netdev - Re: [PATCH] e100 rx: or s and el bits

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Date:	Wed, 02 May 2007 16:21:06 -0400
From:	David Acker <dacker@...net.com>
To:	Milton Miller <miltonm@....com>
CC:	Scott Feldman <sfeldma@...ox.com>, Jeff Garzik <jeff@...zik.org>,
	e1000-devel@...ts.sourceforge.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	John Ronciak <john.ronciak@...el.com>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Auke Kok <auke-jan.h.kok@...el.com>
Subject: Re: [PATCH] e100 rx: or s and el bits

David Acker wrote:
> Milton Miller wrote:
>> In commit d52df4a35af569071fda3f4eb08e47cc7023f094, the description
>> talks about emulating another driver by setting addtional bits and
>> the being unable to test when submitted.  Seeing the & operator to
>> set more bits made me suspicious, and indeed the bits are defined
>> in positive logic:
>>
>>     cb_s      = 0x4000,
>>     cb_el     = 0x8000,
>>
>> So anding those together would be 0.   I'm guessing they should
>> be or'd, but don't have hardware here to test, much like the
>> committed patch.  In fact, I'll let someone else do the compile
>> test too.  I'll update the comment.
>>
> 
> I wonder if this worked for me because the hardware also spun on the 
> link field being NULL?  Since the RU base is also set to 0, the 
> calculated physical address would be 0 as well.  I would imagine if the 
> hardware tried to read/write to very low addresses across PCI, there 
> would be issues.  I will retest with a small receive pool to try to hit 
> the problem.
> I will also run these tests with the new patch and with a smaller 
> receive pool (default is 256) to make the pool run out more often.

So far my testing has shown both the original and the new version of the 
S-bit patch work in that no corruption seemed to occur over long term 
runs.  The previous S-bit patch may have only worked due to something 
specific about how my PCI companion chip handles I/O to low memory 
addresses (from dereferencing a link address of 0).  Perhaps the e100 
handles the NULL link as well, but given that the manual does not seem 
to state what happens when the hardware encounters a buffer with a link 
of 0, I think Milton's fix is the proper way to do it.  The old eepro 
driver did set both bits although it did it with a hardcoded constant.

I will continue testing with slab debug on but that will take longer. 
Has anyone tried this on other platforms?
-Ack
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