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

[Vidya Sagar <vidyas@nvidia.com>]

On 7/25/2023 2:00 PM, Manivannan Sadhasivam wrote:
> External email: Use caution opening links or attachments
> 
> 
> On Tue, Jul 25, 2023 at 01:49:35PM +0530, Vidya Sagar wrote:
>>
>>
>> On 7/25/2023 1:21 PM, Manivannan Sadhasivam wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> 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.
>>
> 
> Ah, I missed the fact that your issue only happens with a PCIe switch. But
> shouldn't this be fixed in the PCI core instead?
> 
> I mean, PCI core should use 128byte in your case for Root port unless it is not
> allowed in the spec (which I doubt).
well, if the RP's DevCtl is set to 256MPS by default, then, the core 
wouldn't do it automatically unless either 'pcie_bus_safe' or 
'pcie_bus_perf' is passed.


> 
> - Mani
> 
>> -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
>>>>
>>>
>>> --
>>> மணிவண்ணன் சதாசிவம்
> 
> --
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