netdev - RE: [PATCH (net.git) 2/4] stmmac: fix and better tune the default buffer sizes

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D0F6CC945@AcuExch.aculab.com>
Date:	Thu, 27 Feb 2014 10:51:22 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Giuseppe Cavallaro' <peppe.cavallaro@...com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH (net.git) 2/4] stmmac: fix and better tune the default
 buffer sizes

From: Giuseppe Cavallaro
> This patch is to fix and tune the default buffer sizes.
> It reduces the default bufsize used by the driver from
> 2048 to 1518 (taking into account the extra 4 bytes in case of VLAN).
...
> -#define DMA_BUFFER_SIZE	BUF_SIZE_4KiB
> -static int buf_sz = DMA_BUFFER_SIZE;

Does this means that the old default was 4k, not the 2k in the
patch description.

> +#ifdef STMMAC_VLAN_TAG_USED
> +#define	DEFAULT_BUFSIZE	(VLAN_ETH_FRAME_LEN + ETH_FCS_LEN)
> +#else
> +#define	DEFAULT_BUFSIZE	(ETH_FRAME_LEN + ETH_FCS_LEN)
> +#endif
...
> +	if (unlikely((buf_sz < DEFAULT_BUFSIZE) || (buf_sz > BUF_SIZE_16KiB)))
> +		buf_sz = DEFAULT_BUFSIZE;

It doesn't seem right to me for the minimum buffer size to
depend on a compile-time option for VLAN.

Also (provided the hardware supports it) the rx buffers (are these
the ones being sized?) need to be aligned on a 4n+2 boundary in
order to avoid a realignment copy later on.
So I'm not sure that some of these sizes are right and/or optimal.

	David



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