Re: [PATCH net-next] docs: networking: document NAPI

[Florian Fainelli <f.fainelli@gmail.com>]

On 3/15/23 16:11, Jakub Kicinski wrote:
> On Wed, 15 Mar 2023 15:52:02 -0700 Stephen Hemminger wrote:
>> On Wed, 15 Mar 2023 15:30:44 -0700
>> Jakub Kicinski <kuba@kernel.org> wrote:
>>
>>> Add basic documentation about NAPI. We can stop linking to the ancient
>>> doc on the LF wiki.
>>>
>>> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
>>> ---
>>> CC: jesse.brandeburg@intel.com
>>> CC: anthony.l.nguyen@intel.com
>>> CC: corbet@lwn.net
>>> CC: linux-doc@vger.kernel.org
>>
>> The one thing missing, is how to handle level vs edge triggered interrupts.
>> For level triggered interrupts, the re-enable is inherently not racy.
>> I.e re-enabling interrupt when packet is present will cause an interrupt.
>> But for devices with edge triggered interrupts, it is often necessary to
>> poll and manually schedule again. Older documentation referred to this
>> as the "rotten packet" problem.
>>
>> Maybe this is no longer a problem for drivers?
>> Or maybe all new hardware uses PCI MSI and is level triggered?
> 
> It's still a problem depending on the exact design of the interrupt
> controller in the chip / tradeoffs the SW wants to make.
> I haven't actually read the LF doc, because I wasn't sure about the
> licenses (sigh). The rotten packet problem does not come up in reviews
> very often, so it wasn't front of mind. I'm not sure I'd be able to
> concisely describe it, actually :S There are many races and conditions
> which can lead to it.

True, though I would put a word in or two about level vs. edge triggered 
anyway, if nothing else, explain essentially what Stephen just provided 
ought to be a good starting point for driver writers to consider the 
possible issue.
-- 
Florian