Re: [PATCH 1/1] drm/xe/eustall: Return EBADFD from read if EU stall registers get reset

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Sun, 07 Dec 2025 22:16:11 -0800, Harish Chegondi wrote:
>

Hi Harish,

> @@ -541,9 +541,24 @@ static ssize_t xe_eu_stall_stream_read_locked(struct xe_eu_stall_data_stream *st
>	size_t total_size = 0;
>	u16 group, instance;
>	unsigned int xecore;
> +	u32 base_reg_value;
>	int ret = 0;
>
>	mutex_lock(&stream->xecore_buf_lock);
> +	/* If a GT or engine reset happens during EU stall data sampling,
> +	 * all EU stall registers get reset to 0 and the cached values of
> +	 * EU stall data buffers' read and write pointers are out of sync
> +	 * with the register values. This can cause invalid data to be
> +	 * returned from read(). To prevent this, check the value of a
> +	 * EU stall base register. If it is zero, return -EBADFD. The
> +	 * user is expected to close the fd and open a new fd.
> +	 */
> +	base_reg_value = xe_gt_mcr_unicast_read_any(gt, XEHPC_EUSTALL_BASE);
> +	if (unlikely(!base_reg_value)) {
> +		xe_gt_dbg(gt, "EU stall base register has been reset to 0\n");
> +		mutex_unlock(&stream->xecore_buf_lock);
> +		return -EBADFD;
> +	}

Since we are introducing an extra register read every read() call here,
does it make sense to first check if there's a real userland need for this?
And actually have a UMD PR which will consume this -EBADFD return value,
before we merge this?

Thanks.
--
Ashutosh