linux-kernel - Re: [PATCH V3] Revert "PCI: tegra194: Enable support for 256 Byte payload"

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Message-ID: <20230627104814.GA3676313@rocinante>
Date:   Tue, 27 Jun 2023 19:48:14 +0900
From:   Krzysztof Wilczyński <kw@...ux.com>
To:     Vidya Sagar <vidyas@...dia.com>
Cc:     lpieralisi@...nel.org, 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 V3] Revert "PCI: tegra194: Enable support for 256 Byte
 payload"

Hello,

> 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 addressed 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".

Applied to controller/tegra194, thank you!

[1/1] Revert "PCI: tegra194: Enable support for 256 Byte payload"
      https://git.kernel.org/pci/pci/c/606295f16360

	Krzysztof