Re: [PATCH V1] Revert "PCI: tegra194: Enable support for 256 Byte payload"

[Vidya Sagar <vidyas@nvidia.com>]

On 7/25/2023 1:21 PM, Manivannan Sadhasivam wrote:
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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.

If the endpiont device that has support only for 128byte MPS is 
connected directly to the root port, then, I agree that the PCIe 
sub-system takes care of changing the MPS to the common lowest MPS, but 
if the endpoint device is connected behind a PCIe switch, then the PCIe 
subsystem doesn't configure the MPS properly.

-Vidya Sagar

> 
> - 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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