports MPS=128).
The issue is present only if there is a switch.

> 
>> 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.
> 
> "Ensure" is a transitive verb, so "... ensures that the basic
> functionality ..." is missing whatever the object should be.
I think I missed to add 'work' in the end after 'hierarchy'. My bad.
> 
> Maybe something like the following?
> 
>    After 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 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.
Thanks, I'll send a new patch with the above commit message.
> 
>> 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>
>> ---
>> 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
>>