On Thu, Feb 10, 2022 at 07:26:32PM +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 v4: > * Use module parameter for runtime choice of enabling this feature. No, this is not the 1990's, module parameters do not work and are not sustainable. They operate on a code-level while you are modifying a device-specific attribute here. Please make this per-device if you really want to be able to somehow turn this on or off. > * 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/hvc_dcc.c | 177 +++++++++++++++++++++++++++++++++++++- > 1 file changed, 174 insertions(+), 3 deletions(-) > > diff --git a/drivers/tty/hvc/hvc_dcc.c b/drivers/tty/hvc/hvc_dcc.c > index 8e0edb7d93fd..535b09441e55 100644 > --- a/drivers/tty/hvc/hvc_dcc.c > +++ b/drivers/tty/hvc/hvc_dcc.c > @@ -2,19 +2,35 @@ > /* Copyright (c) 2010, 2014 The Linux Foundation. All rights reserved. */ > > #include <linux/console.h> > +#include <linux/cpu.h> > +#include <linux/cpumask.h> > #include <linux/init.h> > +#include <linux/kfifo.h> > +#include <linux/moduleparam.h> > #include <linux/serial.h> > #include <linux/serial_core.h> > +#include <linux/spinlock.h> > > #include <asm/dcc.h> > #include <asm/processor.h> > > #include "hvc_console.h" > > +static bool serialize_smp; > +module_param(serialize_smp, bool, 0444); > +MODULE_PARM_DESC(serialize_smp, "Serialize all DCC console input and output to CPU core 0"); > + > /* DCC Status Bits */ > #define DCC_STATUS_RX (1 << 30) > #define DCC_STATUS_TX (1 << 29) > > +#define DCC_INBUF_SIZE 128 > +#define DCC_OUTBUF_SIZE 1024 Why these random sizes? Why is one bigger than the other? Why are they these specific numbers? > + > +static DEFINE_SPINLOCK(dcc_lock); What is this locking? Please document it (didn't checkpatch complain?) > +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 +83,179 @@ static int hvc_dcc_get_chars(uint32_t vt, char *buf, int count) > return i; > } > > +/* > + * Check if the DCC is enabled. If serialize_smp module param 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. > + */ > + if (serialize_smp && smp_processor_id() && dcc_core0_available) Why are you checking smp_processor_id()? Are you sure it is safe to do that here? > + return true; > > /* 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; > + } That's a hard busy loop, are you sure it will always exit? thanks, greg k-h
