This is a note to let you know that I've just added the patch titled serial: amba-pl011: avoid SBSA UART accessing DMACR register to the 6.1-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: serial-amba-pl011-avoid-sbsa-uart-accessing-dmacr-re.patch and it can be found in the queue-6.1 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit d1bc93e8d7924f8b2a7615530cd2dda09b3bee3c Author: Jiamei Xie <jiamei.xie@xxxxxxx> Date: Thu Nov 17 18:32:37 2022 +0800 serial: amba-pl011: avoid SBSA UART accessing DMACR register [ Upstream commit 94cdb9f33698478b0e7062586633c42c6158a786 ] Chapter "B Generic UART" in "ARM Server Base System Architecture" [1] documentation describes a generic UART interface. Such generic UART does not support DMA. In current code, sbsa_uart_pops and amba_pl011_pops share the same stop_rx operation, which will invoke pl011_dma_rx_stop, leading to an access of the DMACR register. This commit adds a using_rx_dma check in pl011_dma_rx_stop to avoid the access to DMACR register for SBSA UARTs which does not support DMA. When the kernel enables DMA engine with "CONFIG_DMA_ENGINE=y", Linux SBSA PL011 driver will access PL011 DMACR register in some functions. For most real SBSA Pl011 hardware implementations, the DMACR write behaviour will be ignored. So these DMACR operations will not cause obvious problems. But for some virtual SBSA PL011 hardware, like Xen virtual SBSA PL011 (vpl011) device, the behaviour might be different. Xen vpl011 emulation will inject a data abort to guest, when guest is accessing an unimplemented UART register. As Xen VPL011 is SBSA compatible, it will not implement DMACR register. So when Linux SBSA PL011 driver access DMACR register, it will get an unhandled data abort fault and the application will get a segmentation fault: Unhandled fault at 0xffffffc00944d048 Mem abort info: ESR = 0x96000000 EC = 0x25: DABT (current EL), IL = 32 bits SET = 0, FnV = 0 EA = 0, S1PTW = 0 FSC = 0x00: ttbr address size fault Data abort info: ISV = 0, ISS = 0x00000000 CM = 0, WnR = 0 swapper pgtable: 4k pages, 39-bit VAs, pgdp=0000000020e2e000 [ffffffc00944d048] pgd=100000003ffff803, p4d=100000003ffff803, pud=100000003ffff803, pmd=100000003fffa803, pte=006800009c090f13 Internal error: ttbr address size fault: 96000000 [#1] PREEMPT SMP ... Call trace: pl011_stop_rx+0x70/0x80 tty_port_shutdown+0x7c/0xb4 tty_port_close+0x60/0xcc uart_close+0x34/0x8c tty_release+0x144/0x4c0 __fput+0x78/0x220 ____fput+0x1c/0x30 task_work_run+0x88/0xc0 do_notify_resume+0x8d0/0x123c el0_svc+0xa8/0xc0 el0t_64_sync_handler+0xa4/0x130 el0t_64_sync+0x1a0/0x1a4 Code: b9000083 b901f001 794038a0 8b000042 (b9000041) ---[ end trace 83dd93df15c3216f ]--- note: bootlogd[132] exited with preempt_count 1 /etc/rcS.d/S07bootlogd: line 47: 132 Segmentation fault start-stop-daemon This has been discussed in the Xen community, and we think it should fix this in Linux. See [2] for more information. [1] https://developer.arm.com/documentation/den0094/c/?lang=en [2] https://lists.xenproject.org/archives/html/xen-devel/2022-11/msg00543.html Fixes: 0dd1e247fd39 (drivers: PL011: add support for the ARM SBSA generic UART) Signed-off-by: Jiamei Xie <jiamei.xie@xxxxxxx> Reviewed-by: Andre Przywara <andre.przywara@xxxxxxx> Link: https://lore.kernel.org/r/20221117103237.86856-1-jiamei.xie@xxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c index 5cdced39eafd..5b97645be667 100644 --- a/drivers/tty/serial/amba-pl011.c +++ b/drivers/tty/serial/amba-pl011.c @@ -1045,6 +1045,9 @@ static void pl011_dma_rx_callback(void *data) */ static inline void pl011_dma_rx_stop(struct uart_amba_port *uap) { + if (!uap->using_rx_dma) + return; + /* FIXME. Just disable the DMA enable */ uap->dmacr &= ~UART011_RXDMAE; pl011_write(uap->dmacr, uap, REG_DMACR);