netdev - Re: [PATCH 1/4] pch_gbe: Fix transmit queue management

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Message-ID: <529A6982.1090206@gmail.com>
Date:	Sat, 30 Nov 2013 23:41:06 +0100
From:	Ondřej Pužman <puzman@...il.com>
To:	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] pch_gbe: Fix transmit queue management

> Please format your commit  message text to 80 columns.

Ok, no problem. I did not know that 80 columns limitation applies also
for commit messages.

> You should be instead  preventing the transmit method from being
> invoked when it might be possible that a request cannot be
> satisfied. This means that at the end of a transmit request, you must
> stop the queue if a packet with the maximum number of possible
> descriptors cannot be satisfied. Then it is impossible for the
> transmit function to be invoked in a situation where it would need to
> fail for lack of available transmit descriptors. This is why drivers
> decided whether to stop their TX queues based upon calculations
> involving MAX_SKB_FRAGS.

Hmm, correct me if I'm wrong ... but the driver and hardware does not
support scatter-gather DMA. So fragmented skbs can't be passed to
ndo_start_xmit function and each skb passed to the function represents
one packet and one descriptor in tx_ring.
I do not think that in this caseit makes sense to to involve
MAX_SKB_FRAGS in any calculation and the queue should be stopped in the
moment when it gets full.
But maybe I am missing something, I am no expert in this kind of stuff.

--
Ondrej Puzman

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