Re: [PATCH v2 2/2] serial: fsl_lpuart: avoid new transfer while DMA is running

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Jan 10, 2015 at 01:08:59AM +0100, Stefan Agner wrote:
> When the UART is in DMA receive mode (RDMAS set) and one character
> just arrived while another interrupt is handled (e.g. TX), the RDRF
> (receiver data register full flag) is set due to the water level of
> 1. But since the DMA will take care of this character, there is no
> need to handle it by calling lpuart_prepare_rx. Handling it leads to
> adding the RX timeout timer twice:
> 
> [   74.336698] Kernel BUG at 80053070 [verbose debug info unavailable]
> [   74.342999] Internal error: Oops - BUG: 0 [#1] ARM0:00.00 khungtaskd
> [   74.347817] Modules linked in:    0 S  0.0  0.0   0:00.00 writeback
> [   74.350926] CPU: 0 PID: 0 Comm: swapper Not tainted 3.19.0-rc3-00001-g39d78e2 #1788
> [   74.358617] Hardware name: Freescale Vybrid VF610 (Device Tree)t
> [   74.364563] task: 807a7678 ti: 8079c000 task.ti: 8079c000 kblockd
> [   74.370002] PC is at add_timer+0x24/0x28.0  0.0   0:00.09 kworker/u2:1
> [   74.373960] LR is at lpuart_int+0x15c/0x3d8
> [   74.378171] pc : [<80053070>]    lr : [<802e0d88>]    psr: a0010193
> [   74.378171] sp : 8079de10  ip : 8079de20  fp : 8079de1c
> [   74.389694] r10: 807d44c0  r9 : 8688c300  r8 : 00000013
> [   74.394943] r7 : 20010193  r6 : 00000000  r5 : 000000a0  r4 : 86997210
> [   74.401498] r3 : ffffa7da  r2 : 80817868  r1 : 86997210  r0 : 86997344
> [   74.408052] Flags: NzCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
> [   74.415489] Control: 10c5387d  Table: 8611c059  DAC: 00000015
> [   74.421265] Process swapper (pid: 0, stack limit = 0x8079c230)
> ...
> 
> Solve this by only execute the receiver path (lpuart_prepare_rx) if
> the DMA receive mode (RDMAS) is not set. Also, make sure the flag is
> cleared on initialization, in case it has been left set.
> 
> This can be best reproduced using UART as a serial console, then
> running top while dd'ing data into the terminal.
> 
> Signed-off-by: Stefan Agner <stefan@agner.ch>
> ---
>  drivers/tty/serial/fsl_lpuart.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)

Same stable question as before.