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

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Message-ID: <20230725083030.GC22139@thinkpad>
Date:   Tue, 25 Jul 2023 14:00:30 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Vidya Sagar <vidyas@...dia.com>
Cc:     lpieralisi@...nel.org, kw@...ux.com, robh@...nel.org,
        bhelgaas@...gle.com, thierry.reding@...il.com,
        jonathanh@...dia.com, Sergey.Semin@...kalelectronics.ru,
        linux-pci@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org, kthota@...dia.com,
        mmaddireddy@...dia.com, sagar.tv@...il.com
Subject: Re: [PATCH V1] Revert "PCI: tegra194: Enable support for 256 Byte
 payload"

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@...dia.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).

- 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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