Re: [PATCH v13 5/5] media: qcom: camss: vfe: Add support for VFE gen4

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Hardware constraint violation due to AUP_UPDATE and enqueued buffer count mismatch on Kaanapali.
--

commit d8f6f74712ec57882b1ea04201952757c549300f
Author: Hangxiang Ma <hangxiang.ma@oss.qualcomm.com>

media: qcom: camss: vfe: Add support for VFE gen4

Add Video Front End (VFE) version gen4 as found on the Kaanapali SoC.

> diff --git a/drivers/media/platform/qcom/camss/camss-vfe.c b/drivers/media/platform/qcom/camss/camss-vfe.c
> index 5baf0e3d4bc46..99630ffa1db57 100644
> --- a/drivers/media/platform/qcom/camss/camss-vfe.c
> +++ b/drivers/media/platform/qcom/camss/camss-vfe.c

[ ... ]

> @@ -551,7 +553,9 @@ int vfe_enable_output_v2(struct vfe_line *line)
>  		output->gen2.active_num++;
>  		ops->vfe_wm_update(vfe, output->wm_idx[0],
>  				   output->buf[i]->addr[0], line);
> -		ops->reg_update(vfe, line->id);
> +
> +		if (!vfe->res->reg_update_after_csid_config)
> +			ops->reg_update(vfe, line->id);
>  	}
>  
>  	spin_unlock_irqrestore(&vfe->output_lock, flags);

Does this implementation violate the Kaanapali hardware constraint regarding
AUP_UPDATE and enqueued buffer count?

The commit message explicitly states that Kaanapali hardware requires the
number of AUP_UPDATE commands to exactly match the number of buffers enqueued
to the write master while it is being enabled.

During STREAMON, VFE dynamically enqueues between 0 and 2 pending buffers
(up to CAMSS_INIT_BUF_COUNT) depending on what userspace has queued. It
skips issuing the VFE register update here because
reg_update_after_csid_config is true.

However, csid_configure_stream() in the CSID driver executes and
unconditionally issues exactly 2 AUP_UPDATEs via a hardcoded loop
(for (k = 0; k < CAMSS_INIT_BUF_COUNT; k++) __csid_aup_update()), completely
ignoring the actual active_num of buffers the VFE enqueued.

Furthermore, if userspace queued fewer than 2 buffers before STREAMON and
queues the remaining buffers later via VIDIOC_QBUF, vfe_queue_buffer_v2()
sees active_num < 2 and unconditionally issues an additional ops->reg_update()
per buffer.

For example, if userspace queues 0 buffers initially, CSID blindly issues
2 AUP_UPDATEs. When 2 buffers are subsequently queued via QBUF,
vfe_queue_buffer_v2() issues 2 more, resulting in 4 AUP_UPDATEs for 2 buffers.

Could this mismatch between AUP_UPDATEs and enqueued buffers cause
desynchronization in the state machine, FIFO underflows, or IOMMU faults?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260508-kaanapali-camss-v13-0-2541d8e55651@oss.qualcomm.com?part=5