Messages in this thread | | | Date | Wed, 6 Aug 2014 10:38:19 -0400 | From | Murali Karicheri <> | Subject | Re: [PATCH v7 4/5] PCI: add PCI controller for keystone PCIe h/w |
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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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-pci" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html 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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