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

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Date:   Tue, 18 Jul 2023 06:09:13 -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 V3] Revert "PCI: tegra194: Enable support for 256 Byte
 payload"

On Tue, Jul 18, 2023 at 08:03:47AM +0530, Vidya Sagar wrote:
> On 7/14/2023 3:09 AM, Bjorn Helgaas wrote:
> > On Mon, Jun 19, 2023 at 03:56:04PM +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 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.
> > 
> > I think this has some irrelevant detail (IIUC the problem should
> > happen even without a switch) and could be more specific (I think the
> > problem case is RP MPS=256, EP only supports MPS=128).
>
> The issue is present only if there is a switch.

So if there's no switch, and an EP that only supports MPS=128, the PCI
core changes the RP MPS setting to 128?  Just based on reading the
code, I thought we would leave RP MPS=256 and EP MPS=128, which would
be a problem.  But maybe the PCI core changes the RP down to MPS=128?

Bjorn