Re: [PATCH 4/4] wcn36xx: Put DXE block into reset before freeing memory

[Bryan O'Donoghue <bryan.odonoghue@linaro.org>]

On 15/10/2021 14:17, Bryan O'Donoghue wrote:
> When deiniting the DXE hardware we should reset the block to ensure there
> is no spurious DMA write transaction from the downstream WCNSS to upstream
> MSM at a skbuff address we will have released.
> 
> This is actually a pretty serious bug. Immediately after the reset we
> release skbs, skbs which are from the perspective of the WCNSS DXE still
> valid addresses for DMA.
> 
> Without first placing the DXE block into reset, it is possible for an
> upstream DMA transaction to write to skbs we have freed.
> 
> We have seen some backtraces from usage in testing on 50k+ devices which
> indicates an invalid RX of an APs beacon to unmapped memory.
> 
> The logical conclusion is that an RX transaction happened to a region of
> memory that was previously valid but was subsequently released.
> 
> The only time such a window of opportunity exists is when we have
> deallocated the skbs attached to the DMA BDs in other words after doing
> wcn36xx_stop().
> 
> If we free the skbs on the DMA channel, we need to make sure we have
> quiesced potential DMA on that channel prior to freeing.
> 
> This patch should eliminate that error.
> 
> Fixes: 8e84c2582169 ("wcn36xx: mac80211 driver for Qualcomm WCN3660/WCN3680 hardware")
> Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
> ---
>   drivers/net/wireless/ath/wcn36xx/dxe.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/net/wireless/ath/wcn36xx/dxe.c b/drivers/net/wireless/ath/wcn36xx/dxe.c
> index e89002502869a..56f605c23f36c 100644
> --- a/drivers/net/wireless/ath/wcn36xx/dxe.c
> +++ b/drivers/net/wireless/ath/wcn36xx/dxe.c
> @@ -1020,6 +1020,8 @@ int wcn36xx_dxe_init(struct wcn36xx *wcn)
>   
>   void wcn36xx_dxe_deinit(struct wcn36xx *wcn)
>   {
> +	int reg_data = 0;
> +
>   	/* Disable channel interrupts */
>   	wcn36xx_dxe_disable_ch_int(wcn, WCN36XX_INT_MASK_CHAN_RX_H);
>   	wcn36xx_dxe_disable_ch_int(wcn, WCN36XX_INT_MASK_CHAN_RX_L);
> @@ -1035,6 +1037,10 @@ void wcn36xx_dxe_deinit(struct wcn36xx *wcn)
>   		wcn->tx_ack_skb = NULL;
>   	}
>   
> +	/* Put the DXE block into reset before freeing memory */
> +	reg_data = WCN36XX_DXE_REG_RESET;
> +	wcn36xx_dxe_write_register(wcn, WCN36XX_DXE_REG_CSR_RESET, reg_data);
> +
>   	wcn36xx_dxe_ch_free_skbs(wcn, &wcn->dxe_rx_l_ch);
>   	wcn36xx_dxe_ch_free_skbs(wcn, &wcn->dxe_rx_h_ch);
>   
> 

Johannes asked me separately if we need to wait for the quiesence to 
complete.

I don't see that downstream but, that doesn't mean we shouldn't do it.

So I'll investigate that.

Also - now that I look at this code, this being the second usage of the 
CSR_RESET like this, also means the reset can be functionally decomposed 
into a routine.

So - I'll look into the first and definitely do the second as a V2