Re: [PATCH] drm/panthor: snapshot firmware interface counts before loops

[Liviu Dudau <liviu.dudau@arm.com>]

On Mon, Jul 20, 2026 at 03:55:18PM +0200, Osama Abdelkader wrote:
> The firmware exposes the global group count and per-group stream count in
> the shared control interface. These values are validated before being used
> as loop bounds, but the memory is shared with the MCU firmware and can be
> changed after validation.

Again, you're making statements that need to be proven here. How is MCU
firmware going to change the content of the memory shared with the driver?

> 
> Read each count once with READ_ONCE() and use the validated snapshot as the
> loop bound. This keeps the loop bounds consistent with the validation and
> prevents the compiler from reloading a firmware-controlled count during
> iteration.

As an optimisation I'm fine with it, so:

Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>

Best regards,
Liviu

> 
> Fixes: 2718d91816ee ("drm/panthor: Add the FW logical block")
> Cc: stable@vger.kernel.org
> Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
> ---
>  drivers/gpu/drm/panthor/panthor_fw.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_fw.c b/drivers/gpu/drm/panthor/panthor_fw.c
> index e2fcbd639c3c..6e6da98d795e 100644
> --- a/drivers/gpu/drm/panthor/panthor_fw.c
> +++ b/drivers/gpu/drm/panthor/panthor_fw.c
> @@ -959,6 +959,7 @@ static int panthor_init_csg_iface(struct panthor_device *ptdev,
>  	u64 shared_section_sz = panthor_kernel_bo_size(ptdev->fw->shared_section->mem);
>  	u64 iface_offset = CSF_GROUP_CONTROL_OFFSET +
>  			   ((u64)csg_idx * glb_iface->control->group_stride);
> +	u32 stream_num;
>  	unsigned int i;
>  
>  	if (iface_offset > shared_section_sz ||
> @@ -972,8 +973,8 @@ static int panthor_init_csg_iface(struct panthor_device *ptdev,
>  	csg_iface->output = iface_fw_to_cpu_addr(ptdev, csg_iface->control->output_va,
>  						 sizeof(*csg_iface->output));
>  
> -	if (csg_iface->control->stream_num < MIN_CS_PER_CSG ||
> -	    csg_iface->control->stream_num > MAX_CS_PER_CSG)
> +	stream_num = READ_ONCE(csg_iface->control->stream_num);
> +	if (stream_num < MIN_CS_PER_CSG || stream_num > MAX_CS_PER_CSG)
>  		return -EINVAL;
>  
>  	if (!csg_iface->input || !csg_iface->output) {
> @@ -990,7 +991,7 @@ static int panthor_init_csg_iface(struct panthor_device *ptdev,
>  		}
>  	}
>  
> -	for (i = 0; i < csg_iface->control->stream_num; i++) {
> +	for (i = 0; i < stream_num; i++) {
>  		int ret = panthor_init_cs_iface(ptdev, csg_idx, i);
>  
>  		if (ret)
> @@ -1015,6 +1016,7 @@ static int panthor_fw_init_ifaces(struct panthor_device *ptdev)
>  {
>  	struct panthor_fw_global_iface *glb_iface = &ptdev->fw->iface.global;
>  	u64 shared_section_sz = panthor_kernel_bo_size(ptdev->fw->shared_section->mem);
> +	u32 group_num;
>  	unsigned int i;
>  
>  	if (!ptdev->fw->shared_section->mem->kmap)
> @@ -1034,17 +1036,17 @@ static int panthor_fw_init_ifaces(struct panthor_device *ptdev)
>  		return -EINVAL;
>  	}
>  
> -	if (glb_iface->control->group_num > MAX_CSGS ||
> -	    glb_iface->control->group_num < MIN_CSGS) {
> +	group_num = READ_ONCE(glb_iface->control->group_num);
> +	if (group_num > MAX_CSGS || group_num < MIN_CSGS) {
>  		drm_err(&ptdev->base, "Invalid number of control groups");
>  		return -EINVAL;
>  	}
>  
> -	for (i = 0; i < glb_iface->control->group_num; i++) {
> +	for (i = 0; i < group_num; i++) {
>  		int ret = panthor_init_csg_iface(ptdev, i);
>  
>  		if (ret)
>  			return ret;
>  	}
>  
>  	drm_info(&ptdev->base, "CSF FW using interface v%d.%d.%d, Features %#x Instrumentation features %#x",
> -- 
> 2.43.0
> 

-- 
====================
| I would like to |
| fix the world,  |
| but they're not |
| giving me the   |
 \ source code!  /
  ---------------
    ¯\_(ツ)_/¯