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Date:	Tue, 5 Jun 2007 11:14:15 -0500
From:	Milton Miller <miltonm@....com>
To:	David Acker <dacker@...net.com>
Cc:	Jeff Garzik <jgarzik@...ox.com>, netdev@...r.kernel.org,
	e1000-devel@...ts.sourceforge.net,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	John Ronciak <john.ronciak@...el.com>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	"Kok, Auke" <auke-jan.h.kok@...el.com>,
	Scott Feldman <sfeldma@...ox.com>
Subject: Re: [PATCH] fix e100 rx path on ARM (was [PATCH] e100 rx: or s and el bits)

On Jun 5, 2007, at 8:34 AM, David Acker wrote:
> Milton Miller wrote:
>> On Jun 1, 2007, at 3:45 PM, David Acker wrote:
>>> Ok, I took a stab at coding and testing these ideas.  Below is a 
>>> patch against 2.6.22-rc3.
>>> Let me know what you think.
>> I think you got most of the ideas.   As Auke noted, your coding style 
>> is showing again.   And your mailer again munged whitespace (fixed by 
>> s/^<space><space>/<space>/ s/^$/<space>/).
> Sorry about the coding style.  I instinctively followed what was there 
> instead of kernel coding convention.  I will look into how whitespace 
> is getting screwed up.

I have to watch my coding style too (I like to indent the closing 
brace).

At least the white space damage seems to be reversable.  More than I 
can say for this mailer.

>>> Find a buffer that is complete with rx->el not set and rx->s0 set.
>>>     It appears that hardware can read the rfd's el-bit, then 
>>> software can clear the rfd el-bit and set the rfd size, and then 
>>> hardware can come in and read the size.
>> Yes, since the size is after the EL flag in the descriptor, this can 
>> happen since the pci read is not atomic.
>>> I am reading the status back, although I don't think that I have to 
>>> in this instance.
>> Actually, you are reading it when the rfd still has EL set.  Since 
>> the cpu will never encounter that case, the if condition is never 
>> satisfied.
> In my tests, every time I found a completed rfd with the el-bit set, 
> the receiver was in the out of resources state.

Yes, if the EL was set, it would be a real hard race to find the 
completed packet with EL but not RNR.   I was trying to refer to where 
you find a completed packet and then check for EL in the RFD.  That is 
what I was claiming can not be observed by the cpu (unless the card 
writes the EL bit back, and not just the status u16).

If the unless ... above is true, then please put a comment that the 
device can write RFD->EL back to 1 if we raced.


>> How about creating a state unknown, for when we think we should check 
>> the device if its running.
>> If we are in this state and then encounter a received packet without 
>> s0 set, we can set it back
>> to running.   We set it when we rx a packet with s0 set.
>> We then move both io_status reads to the caller.
> I can look into that as I clean this up.
>
>
>>> I am testing a version of this code patched against 2.6.18.4 on my 
>>> PXA 255 based system.  I will let you all know how it goes.
> The testing I did so far did well.  I will try to get some more going 
> tonight, hopefully on a cleaned up patch.

Good to hear our expectiations match reality.

>
>
>> I'm assuming this is why the cleanup of the receiver start to always 
>> start on rx_to_clean got dropped again. :-)
> Yep.  I will get that in the next patch.

Ok.

>> Also, I would like a few sentences in the Driver Operation section IV 
>> Receive big comment.  Something like
>> In order to keep updates to the RFD link field from colliding with 
>> hardware writes to mark packets complete, we use the feature that 
>> hardware will not write to a size 0 descriptor and mark the previous 
>> packet as end-of-list (EL).   After updating the link, we remove EL 
>> and only then restore the size such that hardware may use the 
>> previous-to-end RFD.
>> at the end of the first paragraph, and insert software before "no 
>> locking is required" in the second.
> Sounds good to me.
>
> I will see if I can get into a cleaned up patch today and get it out 
> by tomorrow.  Thanks for dealing with me...I have been around kernel 
> code for awhile but posting official patches to linux is new to me.
> -Ack

I've just learned by watching the lists over the last several years.  
Well, and actually writing the odd patch here and there.

It occurs to me that I have been focusing on the code and not the 
changelog.   I'll send a seperate reply on that thread shortly.

One more thing I'll state here ... as per the perfect patch guidelines, 
it is preferred that the meta-discussion about the patch and its 
history go after the change log, seperated from it by a line of "--- " 
so that the patch application scripts can just extract the email 
subject as the title and through the firsst line of --- as the commit 
log.  (This saves some manual editing).

[1] http://kernelnewbies.org/UpstreamMerge

milton

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