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

[Bjorn Helgaas]

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.

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.

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.

Bjorn