[PATCH v2 2/3] gianfar: Implement workaround for eTSEC76 erratum

[David Miller]

From: Anton Vorontsov <avorontsov at mvista.com>
Date: Wed, 30 Jun 2010 20:39:13 +0400

> MPC8313ECE says:
> 
> "For TOE=1 huge or jumbo frames, the data required to generate the
>  checksum may exceed the 2500-byte threshold beyond which the controller
>  constrains itself to one memory fetch every 256 eTSEC system clocks.
> 
>  This throttling threshold is supposed to trigger only when the
>  controller has sufficient data to keep transmit active for the duration
>  of the memory fetches. The state machine handling this threshold,
>  however, fails to take large TOE frames into account. As a result,
>  TOE=1 frames larger than 2500 bytes often see excess delays before start
>  of transmission."
> 
> This patch implements the workaround as suggested by the errata
> document, i.e.:
> 
> "Limit TOE=1 frames to less than 2500 bytes to avoid excess delays due to
>  memory throttling.
>  When using packets larger than 2700 bytes, it is recommended to turn TOE
>  off."
> 
> To be sure, we limit the TOE frames to 2500 bytes, and do software
> checksumming instead.
> 
> Signed-off-by: Anton Vorontsov <avorontsov at mvista.com>

Applied.