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

From: Kalle Valo <[email-masked]>
Date: Thu, 01 Aug 2024 18:07:58 +0300

Baochen Qiang <[email-masked]> writes:

> 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.
>
> Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3
>
> Signed-off-by: Baochen Qiang <[email-masked]>

[...]

> --- a/drivers/net/wireless/ath/ath12k/dp_tx.c
> +++ b/drivers/net/wireless/ath/ath12k/dp_tx.c
> @@ -162,6 +162,60 @@ static int ath12k_dp_prepare_htt_metadata(struct sk_buff *skb)
>  	return 0;
>  }
>  
> +static void ath12k_dp_tx_move_payload(struct sk_buff *skb,
> +				      unsigned long delta,
> +				      bool head)
> +{
> +	unsigned long len = skb->len;
> +
> +	if (head) {
> +		skb_push(skb, delta);
> +		memmove(skb->data, skb->data + delta, len);
> +		skb_trim(skb, len);
> +	} else {
> +		skb_put(skb, delta);
> +		memmove(skb->data + delta, skb->data, len);
> +		skb_pull(skb, delta);
> +	}
> +}

I'm nitpicking, but usually booleans like the head variable here don't
help with readability. Having two separate functions would be easier to
read, but this is fine as it's so small.

> @@ -279,6 +334,23 @@ int ath12k_dp_tx(struct ath12k *ar, struct ath12k_vif *arvif,
>  		goto fail_remove_tx_buf;
>  	}
>  
> +	if (iova_mask &&
> +	    (unsigned long)skb->data & iova_mask) {
> +		ret = ath12k_dp_tx_align_payload(ab, &skb);
> +		if (ret) {
> +			dev_warn_once(ab->dev, "failed to align TX buffer %d\n", ret);

Why dev_warn_once()? I changed it to ath12k_warn() in the pending
branch.

> --- a/drivers/net/wireless/ath/ath12k/hw.h
> +++ b/drivers/net/wireless/ath/ath12k/hw.h
> @@ -96,6 +96,8 @@
>  #define ATH12K_M3_FILE			"m3.bin"
>  #define ATH12K_REGDB_FILE_NAME		"regdb.bin"
>  
> +#define PCIE_MAX_PAYLOAD_SIZE		128

PCIE prefix implies that this is in PCI subsystem. I renamed it to
ATH12K_PCIE_MAX_PAYLOAD_SIZE.

Please check my changes:

https://git.kernel.org/pub/scm/linux/kernel/git/ath/ath.git/commit/?h=pending&id=b603c1e0d94fb1eb0576ef48ebe37c8c1ce86328

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