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SubjectRe: [patch] ns83820 0.17 (Re: Broadcom 5700/5701 Gigabit Ethernet Adapters)
Benjamin LaHaise wrote:

>>3) Seeing "volatile" in your code. Cruft? volatile's meaning change in
>>recent gcc versions implies to me that your code may need some addition
>>rmb/wmb calls perhaps, which are getting hidden via the driver's
>>dependency on a compiler-version-specific implementation of "volatile."
>>
>
>Paranoia during writing. I'll reaudit. That said, volatile behaviour
>is not compiler version specific.
>
gcc 3.1 volatile behavior changes, so, yes, it is...

>>4) Do you really mean to allocate memory for "REAL_RX_BUF_SIZE + 16"?
>> Why not plain old REAL_RX_BUF_SIZE?
>>
>
>The +16 is for alignment (just like the comment says). The hardware
>requires that rx buffers be 64 bit aligned.
>
Cool... just checking. Both RX_BUF_SIZE and REAL_RX_BUF_SIZE are defined as
foo + magic_number

so I wasn't sure if the alignment space was -already- accounted for, in
the definition of RX_BUF_SIZE, thus making the addition op next to
allocations of REAL_RX_BUF_SIZE superfluous. But, I stand corrected,
thanks.

>5) Random question, do you call netif_carrier_{on,off,ok} for link
>> status manipulation? (if not, you should...)
>
>
>Ah, api updates. Added to the todo.
>
More than just api updates... You have a bunch of hack-y logic for when
the link goes down and up, messing around with netif_stop_queue and
netif_wake_queue. That stuff will be simplified or simply go away. The
basic idea is, if netif_carrier_ok(dev) is not true, then the net stack
will not be sending you any packets. So those extra
netif_{stop,wake}_queue calls are superflouous.
We're also about to start sending link up/down messages async-ly via
netlink, so that's even more added value as well.

>>6) skb_mangle_for_davem is pretty gross... curious: what sort of NIC
>>alignment restrictions are there on rx and tx buffers (not descriptors)?
>> None? 32-bit? Ignore CPU alignment for a moment here...
>>
>
>tx descriptors have no alignment restriction, rx descriptors must be
>64 bit aligned. Someone chose not to include the transistors for a
>barrel shifter in the rx engine.
>

Sigh :)

>>7) What are the criteria for netif_wake_queue? If you are waking when
>>the TX is still "mostly full" you probably generate excessive wakeups...
>>
>
>Hrm? Currently it will do a wakeup when at least one packet (8 sg
>descriptors) can be sent. Given that the tx done code is only called
>when a tx desc (every 1/4 or so of the tx queue) or txidle interrupt
>occurs, it shouldn't be that often.
>

Cool. As FYI (_not_ advice on your driver), here's the logic I was
referring to:

dev->hard_start_xmit()
if (free slots < MAX_SKB_FRAGS)
BUG()
queue packet
if (free slots < MAX_SKB_FRAGS)
netif_stop_queue(dev)
foo_interrupt()
if (some tx interrupt)
complete as many TX's as possible
if (netif_queue_stopped && (free slots > (TX_RING_SIZE / 4)))
netif_wake_queue(dev)
But as long as your TX interrupts are well mitigated (and it sounds like
they are), you can get by with your current scheme just fine.

Jeff




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