[rescue] When is 100 less than 10?
jkunz at unixag-kl.fh-kl.de
Wed Apr 27 12:15:31 CDT 2005
On Wed, 27 Apr 2005 10:56:13 -0500
Chad McAuley <chizad at gmail.com> wrote:
> Same here. I've always heard people saying how horrible those RTL8319
> cards are, but I'd be interested in seeing some actual reasons why.
>From the NetBSD Realtek 8129/8139 PCI NIC driver code:
* Default to using PIO access for this driver. On SMP systems,
* there appear to be problems with memory mapped mode: it looks like
* doing too many memory mapped access back to back in rapid succession
* can hang the bus. I'm inclined to blame this on crummy design/construction
* on the part of Realtek. Memory mapped mode does appear to work on
* uniprocessor systems though.
* The RealTek 8139 PCI NIC redefines the meaning of 'low end.' This is
* probably the worst PCI ethernet controller ever made, with the possible
* exception of the FEAST chip made by SMC. The 8139 supports bus-master
* DMA, but it has a terrible interface that nullifies any performance
* gains that bus-master DMA usually offers.
* For transmission, the chip offers a series of four TX descriptor
* registers. Each transmit frame must be in a contiguous buffer, aligned
* on a longword (32-bit) boundary. This means we almost always have to
* do mbuf copies in order to transmit a frame, except in the unlikely
* case where a) the packet fits into a single mbuf, and b) the packet
* is 32-bit aligned within the mbuf's data area. The presence of only
* four descriptor registers means that we can never have more than four
* packets queued for transmission at any one time.
* Reception is not much better. The driver has to allocate a single large
* buffer area (up to 64K in size) into which the chip will DMA received
* frames. Because we don't know where within this region received packets
* will begin or end, we have no choice but to copy data from the buffer
* area into mbufs in order to pass the packets up to the higher protocol
* It's impossible given this rotten design to really achieve decent
* performance at 100Mbps, unless you happen to have a 400MHz PII or
* some equally overmuscled CPU to drive it.
* On the bright side, the 8139 does have a built-in PHY, although
* rather than using an MDIO serial interface like most other NICs, the
* PHY registers are directly accessible through the 8139's register
* space. The 8139 supports autonegotiation, as well as a 64-bit multicast
* The 8129 chip is an older version of the 8139 that uses an external PHY
* chip. The 8129 has a serial MDIO interface for accessing the MII where
* the 8139 lets you directly access the on-board PHY registers. We need
* to select which interface to use depending on the chip type.
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