Re: [PATCH v2] drm/panthor: use local variables for firmware interface counts

[Boris Brezillon]

On Mon,  3 Aug 2026 14:48:06 +0200
Osama Abdelkader <osama.abdelkader@xxxxxxxxx> wrote:

> panthor_fw_init_ifaces() validates the firmware interface group count
> before iterating over the CSG interfaces. panthor_init_csg_iface() does
> the same for the per-group stream count before iterating over the CS
> interfaces.
> 
> Store those validated counts in local variables and use the locals as the
> loop bounds. This avoids reading the same control interface fields twice
> and makes it explicit that the loops use the values that were just
> validated.
> 
> Signed-off-by: Osama Abdelkader <osama.abdelkader@xxxxxxxxx>
> Reviewed-by: Steven Price <steven.price@xxxxxxx>
> ---
> v2:
> - Reword as an optimization/cleanup instead of a firmware trust-boundary fix.
> - Drop the Fixes and stable tags.
> 
>  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);

We need a comment to explain the READ_ONCE(), otherwise new readers
(and given my inability to remember things, I consider myself a new
reader after 2 weeks :-)) will keep wondering why we're forcing the
compiler to read the memory only once. IIUC, that's here to protect
against self-modifying control sections, so maybe say that. Or if we
consider that the FW is trusted/sure, drop the READ_ONCE()...

> +	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",