From 2bc6b218717b9d08f466f88209251d54bc09b207 Mon Sep 17 00:00:00 2001 From: Fuad Tabba Date: Tue, 22 Sep 2026 19:14:30 +0100 Subject: arm64/boot: Disable trapping of PMZR_EL0 writes to EL2 __init_el2_fgt2() writes one mask to both HDFGRTR2_EL2 and HDFGWTR2_EL2. PMZR_EL0 is write-only, so its trap bit, nPMZR_EL0, exists only in HDFGWTR2_EL2 and is therefore never set: a PMZR_EL0 write from the host traps to EL2, where the nVHE hypervisor has no handler and BUG()s. The kernel never writes PMZR_EL0, but kernel.perf_user_access=1 has the PMU driver set PMUSERENR_EL0.UEN for a task with a user-read event, so a write from EL0 reaches the trap and takes the host down without a panic message. Accumulate the HDFGWTR2_EL2 bits separately, as __init_el2_fgt() already does for HDFGWTR_EL2, and set nPMZR_EL0 with the other FEAT_PMUv3p9 bits. Fixes: 858c7bfcb35e1 ("arm64/boot: Enable EL2 requirements for FEAT_PMUv3p9") Cc: stable@vger.kernel.org Signed-off-by: Fuad Tabba Reviewed-by: Anshuman Khandual Reviewed-by: Oliver Upton Signed-off-by: Will Deacon --- Documentation/driver-api/serial/driver.rst | 105 +++++++++++++++++++++++++++++ 1 file changed, 105 insertions(+) create mode 100644 Documentation/driver-api/serial/driver.rst (limited to 'Documentation/driver-api/serial/driver.rst') diff --git a/Documentation/driver-api/serial/driver.rst b/Documentation/driver-api/serial/driver.rst new file mode 100644 index 000000000..c1db6a1a6 --- /dev/null +++ b/Documentation/driver-api/serial/driver.rst @@ -0,0 +1,105 @@ +==================== +Low Level Serial API +==================== + + +This document is meant as a brief overview of some aspects of the new serial +driver. It is not complete, any questions you have should be directed to + + +The reference implementation is contained within amba-pl011.c. + + + +Low Level Serial Hardware Driver +-------------------------------- + +The low level serial hardware driver is responsible for supplying port +information (defined by uart_port) and a set of control methods (defined +by uart_ops) to the core serial driver. The low level driver is also +responsible for handling interrupts for the port, and providing any +console support. + + +Console Support +--------------- + +The serial core provides a few helper functions. This includes +decoding command line arguments (uart_parse_options()). + +There is also a helper function (uart_console_write()) which performs a +character by character write, translating newlines to CRLF sequences. +Driver writers are recommended to use this function rather than implementing +their own version. + + +Locking +------- + +It is the responsibility of the low level hardware driver to perform the +necessary locking using port->lock. There are some exceptions (which +are described in the struct uart_ops listing below.) + +There are two locks. A per-port spinlock, and an overall semaphore. + +From the core driver perspective, the port->lock locks the following +data:: + + port->mctrl + port->icount + port->state->xmit.head (circ_buf->head) + port->state->xmit.tail (circ_buf->tail) + +The low level driver is free to use this lock to provide any additional +locking. + +The port_sem semaphore is used to protect against ports being added/ +removed or reconfigured at inappropriate times. Since v2.6.27, this +semaphore has been the 'mutex' member of the tty_port struct, and +commonly referred to as the port mutex. + + +uart_ops +-------- + +.. kernel-doc:: include/linux/serial_core.h + :identifiers: uart_ops + +Other functions +--------------- + +.. kernel-doc:: drivers/tty/serial/serial_core.c + :identifiers: uart_update_timeout uart_get_baud_rate uart_get_divisor + uart_match_port uart_write_wakeup uart_register_driver + uart_unregister_driver uart_suspend_port uart_resume_port + uart_add_one_port uart_remove_one_port uart_console_write + uart_parse_earlycon uart_parse_options uart_set_options + uart_get_lsr_info uart_handle_dcd_change uart_handle_cts_change + uart_try_toggle_sysrq + +.. kernel-doc:: include/linux/serial_core.h + :identifiers: uart_port_tx_limited uart_port_tx + +Other notes +----------- + +It is intended some day to drop the 'unused' entries from uart_port, and +allow low level drivers to register their own individual uart_port's with +the core. This will allow drivers to use uart_port as a pointer to a +structure containing both the uart_port entry with their own extensions, +thus:: + + struct my_port { + struct uart_port port; + int my_stuff; + }; + +Modem control lines via GPIO +---------------------------- + +Some helpers are provided in order to set/get modem control lines via GPIO. + +.. kernel-doc:: drivers/tty/serial/serial_mctrl_gpio.c + :identifiers: mctrl_gpio_init mctrl_gpio_to_gpiod + mctrl_gpio_set mctrl_gpio_get mctrl_gpio_enable_ms + mctrl_gpio_disable_ms_sync mctrl_gpio_disable_ms_no_sync -- cgit v1.3.1