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;