Re: [PATCH v2 ath-next 2/2] wifi: ath11k: fix HTC rx insufficient length

[Johan Hovold]

On Mon, Mar 10, 2025 at 09:02:17AM +0800, Miaoqing Pan wrote:
> A relatively unusual race condition occurs between host software
> and hardware, where the host sees the updated destination ring head
> pointer before the hardware updates the corresponding descriptor.
> When this situation occurs, the length of the descriptor returns 0.

I still think this description is too vague and it doesn't explain how
this race is even possible. It sounds like there's a bug somewhere in
the driver or firmware, but if this really is an indication the hardware
is broken as your reply here seems to suggest:

	https://lore.kernel.org/lkml/bc187777-588c-4fa0-ba8c-847e91c78d43@xxxxxxxxxxx/

then that too should be highlighted in the commit message (e.g. by
describing this as "working around broken hardware").

> The current error handling method is to increment descriptor tail
> pointer by 1, but 'sw_index' is not updated, causing descriptor and
> skb to not correspond one-to-one, resulting in the following error:
> 
> ath11k_pci 0006:01:00.0: HTC Rx: insufficient length, got 1488, expected 1492
> ath11k_pci 0006:01:00.0: HTC Rx: insufficient length, got 1460, expected 1484
> 
> To address this problem, temporarily skip processing the current
> descriptor and handle it again next time. However, to prevent this
> descriptor from continuously returning 0, use skb cb to set a flag.
> If the length returns 0 again, this descriptor will be discarded.

The ath12k ring-buffer handling looks very similar. Do you need a
corresponding workaround in ath12k_ce_completed_recv_next()? Or are you
sure that this (hardware) bug only affects ath11k devices?
 
>  	*nbytes = ath11k_hal_ce_dst_status_get_length(desc);
> -	if (*nbytes == 0) {
> -		ret = -EIO;
> -		goto err;
> +	if (unlikely(*nbytes == 0)) {
> +		struct ath11k_skb_rxcb *rxcb =
> +			ATH11K_SKB_RXCB(pipe->dest_ring->skb[sw_index]);
> +
> +		/* A relatively unusual race condition occurs between host
> +		 * software and hardware, where the host sees the updated
> +		 * destination ring head pointer before the hardware updates
> +		 * the corresponding descriptor.
> +		 *
> +		 * Temporarily skip processing the current descriptor and handle
> +		 * it again next time. However, to prevent this descriptor from
> +		 * continuously returning 0, set 'is_desc_len0' flag. If the
> +		 * length returns 0 again, this descriptor will be discarded.
> +		 */
> +		if (!rxcb->is_desc_len0) {
> +			rxcb->is_desc_len0 = true;
> +			ret = -EIO;
> +			goto err;
> +		}
>  	}

I'm still waiting for feedback from one user that can reproduce the
ring-buffer corruption very easily, but another user mentioned seeing
multiple zero-length descriptor warnings over the weekend when running
with this patch:

	ath11k_pci 0006:01:00.0: rxed invalid length (nbytes 0, max 2048)

Are there ever any valid reasons for seeing a zero-length descriptor
(i.e. unrelated to the race at hand)? IIUC the warning would only be
printed when processing such descriptors a second time (i.e. when
is_desc_len0 is set).

Johan