linux-kernel - Re: [PATCH v1 5/5] pci: keystone: add pcie driver based on designware core driver

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Message-ID: <CAErSpo667uWybiHOh4tVQDh_JFpF0EbvxKwo_GQm+B1-wMtT1Q@mail.gmail.com>
Date:	Tue, 20 May 2014 11:22:22 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	"Karicheri, Muralidharan" <m-karicheri2@...com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"Strashko, Grygorii" <grygorii.strashko@...com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Jingoo Han <jg1.han@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	Mohit Kumar <mohit.kumar@...com>
Subject: Re: [PATCH v1 5/5] pci: keystone: add pcie driver based on designware
 core driver

On Tue, May 20, 2014 at 11:02 AM, Jason Gunthorpe
<jgunthorpe@...idianresearch.com> wrote:
> On Fri, May 16, 2014 at 08:29:56PM +0000, Karicheri, Muralidharan wrote:
>
>> But pcie_bus_configure_settings just make sure the mrrs for a device
>> is not greater than the max payload size.
>
> Not quite, it first scans the network checking the Maximum Payload Size
> Supported (MPSS) for each device, and chooses the highest supported by
> all as the MPS for all.

The "performance" setting, e.g., "pci=pcie_bus_perf", adds a few
wrinkles by setting MRRS in ways that allow some devices to have
larger MPS than others.  I don't think this is exactly what was
envisioned in the spec, and it is not guaranteed to work if there is
peer-to-peer DMA.  This isn't documented very well; the best I know of
is the changelogs for:

  a1c473aa11e6 pci: Clamp pcie_set_readrq() when using "performance" settings
  b03e7495a862 PCI: Set PCI-E Max Payload Size on fabric

Bjorn
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