netdev - Re: smsc95xx truesize badness

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Date:	Fri, 20 Feb 2015 06:41:09 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Steve Glendinning <steve@...well.net>
Cc:	netdev <netdev@...r.kernel.org>, popcorn mix <popcornmix@...il.com>
Subject: Re: smsc95xx truesize badness

On Thu, 2015-02-19 at 14:14 -0800, Eric Dumazet wrote:
> On Thu, 2015-02-19 at 18:58 +0000, Steve Glendinning wrote:
> 
> > This is triggering warnings for many Raspberry Pi users due to Eric's
> > patch to catch truesize offenders.  It's a fair cop.
> > 
> > Before I submit a patch to remove these two truesize lines, I believe
> > it's safe to just do so but can anyone confirm if removing them will
> > have any adverse impact?
> 
> If you simply remove the lines, skb->truesize might be big enough to
> slow down TCP noticeably.
> 
> I would rather use something like commit
> a9e0aca4b37885b5599e52211f098bd7f565e749
> ("asix: asix_rx_fixup surgery to reduce skb truesizes")
> 
> Idea is to 'copybreak' tiny frames.
> 

On smsc95xx.c, rx_urb_size is set to :

MAX_SINGLE_PACKET_SIZE (2048) on non turbo mode
DEFAULT_HS_BURST_CAP_SIZE (more than 18 KB) on USB_SPEED_HIGH mode
DEFAULT_FS_BURST_CAP_SIZE  (more than 7 KB)

So only non turbo mode would be okay with non copybreak strategy.


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