[U-Boot] [PATCH] smc911x driver frame alignment patch

[Valentin Yakovenkov]

21.04.2010 23:52, Mike Frysinger wrote:
>>> Wrong alignment in smc911x driver when reading a frame from fifo.
>>> Neither smc911x chip nor U-Boot doesn't use IP-alignment, so we don't
>>> need to add anything here.
>>
>> I know you use this driver a lot.  Please comment on this patch.
>
> i really havent a clue what this change is doing, and the changelog doesnt
> make much sense to me.  too many negatives perhaps ...

SMSC911x chips has alignment function to allow frame payload data (which 
comes after 14-bytes ethernet header) to be aligned at some boundary 
when reading it from fifo (usually - 4 bytes boundary).
This is done by inserting fake zeros bytes BEFORE actual frame data when 
reading from SMSC's fifo.
This function controlled by RX_CFG register. There are bits that 
represents amount of fake bytes to be inserted.

Linux uses alignment of 4 bytes. Ethernet frame header is 14 bytes long, 
so we need to add 2 fake bytes to get payload data aligned at 4-bytes 
boundary.
Linux driver does this by adding IP_ALIGNMENT constant (defined at 
skb.h) when calculating fifo data length. But all network subsystem of 
Linux uses this constant too when calculating different offsets.

But u-boot does not use any packet data alignment, so we don't need to 
add anything when calculating fifo data length.
Moreover, driver zeros the RX_CFG register just one line up, so chip 
does not insert any fake data. After calculating data length we always 
got 1 more word to read.

So, at almost every packet read we get an underflow condition at fifo 
and possible corruption of data. Especially at continuous transfers, 
such as tftp.

Just after removing this magic addition, I've got tftp transfer speed as 
it aught to be at 100Mbps. It was really slow before.

Sorry for my english, I'm just a bad russian boy %)

-- 
   WBR, Valentin
   CJSC "NII STT", Russia, Smolensk
   http://www.niistt.ru

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