linux-kernel - RE: [PATCH 06/16] ntb: add check and comment for link up to mw_count and mw_get

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Date:   Thu, 29 Jun 2017 21:27:50 +0000
From:   "Hubbe, Allen" <Allen.Hubbe@...l.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        "linux-ntb@...glegroups.com" <linux-ntb@...glegroups.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "'Jon Mason'" <jdmason@...zu.us>,
        "'Dave Jiang'" <dave.jiang@...el.com>,
        "'Bjorn Helgaas'" <bhelgaas@...gle.com>,
        "'Greg Kroah-Hartman'" <gregkh@...uxfoundation.org>,
        "'Kurt Schwemmer'" <kurt.schwemmer@...rosemi.com>,
        "'Stephen Bates'" <sbates@...thlin.com>,
        "'Serge Semin'" <fancer.lancer@...il.com>
Subject: RE: [PATCH 06/16] ntb: add check and comment for link up to
 mw_count and mw_get_align

From: Logan Gunthorpe
> On 6/29/2017 2:13 PM, Allen Hubbe wrote:
> > Unfortunately, it is to work around hardware errata.  That is not so trivial to fix.
> 
> Can you describe more what the work around is doing? Can you share the
> code? It seems odd that a workaround is based on the alignment
> restrictions of the mws.

Sure, while not making any claim that this is ready for upstream.

It is not a workaround for alignment restrictions of the mws.  It is a restriction to avoid the use of doorbells and scratchpads.  Memory windows are used exclusively.

Read msi-x local <addr,data> and send that to the peer:
https://github.com/ntrdma/ntrdma/blob/master/drivers/ntc/ntc_ntb_msi.c#L603

Transform peer's addr to the memory window region:
https://github.com/ntrdma/ntrdma/blob/master/drivers/ntc/ntc_ntb_msi.c#L622

Append a dma immediate value operation after other operations, to write the data at addr:
https://github.com/ntrdma/ntrdma/blob/master/drivers/ntc/ntc_ntb_msi.c#L1208

Above describes the workaround.

Used in the context of a rdma-over-ntb driver here:
https://github.com/ntrdma/ntrdma/blob/master/drivers/infiniband/hw/ntrdma/ntrdma_qp.c#L1585

> 
> Logan