David Miller wrote:
From: James Chapman <[EMAIL PROTECTED]>
Date: Thu, 6 Sep 2007 15:16:00 +0100
First, do we need to encourage consistency in NAPI poll drivers? A
survey of current NAPI drivers shows different strategies being used
in their poll(). Some such as r8169 do the napi_complete() if poll()
does less work than their allowed budget. Others such as e100 and
tg3 do napi_complete() only if they do no work at all.
Actually, I want to clarify this situation. In reality these
drivers are more consistent than different.
For some chips the cheapest way to figure out if there is more
RX work is simply to see if the amount of work processed is
less than "budget". It's too expensive to recheck the hardware.
On some chips like tg3, it's extremely cheap to see if new work
arrived between the completion of processing the RX queue and
the NAPI completion check, so they do it.
The inconsistencies I see are to do with the conditions that the driver
chooses to exit polled mode, i.e. doing no work in the poll() versus
doing less than budget, and whether txdone processing is done in the
poll or in the interrupt handler. I didn't mean to suggest that
rechecking for more work just before doing the napi_complete() was an
example of inconsistency.
The rest of the RFC talks about polling the device while it might be
idle. The overhead of checking for work varies for each system / device
as you say. Where it is expensive, the driver could optimize that case.
--
James Chapman
Katalix Systems Ltd
http://www.katalix.com
Catalysts for your Embedded Linux software development
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