linux-kernel - Re: [RFC v3 1/2] PCI-Express Non-Transparent Bridge Support

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Message-Id: <20120921.141447.1146043198575242769.davem@davemloft.net>
Date:	Fri, 21 Sep 2012 14:14:47 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	jon.mason@...el.com
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-pci@...r.kernel.org, dave.jiang@...el.com,
	nab@...ingtidesystems.com
Subject: Re: [RFC v3 1/2] PCI-Express Non-Transparent Bridge Support

From: Jon Mason <jon.mason@...el.com>
Date: Fri, 21 Sep 2012 11:09:48 -0700

> A PCI-Express non-transparent bridge (NTB) is a point-to-point PCIe bus
> connecting 2 systems, providing electrical isolation between the two subsystems.
> A non-transparent bridge is functionally similar to a transparent bridge except
> that both sides of the bridge have their own independent address domains.  The
> host on one side of the bridge will not have the visibility of the complete
> memory or I/O space on the other side of the bridge.  To communicate across the
> non-transparent bridge, each NTB endpoint has one (or more) apertures exposed to
> the local system.  Writes to these apertures are mirrored to memory on the
> remote system.  Communications can also occur through the use of doorbell
> registers that initiate interrupts to the alternate domain, and scratch-pad
> registers accessible from both sides.
> 
> The NTB device driver is needed to configure these memory windows, doorbell, and
> scratch-pad registers as well as use them in such a way as they can be turned
> into a viable communication channel to the remote system.  ntb_hw.[ch]
> determines the usage model (NTB to NTB or NTB to Root Port) and abstracts away
> the underlying hardware to provide access and a common interface to the doorbell
> registers, scratch pads, and memory windows.  These hardware interfaces are
> exported so that other, non-mainlined kernel drivers can access these.
> ntb_transport.[ch] also uses the exported interfaces in ntb_hw.[ch] to setup a
> communication channel(s) and provide a reliable way of transferring data from
> one side to the other, which it then exports so that "client" drivers can access
> them.  These client drivers are used to provide a standard kernel interface
> (i.e., Ethernet device) to NTB, such that Linux can transfer data from one
> system to the other in a standard way.
> 
> Signed-off-by: Jon Mason <jon.mason@...el.com>

I really don't see anything that requires the TX networking processing
to occur in a tasklet.  I see no sleeping, strange locking, or
anything like that.

And if that's the case, it's just pure overhead.
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