Re: [PATCH] powerpc/fadump: Add timeout to RTAS busy-wait loops

[Sourabh Jain <sourabhjain@linux.ibm.com>]

Hello Adriano

On 06/04/26 11:45, Adriano Vero wrote:
> The ibm,configure-kernel-dump RTAS call sites in
> rtas_fadump_register(), rtas_fadump_unregister(), and
> rtas_fadump_invalidate() polled indefinitely while firmware returned
> a busy status. A misbehaving or hung firmware could stall these paths
> forever, blocking fadump registration at boot or preventing clean
> teardown.

I agree that it is a good idea to avoid calling rtas_call for
fadump operations indefinitely. However, so far I have not come
across a case where the kernel gets stuck during fadump
registration, unregistration, or invalidation due to phyp/RTAS
continuously returning a wait time on an LPAR.

That said, since fadump support has recently been extended to
QEMU, this change might possibly prove useful in that environment.

>
> Track the accumulated delay in a total_wait counter and bail out with
> -ETIMEDOUT if it reaches RTAS_FADUMP_MAX_WAIT_MS (60 seconds)

What is the rationale behind choosing a 60-second limit?


> before
> firmware signals completion. This follows the bounded busy-wait pattern
> used in rtas-rtc.c.
>
> Signed-off-by: Adriano Vero <litaliano00.contact@gmail.com>
> ---
>   arch/powerpc/platforms/pseries/rtas-fadump.c | 37 ++++++++++++++------
>   arch/powerpc/platforms/pseries/rtas-fadump.h |  6 ++++
>   2 files changed, 33 insertions(+), 10 deletions(-)
>
> diff --git a/arch/powerpc/platforms/pseries/rtas-fadump.c b/arch/powerpc/platforms/pseries/rtas-fadump.c
> index eceb32893..b165f165c 100644
> --- a/arch/powerpc/platforms/pseries/rtas-fadump.c
> +++ b/arch/powerpc/platforms/pseries/rtas-fadump.c
> @@ -181,7 +181,7 @@ static u64 rtas_fadump_get_bootmem_min(void)
>   
>   static int rtas_fadump_register(struct fw_dump *fadump_conf)
>   {
> -	unsigned int wait_time, fdm_size;
> +	unsigned int wait_time, total_wait, fdm_size;
>   	int rc, err = -EIO;
>   
>   	/*
> @@ -192,15 +192,20 @@ static int rtas_fadump_register(struct fw_dump *fadump_conf)
>   	fdm_size = sizeof(struct rtas_fadump_section_header);
>   	fdm_size += be16_to_cpu(fdm.header.dump_num_sections) * sizeof(struct rtas_fadump_section);
>   
> -	/* TODO: Add upper time limit for the delay */
> +	total_wait = 0;
>   	do {
>   		rc =  rtas_call(fadump_conf->ibm_configure_kernel_dump, 3, 1,
>   				NULL, FADUMP_REGISTER, &fdm, fdm_size);
>   
>   		wait_time = rtas_busy_delay_time(rc);
> -		if (wait_time)
> +		if (wait_time) {
> +			if (total_wait >= RTAS_FADUMP_MAX_WAIT_MS) {
> +				pr_err("Timed out waiting for firmware to register fadump\n");
> +				return -ETIMEDOUT;
> +			}
> +			total_wait += wait_time;
>   			mdelay(wait_time);
> -
> +		}
>   	} while (wait_time);
>   
>   	switch (rc) {
> @@ -234,18 +239,24 @@ static int rtas_fadump_register(struct fw_dump *fadump_conf)
>   
>   static int rtas_fadump_unregister(struct fw_dump *fadump_conf)
>   {
> -	unsigned int wait_time;
> +	unsigned int wait_time, total_wait;
>   	int rc;
>   
> -	/* TODO: Add upper time limit for the delay */
> +	total_wait = 0;
>   	do {
>   		rc =  rtas_call(fadump_conf->ibm_configure_kernel_dump, 3, 1,
>   				NULL, FADUMP_UNREGISTER, &fdm,
>   				sizeof(struct rtas_fadump_mem_struct));
>   
>   		wait_time = rtas_busy_delay_time(rc);
> -		if (wait_time)
> +		if (wait_time) {
> +			if (total_wait >= RTAS_FADUMP_MAX_WAIT_MS) {
> +				pr_err("Timed out waiting for firmware to unregister fadump\n");
> +				return -ETIMEDOUT;
> +			}
> +			total_wait += wait_time;
>   			mdelay(wait_time);
> +		}
>   	} while (wait_time);
>   
>   	if (rc) {
> @@ -259,18 +270,24 @@ static int rtas_fadump_unregister(struct fw_dump *fadump_conf)
>   
>   static int rtas_fadump_invalidate(struct fw_dump *fadump_conf)
>   {
> -	unsigned int wait_time;
> +	unsigned int wait_time, total_wait;
>   	int rc;
>   
> -	/* TODO: Add upper time limit for the delay */
> +	total_wait = 0;
>   	do {
>   		rc =  rtas_call(fadump_conf->ibm_configure_kernel_dump, 3, 1,
>   				NULL, FADUMP_INVALIDATE, fdm_active,
>   				sizeof(struct rtas_fadump_mem_struct));
>   
>   		wait_time = rtas_busy_delay_time(rc);
> -		if (wait_time)
> +		if (wait_time) {
> +			if (total_wait >= RTAS_FADUMP_MAX_WAIT_MS) {
> +				pr_err("Timed out waiting for firmware to invalidate fadump\n");
> +				return -ETIMEDOUT;
> +			}
> +			total_wait += wait_time;
>   			mdelay(wait_time);
> +		}
>   	} while (wait_time);

This do...while loop is almost identical in all three places.
Would it make sense to introduce a helper function to wrap the
rtas_call, along with handling the wait time and timeout?

- Sourabh Jain

>   	if (rc) {
> diff --git a/arch/powerpc/platforms/pseries/rtas-fadump.h b/arch/powerpc/platforms/pseries/rtas-fadump.h
> index c109abf6b..65fdab7b5 100644
> --- a/arch/powerpc/platforms/pseries/rtas-fadump.h
> +++ b/arch/powerpc/platforms/pseries/rtas-fadump.h
> @@ -41,6 +41,12 @@
>   #define MAX_SECTIONS				10
>   #define RTAS_FADUMP_MAX_BOOT_MEM_REGS		7
>   
> +/*
> + * Maximum time to wait for firmware to respond to an
> + * ibm,configure-kernel-dump RTAS call before giving up.
> + */
> +#define RTAS_FADUMP_MAX_WAIT_MS			60000U
> +
>   /* Kernel Dump section info */
>   struct rtas_fadump_section {
>   	__be32	request_flag;