Re: [PATCH 1/4] remoteproc: core: Reset freed vring entries to FW_RSC_ADDR_ANY

[Mathieu Poirier <mathieu.poirier@linaro.org>]

On Tue, Sep 29, 2026 at 03:54:50PM +0800, Yonghao Zhang wrote:
> rproc_free_vring() resets a vring entry in the resource table as it
> releases it.  Resetting da to 0 leaves the entry looking like a vring
> at device address 0; write FW_RSC_ADDR_ANY instead, the marker for an
> unallocated address, which is what the entry means from then on.
> 

The above is enough to describe what this patch does.  An additional line will
also be enough to justify the modifiation of the comment (see below).

> That matters on the error paths of rproc_start() and rproc_attach(),
> where rproc_free_vring() runs while the remote processor is already
> running: the failure originates in rp_find_vq(), called through
> virtio_find_vqs() from e.g. rpmsg_probe(), which the core only
> reaches through rproc_start_subdevices(), after ops->start() or
> ops->attach() have completed.  At that point table_ptr is the
> resource table installed in remote processor memory, and whatever
> the reset writes is visible to the remote side.  Whether and when a
> running processor reads the entry cannot be known, so the value must
> be safe for one: 0 is a plausible device address, FW_RSC_ADDR_ANY is
> not.  On the teardown paths, stop and detach, the write lands in the
> cached table or in a copy of the installed one and reaches no one.
> 
> The comment replaced along with it was written for a call graph that
> no longer exists.  It described the teardown callers only:
> rproc_stop() has run, table_ptr points at the cached table, and that
> table is NULL for a processor started by another entity, so there is
> nothing to clear.  Two facts have overtaken it:
> 
> - rp_find_vq() has been calling rproc_free_vring() when
>   vring_new_virtqueue() fails since commit 6db20ea8d850 ("remoteproc:
>   allocate vrings on demand, free when not needed"), and that call
>   was present, unchanged, when commit 9dc9507f1880 ("remoteproc:
>   Properly deal with the resource table when detaching") landed: it
>   runs from rproc_start_subdevices(), after ops->start() or
>   ops->attach() have completed, with table_ptr at the table installed
>   in remote processor memory and the processor running.  The comment
>   never described this caller, not on the day it was written.

Are comments supposed to describe all the paths that lead to a function?

> 
> - commit 9dc9507f1880 ("remoteproc: Properly deal with the resource
>   table when detaching") and commit 8088dd4d9316 ("remoteproc: Properly
>   deal with the resource table when stopping") later taught the
>   detach/stop paths to take a kmemdup() copy of the installed table for
>   processors started by another entity, so the NULL case the guard was
>   deciding between is gone: table_ptr is always valid where the teardown
>   callers run.  (Those callers have since moved to the rproc-virtio
>   platform driver, 9c31255ce5fe.)
> 
> Fixes: c0d631570ad5 ("remoteproc: set vring addresses in resource table")
> Fixes: 4d3ebb3b9990 ("remoteproc: Refactor function rproc_free_vring()")
> Signed-off-by: Yonghao Zhang <hyz3367@gmail.com>
> ---
>  drivers/remoteproc/remoteproc_core.c | 40 +++++++++++++++++++---------
>  1 file changed, 28 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/remoteproc/remoteproc_core.c b/drivers/remoteproc/remoteproc_core.c
> index 263e12f022ea..123aadb467a0 100644
> --- a/drivers/remoteproc/remoteproc_core.c
> +++ b/drivers/remoteproc/remoteproc_core.c
> @@ -403,6 +403,33 @@ rproc_parse_vring(struct rproc_vdev *rvdev, struct fw_rsc_vdev *rsc, int i)
>  	return 0;
>  }
>  
> +/*
> + * rproc_free_vring() runs while a remote processor is being torn down
> + * (rproc_stop()/rproc_detach() paths) or while a boot or attach attempt
> + * is failing.  Whether the reset below can reach the remote processor
> + * depends on what rproc->table_ptr refers to at that point:
> + *
> + * - teardown paths: the call comes from the rvdev cleanup in
> + *   rproc_resource_cleanup(), by which time rproc_stop()/__rproc_detach()
> + *   have switched table_ptr to the table the core was booted with, or
> + *   to a copy of the installed table when the remote processor was
> + *   started by another entity (rproc_reset_rsc_table_on_{stop,detach}()),
> + *   so the write cannot reach the remote processor.
> + *
> + * - error paths of rproc_start()/rproc_attach(): the failure originates
> + *   in rp_find_vq() (virtio_find_vqs() failing in e.g. rpmsg_probe()),
> + *   which rproc_start_subdevices() calls only once ops->start() or
> + *   ops->attach() have completed.  table_ptr still points at the
> + *   resource table installed in remote processor memory and the remote
> + *   processor is already running.
> + *
> + * Especially in the last case the reset has to tell the remote
> + * processor that the vring entry is not usable: with da set to the
> + * invalid FW_RSC_ADDR_ANY a running remote processor sees the address
> + * as unallocated, and notifyid is invalidated along with it.

Why is there a need to describe all that?

> + *
> + * Reset the virtio device section only if there is a table to work with.
> + */
>  void rproc_free_vring(struct rproc_vring *rvring)
>  {
>  	struct rproc *rproc = rvring->rvdev->rproc;
> @@ -411,20 +438,9 @@ void rproc_free_vring(struct rproc_vring *rvring)
>  
>  	idr_remove(&rproc->notifyids, rvring->notifyid);
>  
> -	/*
> -	 * At this point rproc_stop() has been called and the installed resource
> -	 * table in the remote processor memory may no longer be accessible. As
> -	 * such and as per rproc_stop(), rproc->table_ptr points to the cached
> -	 * resource table (rproc->cached_table).  The cached resource table is
> -	 * only available when a remote processor has been booted by the
> -	 * remoteproc core, otherwise it is NULL.

Just remove the last sentence if it has become obsolete.   

> -	 *
> -	 * Based on the above, reset the virtio device section in the cached
> -	 * resource table only if there is one to work with.
> -	 */
>  	if (rproc->table_ptr) {
>  		rsc = (void *)rproc->table_ptr + rvring->rvdev->rsc_offset;
> -		rsc->vring[idx].da = 0;
> +		rsc->vring[idx].da = FW_RSC_ADDR_ANY;

I agree with this change.

>  		rsc->vring[idx].notifyid = -1;
>  	}
>  }
> -- 
> 2.34.1
>