netdev - Re: [PATCH] wifi: ath12k: use 128 bytes aligned iova in transmit path for WCN7850

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Message-ID: <1aad7e68-c2d4-4b77-b526-5580860d7e3e@quicinc.com>
Date: Mon, 22 Jul 2024 15:02:43 +0800
From: Baochen Qiang <quic_bqiang@...cinc.com>
To: Jeff Johnson <quic_jjohnson@...cinc.com>, <ath12k@...ts.infradead.org>
CC: <linux-wireless@...r.kernel.org>, <kernel@...cinc.com>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH] wifi: ath12k: use 128 bytes aligned iova in transmit path
 for WCN7850



On 7/22/2024 2:54 PM, Baochen Qiang wrote:
> 
> 
> On 7/19/2024 10:10 PM, Jeff Johnson wrote:
>> On 7/14/2024 7:38 PM, Baochen Qiang wrote:
>>> In transmit path, it is likely that the iova is not aligned to PCIe TLP
>>> max payload size, which is 128 for WCN7850. Normally in such cases hardware
>>> is expected to split the packet into several parts in a manner such that
>>> they, other than the first one, have aligned iova. However due to hardware
>>> limitations, WCN7850 does not behave like that properly with some specific
>>> unaligned iova in transmit path. This easily results in target hang in a
>>> KPI transmit test: packet send/receive failure, WMI command send timeout
>>> etc. Also fatal error seen in PCIe level:
>>>
>>> 	...
>>> 	Capabilities: ...
>>> 		...
>>> 		DevSta: ... FatalErr+ ...
>>> 		...
>>> 	...
>>>
>>> Work around this by manually moving/reallocating payload buffer such that
>>> we can map it to a 128 bytes aligned iova. The moving requires sufficient
>>> head room or tail room in skb: for the former we can do ourselves a favor
>>> by asking some extra bytes when registering with mac80211, while for the
>>> latter we can do nothing.
>>>
>>> Moving/reallocating buffer consumes additional CPU cycles, but the good news
>>> is that an aligned iova increases PCIe efficiency. In my tests on some X86
>>> platforms the KPI results are almost consistent.
>>>
>>> Since this is seen only with WCN7850, add a new hardware parameter to
>>> differentiate from others.
>>
>> I asked for expert opinion on this patch and received the following response.
>> Baochen, can you take a look at this suggestion?
>>
>>> Aligning headers is sometimes done, but it appears the driver
>>> doesn't support scatter gather? I think the author may want to advertise
> right, ath12k does not support SG currently.
> 
>>> scatter and linearize manually in the driver, to a correct offset.
> is there an existing skb API or API combinations which can do that for me? I checked __skb_linearize() and it does not take an 'offset' argument.
or do I need to implement it myself from a very low level basis? like (if required) allocating skb structure, allocating/aligning payload buffer, copying/freeing paged frag/frag list, etc..

> 
>>> Because now core is linearizing the skb in validate_xmit_skb()
>>> and then the driver moves it a second time..
>>
>>
>