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

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Date:   Fri, 19 Aug 2016 21:24:30 +0000
From:   Claudiu Manoil <claudiu.manoil@....com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     Zefir Kurtisi <zefir.kurtisi@...atec.com>,
        "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

Hi Andrew,

From: Andrew Lunn <andrew@...n.ch>
Sent: Friday, August 19, 2016 7:39 PM
To: Claudiu Manoil
Cc: Zefir Kurtisi; netdev@...r.kernel.org; claudiu.manoil@...escale.com
Subject: Re: [PATCH] gianfar: prevent fragmentation in DSA environments

> >> -#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.

Hi Claudiu

Thanks for pointing this out. This makes RXBUF_ALIGNMENT even worse
for understabability, since you say multiples of 64 is important, not
alignment.

How about

/* Prevent fragmentation by HW when using extra headers like DSA,
while respecting the multiple of 64 requirement. */

#define GFAR_RXB_SIZE roundup(1536 + 8, 64)

[Claudiu]
Nice improvement.
But what's so special about 8?  I thought only 4 bytes were missing :)
At least 1536 is the default size of the MRBLR register, as specified
in the h/w ref manual.  Is there some recommended standard size
to accommodate most (if not all) headers, to refer to?