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Re: NAPI for eepro100

To: saw@xxxxxxxxxxxxx
Subject: Re: NAPI for eepro100
From: "David S. Miller" <davem@xxxxxxxxxx>
Date: Thu, 13 Jun 2002 01:47:00 -0700 (PDT)
Cc: fxzhang@xxxxxxxxx, linux-mips@xxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxx, jgarzik@xxxxxxxxxxxxxxxx
In-reply-to: <20020613125753.A23693@xxxxxxxxxxxxxxxxx>
References: <3D07D270.5060902@xxxxxxxxxxxxxxxx> <20020612.160532.134201977.davem@xxxxxxxxxx> <20020613125753.A23693@xxxxxxxxxxxxxxxxx>
Sender: owner-netdev@xxxxxxxxxxx
   From: Andrey Savochkin <saw@xxxxxxxxxxxxx>
   Date: Thu, 13 Jun 2002 12:57:53 +0400

   On Wed, Jun 12, 2002 at 04:05:32PM -0700, David S. Miller wrote:
   > No, it's worse than that.
   > 
   > See how non-consistent memory is used by the eepro100 driver
   > for descriptor bits?  The skb->tail bits?
   > 
   > That is very problematic.
   
   What's the problem?
   If it isn't allowed to do, then what is the meaning of PCI_DMA_BIDIRECTIONAL
   mappings?

It's slow.  Not wrong, just inefficient.

Descriptors were meant to be done using consistent mappings, not
"pci_map_*()"'d memory.  The latter is meant to be used for long
linear DMA transfers to/from the device.  It is not meant for things
the cpu pokes small bits of data in and out of, that is what
consistent DMA memory is for.

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