netdev - RE: [PATCH 1/5] e1000e: reset MAC-PHY interconnect on 82577/82578

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Date:	Tue, 16 Dec 2014 10:48:26 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Zhu Yanjun' <zyjzyj2000@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"w@....eu" <w@....eu>
CC:	Zhu Yanjun <Yanjun.Zhu@...driver.com>,
	Bruce Allan <bruce.w.allan@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: RE: [PATCH 1/5] e1000e: reset MAC-PHY interconnect on 82577/82578

From: Zhu Yanjun
> 2.6.x kernels require a similar logic change as commit 6dfaa76
> [e1000e: reset MAC-PHY interconnect on 82577/82578] introduces
> for newer kernels.
> 
> During Sx->S0 transitions, the interconnect between the MAC and PHY on
> 82577/82578 can remain in SMBus mode instead of transitioning to the
> PCIe-like mode required during normal operation.  Toggling the LANPHYPC
> Value bit essentially resets the interconnect forcing it to the correct
> mode.
> 
> after review of all intel drivers, found several instances where
> drivers had the incorrect pattern of:
> memory mapped write();
> delay();
> 
> which should always be:
> memory mapped write();
> write flush(); /* aka memory mapped read */
> delay();

Probably not only the intel drivers.
I'd bet that a lot of the delay() calls are wrong.
 
> explanation:
> The reason for including the flush is that writes can be held
> (posted) in PCI/PCIe bridges, but the read always has to complete
> synchronously and therefore has to flush all pending writes to a
> device.  If a write is held and followed by a delay, the delay
> means nothing because the write may not have reached hardware
> (maybe even not until the next read)

It might even be that the 'write flush' (ie a read) guarantees to
generates a long enough delay that the delay() itself isn't needed.

	David




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