Subject: Re: [PATCH] gianfar: prevent fragmentation in DSA environments

From: Andrew Lunn <[email-masked]>
Date: Fri, 19 Aug 2016 16:59:00 +0200

On Fri, Aug 19, 2016 at 11:16:14AM +0200, Zefir Kurtisi wrote:
> The eTSEC register MRBLR defines the maximum space in
> the RX buffers and is set to 1536 by gianfar. This
> reasonably covers the common use case where the MTU
> is kept at default 1500.
> 
> Alas, if the eTSEC is attached to a DSA enabled switch,
> the DSA header extension causes every maximum sized
> frame to be fragmented by the hardware (1536 + 4).
> 
> This patch increases the maximum RX buffer size by
> RBUF_ALIGNMENT (64) octets. Since the driver uses a
> half-page memory schema, this change does not
> increase allocated memory but allows the hardware to
> use 1600 bytes of the totally available 2048.
> 
> Signed-off-by: Zefir Kurtisi <[email-masked]>
> ---
>  drivers/net/ethernet/freescale/gianfar.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/freescale/gianfar.h b/drivers/net/ethernet/freescale/gianfar.h
> index 373fd09..02b794b 100644
> --- a/drivers/net/ethernet/freescale/gianfar.h
> +++ b/drivers/net/ethernet/freescale/gianfar.h
> @@ -100,7 +100,8 @@ extern const char gfar_driver_version[];
>  #define DEFAULT_RX_LFC_THR  16
>  #define DEFAULT_LFC_PTVVAL  4
>  
> -#define GFAR_RXB_SIZE 1536
> +/* prevent fragmenation by HW in DSA environments */
> +#define GFAR_RXB_SIZE (1536 + RXBUF_ALIGNMENT)

Hi Zefir

Using RXBUF_ALIGNMENT suggests this has something to do with
alignment, not extra headers.

How about

/* Prevent fragmenation by HW when using extra headers like DSA */
#define GFAR_RXB_SIZE (1536 + 8)

	Andrew