On 7/18/2023 4:39 PM, Bjorn Helgaas wrote:
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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
Yes. The code after the if condition here takes care of that.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/probe.c?h=v6.5-rc2#n2049
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
