netdev - RE: [PATCH] gianfar: prevent fragmentation in DSA environments

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Date:   Fri, 19 Aug 2016 15:49:17 +0000
From:   Claudiu Manoil <claudiu.manoil@....com>
To:     Andrew Lunn <andrew@...n.ch>,
        Zefir Kurtisi <zefir.kurtisi@...atec.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "claudiu.manoil@...escale.com" <claudiu.manoil@...escale.com>
Subject: RE: [PATCH] gianfar: prevent fragmentation in DSA environments

>-----Original Message-----
>From: Andrew Lunn [mailto:andrew@...n.ch]
>Sent: Friday, August 19, 2016 5:59 PM
>To: Zefir Kurtisi <zefir.kurtisi@...atec.com>
>Cc: netdev@...r.kernel.org; claudiu.manoil@...escale.com
>Subject: Re: [PATCH] gianfar: prevent fragmentation in DSA environments
>
>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 <zefir.kurtisi@...atec.com>
>> ---
>>  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)
>

MRBL (Maximum receive buffer length) must be multiple of 64.
Please consult de hardware documentation at least before prosing
changes to the driver.

Claudiu