[PATCH] tty: hvc: dcc: Detect JTAG connection only on CPU core 0

[PATCH] tty: hvc: dcc: Detect JTAG connection only on CPU core 0

[shatyuka]

From: Shatyuka <shatyuka@qq.com>

On SMP, migration can occur inside hvc_dcc_check() during
device_initcall, so the CPU writing the probe character and the CPU
reading the TXfull status may not be the same, which can hang the
system.

Commit 4419da5d5d4b1788568b ("tty: hvc: dcc: Bind driver to CPU core0
for reads and writes") cached the DCC availability detected on core 0
during console_initcall. However, this is not sufficient: if DCC is
not detected as available and another CPU later calls
hvc_dcc_check(), the check is not actually performed on core 0.

Multiple checks are unnecessary: nobody is going to plug/unplug JTAG
during kernel boot and expect it to work. Cache the return value of
hvc_dcc_check() from device_initcall and reuse the cached value during
console_initcall. This avoids complex SMP-related code and ensures the
result is always from core 0.

Signed-off-by: Shatyuka <shatyuka@qq.com>
---
 drivers/tty/hvc/hvc_dcc.c | 30 ++++++++----------------------
 1 file changed, 8 insertions(+), 22 deletions(-)

diff --git a/drivers/tty/hvc/hvc_dcc.c b/drivers/tty/hvc/hvc_dcc.c
index dfc5c9c38f07..fc8dcbce9b5b 100644
--- a/drivers/tty/hvc/hvc_dcc.c
+++ b/drivers/tty/hvc/hvc_dcc.c
@@ -29,6 +29,9 @@ static DEFINE_SPINLOCK(dcc_lock);
 static DEFINE_KFIFO(inbuf, u8, DCC_INBUF_SIZE);
 static DEFINE_KFIFO(outbuf, u8, DCC_OUTBUF_SIZE);
 
+/* Cached result of hvc_dcc_check() */
+static bool dcc_core0_available;
+
 static void dcc_uart_console_putchar(struct uart_port *port, u8 ch)
 {
 	while (__dcc_getstatus() & DCC_STATUS_TX)
@@ -90,36 +93,18 @@ static ssize_t hvc_dcc_get_chars(uint32_t vt, u8 *buf, size_t count)
 }
 
 /*
- * 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.
+ * Check if the DCC is enabled. This function will be called once on core 0.
  */
 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)) {
-			dcc_core0_available = true;
+		if (!(__dcc_getstatus() & DCC_STATUS_TX))
 			return true;
-		}
 	}
 
 	return false;
@@ -265,7 +250,8 @@ static int __init hvc_dcc_console_init(void)
 {
 	int ret;
 
-	if (!hvc_dcc_check())
+	dcc_core0_available = hvc_dcc_check();
+	if (!dcc_core0_available)
 		return -ENODEV;
 
 	/* Returns -1 if error */
@@ -279,7 +265,7 @@ static int __init hvc_dcc_init(void)
 {
 	struct hvc_struct *p;
 
-	if (!hvc_dcc_check())
+	if (!dcc_core0_available)
 		return -ENODEV;
 
 	if (IS_ENABLED(CONFIG_HVC_DCC_SERIALIZE_SMP)) {
-- 
2.36.1

Re: [PATCH] tty: hvc: dcc: Detect JTAG connection only on CPU core 0

[Greg Kroah-Hartman]

On Mon, Oct 12, 2026 at 11:17:33PM +0800, shatyuka@qq.com wrote:
> From: Shatyuka <shatyuka@qq.com>

Real name please.

> 
> On SMP, migration can occur inside hvc_dcc_check() during
> device_initcall, so the CPU writing the probe character and the CPU
> reading the TXfull status may not be the same, which can hang the
> system.
> 
> Commit 4419da5d5d4b1788568b ("tty: hvc: dcc: Bind driver to CPU core0
> for reads and writes") cached the DCC availability detected on core 0
> during console_initcall. However, this is not sufficient: if DCC is
> not detected as available and another CPU later calls
> hvc_dcc_check(), the check is not actually performed on core 0.
> 
> Multiple checks are unnecessary: nobody is going to plug/unplug JTAG
> during kernel boot and expect it to work. Cache the return value of
> hvc_dcc_check() from device_initcall and reuse the cached value during
> console_initcall. This avoids complex SMP-related code and ensures the
> result is always from core 0.
> 
> Signed-off-by: Shatyuka <shatyuka@qq.com>
> ---
>  drivers/tty/hvc/hvc_dcc.c | 30 ++++++++----------------------
>  1 file changed, 8 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/tty/hvc/hvc_dcc.c b/drivers/tty/hvc/hvc_dcc.c
> index dfc5c9c38f07..fc8dcbce9b5b 100644
> --- a/drivers/tty/hvc/hvc_dcc.c
> +++ b/drivers/tty/hvc/hvc_dcc.c
> @@ -29,6 +29,9 @@ static DEFINE_SPINLOCK(dcc_lock);
>  static DEFINE_KFIFO(inbuf, u8, DCC_INBUF_SIZE);
>  static DEFINE_KFIFO(outbuf, u8, DCC_OUTBUF_SIZE);
>  
> +/* Cached result of hvc_dcc_check() */
> +static bool dcc_core0_available;

Shouldn't this be per-device?

And how was this tested?

thanks,

greg k-h

Re: [PATCH] tty: hvc: dcc: Detect JTAG connection only on CPU core 0

[shatyuka]

Thanks for the review.

> Please use your real name in Signed-off-by.

Sorry about that. I'll send a v2 patch with my real name.

> > +/* Cached result of hvc_dcc_check() */
> > +static bool dcc_core0_available;
> 
> Shouldn't this be per-device?

There is only one HVC DCC instance, hvc0. No need to store this per
device.

> And how was this tested?

I tested this on an SMP system with no JTAG connected:

  # stty -F /dev/hvc0
  stty: can't open '/dev/hvc0': No such device

With the SMP migration issue present, the same command succeeds and
prints the terminal characteristics, because it falsely detects DCC
as available.

Best regards,
Jiannan Du