Re: [PATCH] PCI: Document PCIE BUS MPS parameters

[Andrew Murray <andrew.murray@arm.com>]

On Wed, Jan 23, 2013 at 08:01:36AM +0000, Yijing Wang wrote:
> Document PCIE BUS MPS parameters pcie_bus_tune_off, pcie_bus_safe,
> pcie_bus_peer2peer, pcie_bus_perf into Documentation/kernel-parameters.txt.
> These parameters were introduced by Jon Mason <mason@myri.com> at
> commit 5f39e6705 and commit b03e7495a8. Document these into 
> kernel-parameters.txt help users to understand and use the parameters.
> 
> Signed-off-by: Yijing Wang <wangyijing@huawei.com>
> ---
>  Documentation/kernel-parameters.txt |   13 +++++++++++++
>  1 files changed, 13 insertions(+), 0 deletions(-)
> 
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 363e348..4dfa8d2 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -2227,6 +2227,19 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>  				This sorting is done to get a device
>  				order compatible with older (<= 2.4) kernels.
>  		nobfsort	Don't sort PCI devices into breadth-first order.
> +		pcie_bus_tune_off	[X86] Disable PCI-E MPS turning and using
> +				the BIOS configured MPS defaults.
> +		pcie_bus_safe	[X86] Use the smallest common denominator MPS
> +				of the entire tree below a root complex for every device
> +				on that fabric. Can avoid inconsistent mps problem caused
> +				by hotplug.
> +		pcie_bus_perf	[X86] Configure pcie device MPS to the largest
> +				allowable MPS based on its parent bus.Improve performance
> +				as much as possible.
> +		pcie_bus_peer2peer	[X86] Make the system wide MPS the smallest
> +				possible value (128B).This configuration could prevent it
> +				from working by having the MPS on one root port different
> +				than the MPS on another.
>  		cbiosize=nn[KMG]	The fixed amount of bus space which is
>  				reserved for the CardBus bridge's IO window.
>  				The default value is 256 bytes.
>
I was searching for documentation on this the other day.

It's not just X86 that use these options, PowerPC and Tile also use them (grep
for users of pcie_bus_configure_settings). I've also noticed a call to it from
hotplug as well...

In addition these options also have an effect on MRRS - I've not figured out
what effect they have, but you can look in drivers/pci/probe.c at the
pcie_write_mrrs function.

Andrew Murray