Re: [PATCH 1/1] serial: 8250: Prevent starting up DMA Rx on THRI interrupt

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 3/17/23 11:30, Ilpo Järvinen wrote:
> Hans de Goede reported Bluetooth adapters (HCIs) connected over an UART
> connection failed due corrupted Rx payload. The problem was narrowed
> down to DMA Rx starting on UART_IIR_THRI interrupt. The problem occurs
> despite LSR having DR bit set, which is precondition for attempting to
> start DMA Rx in the first place.
> 
> From a debug patch:
> [x.807834] 8250irq: iir=cc lsr+saved=60 received=0/15 ier=0f dma_t/rx/err=0/0/0
> [x.808676] 8250irq: iir=c2 lsr+saved=61 received=0/0 ier=0f dma_t/rx/err=0/0/0
> [x.808776] 8250irq: iir=cc lsr+saved=60 received=1/12 ier=0d dma_t/rx/err=0/1/0
> [x.808870] Bluetooth: hci0: Frame reassembly failed (-84)
> 
> In the debug snippet, received field indicates 1 byte was transferred
> over DMA and 12 bytes after that with the non-DMA Rx. The sole byte DMA
> handled was corrupted (gets zeroed) which leads to the HCI failure.
> 
> This problem became apparent after commit e8ffbb71f783 ("serial: 8250:
> use THRE & __stop_tx also with DMA") changed Tx stop behavior. Tx stop
> is now triggered from a THRI interrupt.
> 
> Despite that this problem looks like a HW bug, this fix is not adding
> UART_BUG_xx flag to the driver beucase it seems useful in general to
> avoid starting DMA when there are only a few bytes to transfer.
> Skipping DMA for small transfers avoids the extra overhead DMA incurs.
> 
> Thus, don't setup DMA Rx on UART_IIR_THRI but leave it to a subsequent
> interrupt which has Rx a related IIR value.
> 
> By returning false from handle_rx_dma(), the DMA vs non-DMA decision is
> postponed until either UART_IIR_RDI (FIFO threshold worth of bytes
> awaiting) or UART_IIR_TIMEOUT (inter-character timeout) triggers at a
> later time which allows better to discern whether the number of bytes
> warrants starting DMA or not.
> 
> Reported-by: Hans de Goede <hdegoede@redhat.com>
> Tested-by: Hans de Goede <hdegoede@redhat.com>
> Fixes: e8ffbb71f783 ("serial: 8250: use THRE & __stop_tx also with DMA")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

Thanks, patch looks good to me:

Acked-by: Hans de Goede <hdegoede@redhat.com>

Regards,

Hans

> ---
>  drivers/tty/serial/8250/8250_port.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index fa43df05342b..3ba9c8b93ae6 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -1903,6 +1903,17 @@ EXPORT_SYMBOL_GPL(serial8250_modem_status);
>  static bool handle_rx_dma(struct uart_8250_port *up, unsigned int iir)
>  {
>  	switch (iir & 0x3f) {
> +	case UART_IIR_THRI:
> +		/*
> +		 * Postpone DMA or not decision to IIR_RDI or IIR_RX_TIMEOUT
> +		 * because it's impossible to do an informed decision about
> +		 * that with IIR_THRI.
> +		 *
> +		 * This also fixes one known DMA Rx corruption issue where
> +		 * DR is asserted but DMA Rx only gets a corrupted zero byte
> +		 * (too early DR?).
> +		 */
> +		return false;
>  	case UART_IIR_RDI:
>  		if (!up->dma->rx_running)
>  			break;