Re: [PATCH] ath10k: add modparam 'hw_csum' to make HW checksum configurable

[Peter Oh <poh@codeaurora.org>]

On 12/16/2015 12:53 PM, Felix Fietkau wrote:
> On 2015-12-16 21:46, Peter Oh wrote:
>> On 12/16/2015 12:35 PM, Felix Fietkau wrote:
>>> On 2015-12-16 21:29, Peter Oh wrote:
>>>> On 12/16/2015 10:27 AM, Felix Fietkau wrote:
>>>>> On 2015-12-16 19:20, Peter Oh wrote:
>>>>>> Some hardwares such as QCA988X and QCA99X0 doesn't have
>>>>>> capability of checksum offload when frame formats are not
>>>>>> suitable for it such as Mesh frame.
>>>>>> Hence add a module parameter, hw_csum, to make checksum offload
>>>>>> configurable during module registration time.
>>>>>>
>>>>>> Signed-off-by: Peter Oh <poh@qca.qualcomm.com>
>>>>> How about instead of inventing yet another crappy module parameter, you
>>>>> call skb_checksum_help() in the driver in cases where the hardware is
>>>>> unable to offload the checksum calculation.
>>>>>
>>>>> That way the user has to worry about less driver specific hackery ;)
>>>> That will be good option for hardware not supporting HW checksum, but I
>>>> mind that using the function will add more workload per every packet on
>>>> critical data path when HW supports checksum resulting in throughput down.
>>> I didn't mean calling it for every single frame in the data path.
>>> What I'm suggesting is calling it selectively only for mesh frames, or
>>> any other frames that the hardware cannot offload, and leaving the rest
>>> for the hardware to process.
>>>
>>> There should be no performance difference between disabling checksum
>>> offload and calling skb_checksum_help from the driver.
>> To call it selectively for Mesh frame or interface, we need to add it on
>> mac80211 layer such as ieee80211_build_hdr() since driver layer does not
>> care the interface type in data path.
> No need to change mac80211 - it only touches the headers, and
> skb_checksum_help does not care about that. The skb has enough
> information for it to find the right range to calculate the checksum and
> the place to store it.
If mentioned to use the function to mesh frame only without touching 
mac80211, then how do you suggest it to apply it only to mesh frame 
without interfere other data frames?
Can you share your example?
>
>> In that case it will also introduce throughput degrade to HW that
>> supports HW checksum for Mesh.
> This doesn't make any sense to me. Are you saying that there's no way
> for the driver to detect the cases where the hardware cannot do checksum
> offloading?
I'm saying the case that HW supports checksum except for specific frame 
such as Mesh and to make driver support both case dynamically at code 
level, it requires extra codes which need to check if the frame is Mesh 
or not. Since this approach requires extra workload especially in data 
path, it will degrade driver's performance.
>   How is the user supposed to know when to change that module
> parameter? Trial and error?
>
> - Felix