Re: [PATCH 0/4] PCI: replace dublicated MRRS limit quirks

[Art Nikpal]

> The PCI core is the correct place to handle MPS/MRRS because their> behavior is defined by the PCIe spec.
>Quirks are the way to work around this defect in the Synopsys PCIe IP.

> don't want it to make
> maintenance of the generic MPS/MRRS code harder.

Trying summarize ( every one must use separate quirk )

which file is right place for for meson_mrrs_limit_quirk()

- pci/controller/dwc/pci-meson.c or
- pci/quirks.c

rewrited quirk just for meson:

static void meson_mrrs_limit_quirk(struct pci_dev *dev)
{
    struct pci_bus *bus = dev->bus;
    int mrrs, mrrs_limit = 256;
    static const struct pci_device_id bridge_devids[] = {
        { PCI_DEVICE(PCI_VENDOR_ID_SYNOPSYS, PCI_DEVICE_ID_SYNOPSYS_HAPSUSB3) },
        { 0, },
    };

    /* look for the matching bridge */
    while (!pci_is_root_bus(bus)) {
        /*
         * 256 bytes maximum read request size. They can't handle
         * anything larger than this. So force this limit on
         * any devices attached under these ports.
         */
        if (!pci_match_id(bridge_devids, bus->self)){
            bus = bus->parent;
            continue;
        }
        mrrs = pcie_get_readrq(dev);
        if (mrrs > mrrs_limit) {
            pci_info(dev, "limiting MRRS %d to %d\n", mrrs, mrrs_limit);
            pcie_set_readrq(dev, mrrs_limit);
        }
        break;
    }
}
DECLARE_PCI_FIXUP_ENABLE(PCI_ANY_ID, PCI_ANY_ID, meson_mrrs_limit_quirk);


On Thu, Jul 8, 2021 at 12:57 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>
> On Wed, Jul 07, 2021 at 06:43:13PM +0200, Neil Armstrong wrote:
> > On 07/07/2021 17:54, Bjorn Helgaas wrote:
> > > On Tue, Jul 06, 2021 at 11:54:05AM +0200, Neil Armstrong wrote:
> > >> In their Designware PCIe controller driver, amlogic sets the
> > >> Max_Payload_Size & Max_Read_Request_Size to 256:
> > >> https://elixir.bootlin.com/linux/latest/source/drivers/pci/controller/dwc/pci-meson.c#L260
> > >> https://elixir.bootlin.com/linux/latest/source/drivers/pci/controller/dwc/pci-meson.c#L276
> > >> in their root port PCIe Express Device Control Register.
> > >>
> > >> Looking at the Synopsys DW-PCIe Databook, Max_Payload_Size &
> > >> Max_Read_Request_Size are used to decompose into AXI burst, but it
> > >> seems the Max_Payload_Size & Max_Read_Request_Size are set by
> > >> default to 512 but the internal Max_Payload_Size_Supported is set to
> > >> 256, thus changing these values to 256 at runtime to match and
> > >> optimize bandwidth.
> > >>
> > >> It's said, "Reducing Outbound Decomposition" :
> > >>  - "Ensure that your application master does not generate bursts of
> > >>    size greater than or equal to Max_Payload_Size"
> > >>
> > >>  - "Program your PCIe system with a larger value of Max_Payload_Size
> > >>    without exceeding Max_Payload_Size_Supported"
> > >>
> > >>  - "Program your PCIe system with a larger value of Max_Read_Request
> > >>    without exceeding Max_Payload_Size_Supported:
> > >>
> > >> So leaving 512 in Max_Payload_Size & Max_Read_Request leads to
> > >> Outbound Decomposition which decreases PCIe link and degrades the
> > >> AXI bus by doubling the bursts, leading to this fix to avoid
> > >> overflowing the AXI bus.
> > >>
> > >> So it seems to be still needed, I assume this *should* be handled in
> > >> the core somehow to propagate these settings to child endpoints to
> > >> match the root port Max_Payload_Size & Max_Read_Request sizes.
> > >>
> > >> Maybe by adding a core function to set these values instead of using
> > >> the dw_pcie_find_capability() & dw_pcie_write/readl_dbi() helpers
> > >> and set a state on the root port to propagate the value ?
> > >
> > > I don't have the Synopsys DW-PCIe Databook, so I'm lacking any
> > > context.  The above *seems* to say that MPS/MRRS settings affect AXI
> > > bus usage.
> >
> > It does when the TLPs are directed to the RC.
>
> That's a defect in the RC.
>
> > > The MPS and MRRS registers are defined to affect traffic on *PCIe*.  If
> > > a platform uses MPS and MRRS values to optimize transfers on non-PCIe
> > > links, that's a problem because the PCI core code that manages MPS and
> > > MRRS has no knowledge of those non-PCIe parts of the system.
> >
> > Yes and no, it only affects PCIe in P2P, in non-P2P is will certainly affect
> > transfers on the internal SoC/Processor/Chip internal bus/fabric.
> >
> > > You might be able to deal with this in Synopsys-specific code somehow,
> > > but it's going to be a bit of a hassle because I don't want it to make
> > > maintenance of the generic MPS/MRRS code harder.
> >
> > I understand, but this is why these quirks are currently implemented in the
> > controller driver and only applies when the controller has been probed
> > and to each endpoint detected on this particular controller.
> >
> > So we may continue having separate quirks for each controller if the core
> > isn't the right place to handle MPS/MRRS.
>
> The PCI core is the correct place to handle MPS/MRRS because their
> behavior is defined by the PCIe spec.
>
> Quirks are the way to work around this defect in the Synopsys PCIe IP.
>
> Bjorn