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

Murali Karicheri
2014-08-06T14:39:38Z

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