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

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Message-ID: <20230608163346.GA1204586@bhelgaas>
Date:   Thu, 8 Jun 2023 11:33:46 -0500
From:   Bjorn Helgaas <helgaas@...nel.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 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".

Please add blank lines between paragraphs and wrap to fill 75 columns.
Also add a period at the end of the very first sentence.

s/addresed/addressed/

I guess that without 4fb8e46c1bc4, Linux configured everything with
128 byte MPS, and 4fb8e46c1bc4 was intended as an optimization to
allow 256 byte MPS.

If the Root Port advertises Max_Payload_Size Supported as 256 bytes in
DevCap, and the PCI core doesn't configure MPS=256 when possible, I'd
argue that should be fixed in the PCI core without a driver change
like this.

Bjorn