On Thu, Mar 10, 2022 at 08:56:36AM +0530, Sai Prakash Ranjan wrote: > From: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx> > > Some debuggers, such as Trace32 from Lauterbach GmbH, do not handle > reads/writes from/to DCC on secondary cores. Each core has its > own DCC device registers, so when a core reads or writes from/to DCC, > it only accesses its own DCC device. Since kernel code can run on > any core, every time the kernel wants to write to the console, it > might write to a different DCC. > > In SMP mode, Trace32 creates multiple windows, and each window shows > the DCC output only from that core's DCC. The result is that console > output is either lost or scattered across windows. > > Selecting this option will enable code that serializes all console > input and output to core 0. The DCC driver will create input and > output FIFOs that all cores will use. Reads and writes from/to DCC > are handled by a workqueue that runs only core 0. > > Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx> > Acked-by: Adam Wallis <awallis@xxxxxxxxxxxxxx> > Signed-off-by: Timur Tabi <timur@xxxxxxxxxxxxxx> > Signed-off-by: Elliot Berman <eberman@xxxxxxxxxxxxxx> > Signed-off-by: Sai Prakash Ranjan <quic_saipraka@xxxxxxxxxxx> > --- > > Changes in v6: > * Disable CPU hotplug when CONFIG_HVC_DCC_SERIALIZE_SMP=y. > > Changes in v5: > * Use get_cpu() and put_cpu() for CPU id check in preemptible context. > * Revert back to build time Kconfig. > * Remove unnecessary hotplug locks, they result in sleeping in atomic context bugs. > * Add a comment for the spinlock. > > Changes in v4: > * Use module parameter for runtime choice of enabling this feature. > * Use hotplug locks to avoid race between cpu online check and work schedule. > * Remove ifdefs and move to common ops. > * Remove unnecessary check for this configuration. > * Use macros for buf size instead of magic numbers. > * v3 - https://lore.kernel.org/lkml/20211213141013.21464-1-quic_saipraka@xxxxxxxxxxx/ > > Changes in v3: > * Handle case where core0 is not online. > > Changes in v2: > * Checkpatch warning fixes. > * Use of IS_ENABLED macros instead of ifdefs. > > --- > drivers/tty/hvc/Kconfig | 20 +++++ > drivers/tty/hvc/hvc_dcc.c | 175 +++++++++++++++++++++++++++++++++++++- > 2 files changed, 192 insertions(+), 3 deletions(-) > > diff --git a/drivers/tty/hvc/Kconfig b/drivers/tty/hvc/Kconfig > index 8d60e0ff67b4..62560cd0c04d 100644 > --- a/drivers/tty/hvc/Kconfig > +++ b/drivers/tty/hvc/Kconfig > @@ -87,6 +87,26 @@ config HVC_DCC > driver. This console is used through a JTAG only on ARM. If you don't have > a JTAG then you probably don't want this option. > > +config HVC_DCC_SERIALIZE_SMP > + bool "Use DCC only on CPU core 0" > + depends on SMP && HVC_DCC > + help > + Some debuggers, such as Trace32 from Lauterbach GmbH, do not handle > + reads/writes from/to DCC on more than one CPU core. Each core has its > + own DCC device registers, so when a CPU core reads or writes from/to > + DCC, it only accesses its own DCC device. Since kernel code can run on > + any CPU core, every time the kernel wants to write to the console, it > + might write to a different DCC. > + > + In SMP mode, Trace32 creates multiple windows, and each window shows > + the DCC output only from that core's DCC. The result is that console > + output is either lost or scattered across windows. Why are we documenting, and supporting, a closed source userspace tool with kernel changes? Does this advertisement deserve to be in the kernel source tree? And why can't they just fix their tool if this is such a big issue? Why does this only affect this one platform and not all other smp systems? > + > + Selecting this option will enable code that serializes all console > + input and output to CPU core 0. The DCC driver will create input and > + output FIFOs that all cores will use. Reads and writes from/to DCC > + are handled by a workqueue that runs only CPU core 0. No need to describe implementation details in a help text, right? What happens when we change this to not be a workqueue? > + > config HVC_RISCV_SBI > bool "RISC-V SBI console support" > depends on RISCV_SBI_V01 > diff --git a/drivers/tty/hvc/hvc_dcc.c b/drivers/tty/hvc/hvc_dcc.c > index 8e0edb7d93fd..e87b82e873d7 100644 > --- a/drivers/tty/hvc/hvc_dcc.c > +++ b/drivers/tty/hvc/hvc_dcc.c > @@ -2,9 +2,14 @@ > /* Copyright (c) 2010, 2014 The Linux Foundation. All rights reserved. */ No copyright update? > > #include <linux/console.h> > +#include <linux/cpu.h> > +#include <linux/cpumask.h> > #include <linux/init.h> > +#include <linux/kfifo.h> > #include <linux/serial.h> > #include <linux/serial_core.h> > +#include <linux/smp.h> > +#include <linux/spinlock.h> > > #include <asm/dcc.h> > #include <asm/processor.h> > @@ -15,6 +20,15 @@ > #define DCC_STATUS_RX (1 << 30) > #define DCC_STATUS_TX (1 << 29) > > +#define DCC_INBUF_SIZE 128 > +#define DCC_OUTBUF_SIZE 1024 > + > +/* Lock to serialize access to DCC fifo */ > +static DEFINE_SPINLOCK(dcc_lock); > + > +static DEFINE_KFIFO(inbuf, unsigned char, DCC_INBUF_SIZE); > +static DEFINE_KFIFO(outbuf, unsigned char, DCC_OUTBUF_SIZE); > + > static void dcc_uart_console_putchar(struct uart_port *port, int ch) > { > while (__dcc_getstatus() & DCC_STATUS_TX) > @@ -67,24 +81,176 @@ static int hvc_dcc_get_chars(uint32_t vt, char *buf, int count) > return i; > } > > +/* > + * Check if the DCC is enabled. If CONFIG_HVC_DCC_SERIALIZE_SMP is enabled, > + * then we assume then this function will be called first on core0. That way, > + * dcc_core0_available will be true only if it's available on core0. > + */ > static bool hvc_dcc_check(void) > { > unsigned long time = jiffies + (HZ / 10); > + static bool dcc_core0_available; > + > + /* > + * If we're not on core 0, but we previously confirmed that DCC is > + * active, then just return true. > + */ > + int cpu = get_cpu(); > + > + if (IS_ENABLED(CONFIG_HVC_DCC_SERIALIZE_SMP) && cpu && dcc_core0_available) { > + put_cpu(); > + return true; > + } > + > + put_cpu(); > > /* Write a test character to check if it is handled */ > __dcc_putchar('\n'); > > while (time_is_after_jiffies(time)) { > - if (!(__dcc_getstatus() & DCC_STATUS_TX)) > + if (!(__dcc_getstatus() & DCC_STATUS_TX)) { > + dcc_core0_available = true; > return true; > + } > } > > return false; > } > > +/* > + * Workqueue function that writes the output FIFO to the DCC on core 0. > + */ > +static void dcc_put_work(struct work_struct *work) > +{ > + unsigned char ch; > + unsigned long irqflags; > + > + spin_lock_irqsave(&dcc_lock, irqflags); > + > + /* While there's data in the output FIFO, write it to the DCC */ > + while (kfifo_get(&outbuf, &ch)) > + hvc_dcc_put_chars(0, &ch, 1); > + > + /* While we're at it, check for any input characters */ > + while (!kfifo_is_full(&inbuf)) { > + if (!hvc_dcc_get_chars(0, &ch, 1)) > + break; > + kfifo_put(&inbuf, ch); > + } > + > + spin_unlock_irqrestore(&dcc_lock, irqflags); > +} > + > +static DECLARE_WORK(dcc_pwork, dcc_put_work); > + > +/* > + * Workqueue function that reads characters from DCC and puts them into the > + * input FIFO. > + */ > +static void dcc_get_work(struct work_struct *work) > +{ > + unsigned char ch; > + unsigned long irqflags; > + > + /* > + * Read characters from DCC and put them into the input FIFO, as > + * long as there is room and we have characters to read. > + */ > + spin_lock_irqsave(&dcc_lock, irqflags); > + > + while (!kfifo_is_full(&inbuf)) { > + if (!hvc_dcc_get_chars(0, &ch, 1)) > + break; > + kfifo_put(&inbuf, ch); > + } > + spin_unlock_irqrestore(&dcc_lock, irqflags); > +} > + > +static DECLARE_WORK(dcc_gwork, dcc_get_work); > + > +/* > + * Write characters directly to the DCC if we're on core 0 and the FIFO > + * is empty, or write them to the FIFO if we're not. > + */ > +static int hvc_dcc0_put_chars(u32 vt, const char *buf, int count) > +{ > + int len; > + unsigned long irqflags; > + > + if (!IS_ENABLED(CONFIG_HVC_DCC_SERIALIZE_SMP)) > + return hvc_dcc_put_chars(vt, buf, count); > + > + spin_lock_irqsave(&dcc_lock, irqflags); > + if (smp_processor_id() || (!kfifo_is_empty(&outbuf))) { > + len = kfifo_in(&outbuf, buf, count); > + spin_unlock_irqrestore(&dcc_lock, irqflags); > + > + /* > + * We just push data to the output FIFO, so schedule the > + * workqueue that will actually write that data to DCC. > + * CPU hotplug is disabled so CPU0 cannot be offlined > + * after the cpu online check. How is cpu hotplug disabled here? > + */ > + if (cpu_online(0)) What happens if the cpu state changes right after checking it? > + schedule_work_on(0, &dcc_pwork); What happens if cpu 0 never comes back? > + > + return len; > + } > + > + /* > + * If we're already on core 0, and the FIFO is empty, then just > + * write the data to DCC. > + */ > + len = hvc_dcc_put_chars(vt, buf, count); > + spin_unlock_irqrestore(&dcc_lock, irqflags); > + > + return len; > +} > + > +/* > + * Read characters directly from the DCC if we're on core 0 and the FIFO > + * is empty, or read them from the FIFO if we're not. > + */ > +static int hvc_dcc0_get_chars(u32 vt, char *buf, int count) > +{ > + int len; > + unsigned long irqflags; > + > + if (!IS_ENABLED(CONFIG_HVC_DCC_SERIALIZE_SMP)) > + return hvc_dcc_get_chars(vt, buf, count); > + > + spin_lock_irqsave(&dcc_lock, irqflags); > + > + if (smp_processor_id() || (!kfifo_is_empty(&inbuf))) { > + len = kfifo_out(&inbuf, buf, count); > + spin_unlock_irqrestore(&dcc_lock, irqflags); > + > + /* > + * If the FIFO was empty, there may be characters in the DCC > + * that we haven't read yet. Schedule a workqueue to fill > + * the input FIFO, so that the next time this function is > + * called, we'll have data. CPU hotplug is disabled so CPU0 > + * cannot be offlined after the cpu online check. Again, how is cpu hotplug disabled? > + */ > + if (!len && cpu_online(0)) > + schedule_work_on(0, &dcc_gwork); > + > + return len; > + } > + > + /* > + * If we're already on core 0, and the FIFO is empty, then just > + * read the data from DCC. > + */ > + len = hvc_dcc_get_chars(vt, buf, count); > + spin_unlock_irqrestore(&dcc_lock, irqflags); > + > + return len; > +} > + > static const struct hv_ops hvc_dcc_get_put_ops = { > - .get_chars = hvc_dcc_get_chars, > - .put_chars = hvc_dcc_put_chars, > + .get_chars = hvc_dcc0_get_chars, > + .put_chars = hvc_dcc0_put_chars, > }; > > static int __init hvc_dcc_console_init(void) > @@ -108,6 +274,9 @@ static int __init hvc_dcc_init(void) > if (!hvc_dcc_check()) > return -ENODEV; > > + if (IS_ENABLED(CONFIG_HVC_DCC_SERIALIZE_SMP)) > + cpu_hotplug_disable(); Ah. And you never enable it again? This feels very very wrong. Did you just ensure that you can never use this option in a real system? For this reason alone I can't take this change, as you just increased the power consumption of any system with this enabled :( greg k-h
