[U-Boot] [PATCH v2] smc911x driver frame alignment patch

[Ben Warren]

  Valentin,
On 4/22/2010 10:40 PM, Valentin Yakovenkov wrote:
> SMSC911x chips have 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. 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 at the beginig. So calculated data length
> is always bigger by 1 word.
>
> It seems that 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.
>
> It seems that fifo underflow occurs only when using byte packing on
> 32-bit blackfin bus (may be because of very small delay between reads).
>
>
>
> Signed-off-by: Valentin Yakovenkov<yakovenkov at niistt.ru>
> diff -r 7dc8ff189175 a/drivers/net/smc911x.c
> --- a/drivers/net/smc911x.c	Mon Mar 29 11:08:55 2010 +0400
> +++ b/drivers/net/smc911x.c	Mon Apr 19 10:46:02 2010 +0400
> @@ -220,7 +220,7 @@
>
>   		smc911x_reg_write(dev, RX_CFG, 0);
>
> -		tmplen = (pktlen + 2+ 3) / 4;
> +		tmplen = (pktlen + 3) / 4;
>   		while (tmplen--)
>   			*data++ = pkt_data_pull(dev, RX_DATA_FIFO);
>
> --
>    WBR, Valentin
>    CJSC "NII STT", Russia, Smolensk
>    http://www.niistt.ru
>
>    
Applied to net repo.

thanks,
Ben