linux-kernel - Re: [PATCH v7 4/5] PCI: add PCI controller for keystone PCIe h/w

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Date:	Wed, 6 Aug 2014 10:38:19 -0400
From:	Murali Karicheri <m-karicheri2@...com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
CC:	Bjorn Helgaas <bhelgaas@...gle.com>,
	Richard Zhu <r65037@...escale.com>,
	Marek Vasut <marex@...x.de>,
	Randy Dunlap <rdunlap@...radead.org>,
	Russell King <linux@....linux.org.uk>,
	Pawel Moll <pawel.moll@....com>, Arnd Bergmann <arnd@...db.de>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	<linux-pci@...r.kernel.org>, Jingoo Han <jg1.han@...sung.com>,
	<linux-kernel@...r.kernel.org>,
	Kishon Vijay Abraham I <kishon@...com>,
	Rob Herring <robh+dt@...nel.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Kumar Gala <galak@...eaurora.org>,
	Grant Likely <grant.likely@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v7 4/5] PCI: add PCI controller for keystone PCIe h/w

On 07/30/2014 04:05 PM, Jason Gunthorpe wrote:
> On Wed, Jul 30, 2014 at 03:34:44PM -0400, Murali Karicheri wrote:
>
>> • Maximum remote read request size of 256 bytes
>
> The PCI spec simply does not permit a completer to impose this
> limitation.
>
> See spec 2.3.1.1
>
> It is not an option to error a read request because it is too
> long. All requests must be completed. All completions must be segmented
> according to the RCB and Max_Payload_Size.
>
>> completion packets. So it seems to be able to segment up to maximum
>> 256 bytes of read request. Where do I find the requirement in PCI
>> spec that "read response segmentation at the host bridge does not
>> work" ?
>
> You just said it. Segmenting up to only 256 bytes is not a limitation
> the completer can impose. Such a device is non-conformant.
>
>>> So, the only fix is that every downstream device must always have a
>>> MRSS set to less than the MPS of the host bridge.
>>
>> Why this can't be the default behavior in the PCI core? Any cons?
>
> It increases read request traffic and read response latency on the bus
> for large transfers.
>
> The MRSS is intented as a performance tuning knob, it is not something
> that ever needs to be set properly for correct bus operation.
>
> Jason
> --
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Jason, Bjorn,

It appears that Keystone PCI has a hardware limitation that can't
handle read request size of greater than 256 bytes and a quirk is the
way to handle this hardware limitation. I will be sending a patch to
address this

Murali

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