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SubjectRe: [PATCH] TTY: hvc_dcc: probe for a JTAG connection before registering
On 09/23/2013 01:30 PM, Rob Herring wrote:
> From: Rob Herring <rob.herring@calxeda.com>
>
> Enabling the ARM DCC console and using without a JTAG connection will
> simply hang the system. Since distros like to turn on all options, this
> is a reoccurring problem to debug. We can do better by checking if
> anything is attached and handling characters. There is no way to probe
> this, so send a newline and check that it is handled.
>
> Cc: Paolo Pisati <paolo.pisati@canonical.com>
> Cc: Tim Gardner <tim.gardner@canonical.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Jiri Slaby <jslaby@suse.cz>
> Signed-off-by: Rob Herring <rob.herring@calxeda.com>
> ---
> drivers/tty/hvc/hvc_dcc.c | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/drivers/tty/hvc/hvc_dcc.c b/drivers/tty/hvc/hvc_dcc.c
> index 44fbeba..44c8aff 100644
> --- a/drivers/tty/hvc/hvc_dcc.c
> +++ b/drivers/tty/hvc/hvc_dcc.c
> @@ -86,6 +86,21 @@ static int hvc_dcc_get_chars(uint32_t vt, char *buf, int count)
> return i;
> }
>
> +static bool hvc_dcc_check(void)
> +{
> + unsigned long time = jiffies;
> +
> + /* Write a test character to check if it is handled */
> + __dcc_putchar('\n');
> +
> + while (jiffies < time + (HZ / 10)) {
> + if (!(__dcc_getstatus() & DCC_STATUS_TX))
> + return true;
> + }

I think you should use one of the jiffies time comparison functions, e.g.,

time = jiffies + (HZ / 10);
while (time_before(jiffies,time) {
...
}

What is the possibility that __dcc_getstatus() will return garbage if no
JTAG is connected ? In that event you're likely to get a false positive
on DCC_STATUS_TX.

rtg


--
Tim Gardner tim.gardner@canonical.com


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