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

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Message-ID: <c26af1cb-747a-462d-8e13-af948bbb8171@quicinc.com>
Date: Fri, 19 Jul 2024 07:10:36 -0700
From: Jeff Johnson <quic_jjohnson@...cinc.com>
To: Baochen Qiang <quic_bqiang@...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 7/14/2024 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
> scatter and linearize manually in the driver, to a correct offset.
> Because now core is linearizing the skb in validate_xmit_skb()
> and then the driver moves it a second time..