linux-kernel - Re: [PATCH] microblaze: Add KGDB support

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Date:	Wed, 28 Jul 2010 06:58:59 -0500
From:	Jason Wessel <jason.wessel@...driver.com>
To:	Michal Simek <monstr@...str.eu>
CC:	John Williams <john.williams@...alogix.com>,
	"Edgar E. Iglesias" <edgar.iglesias@...alogix.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] microblaze: Add KGDB support

On 07/28/2010 04:29 AM, Michal Simek wrote:
> Kgdb uses brki r16, 0x18 instruction to call
> low level _debug_exception function which save
> current state to pt_regs and call microblaze_kgdb_break
> function. _debug_exception should be called only from
> the kernel space. User space calling is not supported
> because user application debugging uses different handling.
>
> pt_regs_to_gdb_regs loads additional special registers
> which can't be changed
>
>  * Enable KGDB in Kconfig
>  * Remove ancient not-tested KGDB support
>  * Remove ancient _debug_exception code from entry.S
>
> Only MMU KGDB support is supported.
>
>   
There is only one place I saw a problem and if you fix it, you can add:

Acked-by: Jason Wessel <jason.wessel@...driver.com>

> +
> +int kgdb_arch_handle_exception(int vector, int signo, int err_code,
> +			       char *remcom_in_buffer, char *remcom_out_buffer,
> +			       struct pt_regs *regs)
> +{
> +	char *ptr;
> +	unsigned long address;
> +	int cpu = smp_processor_id();
> +
> +	switch (remcom_in_buffer[0]) {
> +	case 's':
> +	case 'c':
> +		/* handle the optional parameter */
> +		ptr = &remcom_in_buffer[1];
> +		if (kgdb_hex2long(&ptr, &address))
> +			regs->pc = address;
> +		atomic_set(&kgdb_cpu_doing_single_step, -1);
> +		if (remcom_in_buffer[0] == 's')
> +			atomic_set(&kgdb_cpu_doing_single_step, cpu);
> +
> +		return 0;
> +	}
>   

As far as I can see there is no code in the arch specific portion to
make use of single stepping.  It needs to be implemented using hardware
assist, or a software breakpoint strategy, else you simply omit the 's'
case from statement so that kdb will not enable single stepping at run time.

Example:

	switch (remcom_in_buffer[0]) {
	case 'c':
		/* handle the optional parameter */
		ptr = &remcom_in_buffer[1];
		if (kgdb_hex2long(&ptr, &address))
			regs->pc = address;

		return 0;
	}


You can also remove the kgdb_cpu_doing_single_step operations because
this arch does not currently implement the kgdb single stepping.


Cheers,
Jason.
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