Re: [PATCH v1 2/2] drm/xe: Mutual exclusivity between CCS-mode and PF

[Michal Wajdeczko <michal.wajdeczko@intel.com>]

On 11/28/2025 6:10 PM, Nareshkumar Gollakoti wrote:
> Due to SLA agreement between PF and VFs,the alternate CCS-mode
> cannot be changed when VFs are already enabled.
> Similarly, enabling VFs is not permitted when the alternate
> CCS-mode is active.
> 
> Signed-off-by: Nareshkumar Gollakoti <naresh.kumar.g@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_gt_ccs_mode.c | 59 ++++++++++++++++++++++-------
>  drivers/gpu/drm/xe/xe_gt_ccs_mode.h | 12 ++++++
>  2 files changed, 57 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_gt_ccs_mode.c b/drivers/gpu/drm/xe/xe_gt_ccs_mode.c
> index e146e00b0ca2..6652c468e1be 100644
> --- a/drivers/gpu/drm/xe/xe_gt_ccs_mode.c
> +++ b/drivers/gpu/drm/xe/xe_gt_ccs_mode.c
> @@ -13,6 +13,7 @@
>  #include "xe_gt_sysfs.h"
>  #include "xe_mmio.h"
>  #include "xe_sriov.h"
> +#include "xe_sriov_pf.h"
>  
>  static void __xe_gt_apply_ccs_mode(struct xe_gt *gt, u32 num_engines)
>  {
> @@ -108,6 +109,36 @@ ccs_mode_show(struct device *kdev,
>  	return sysfs_emit(buf, "%u\n", gt->ccs_mode);
>  }
>  
> +static int gt_prepare_ccs_mode_enabling(struct xe_gt *gt)
> +{
> +	struct xe_device *xe = gt_to_xe(gt);
> +
> +	if (!IS_SRIOV_PF(xe))
> +		return 0;
> +
> +	/*
> +	 * We can't change CCS-mode when VFs are already enabled
> +	 * and we must prevent enabling VFs when alternate
> +	 * CCS-mode is active

nit: as it might be little hard to understand below logic, maybe add:

	 * Try to lockdown the PF only if CCS is still in default mode,
	 * will unlock PF after CCS is changed to the default mode again.

> +	 */
> +	if (xe_gt_ccs_mode_default(gt))
> +		return xe_sriov_pf_lockdown(xe);
> +
> +	return 0;
> +}
> +
> +static void gt_finish_ccs_mode_enabling(struct xe_gt *gt)
> +{
> +	struct xe_device *xe = gt_to_xe(gt);
> +
> +	if (!IS_SRIOV_PF(xe))
> +		return;
> +
> +	/* Allow enabling VFs, if CCS-mode changed to default mode */
> +	if (xe_gt_ccs_mode_default(gt))
> +		xe_sriov_pf_end_lockdown(xe);
> +}
> +
>  static ssize_t
>  ccs_mode_store(struct device *kdev, struct device_attribute *attr,
>  	       const char *buff, size_t count)
> @@ -117,12 +148,6 @@ ccs_mode_store(struct device *kdev, struct device_attribute *attr,
>  	u32 num_engines, num_slices;
>  	int ret;
>  
> -	if (IS_SRIOV(xe)) {
> -		xe_gt_dbg(gt, "Can't change compute mode when running as %s\n",
> -			  xe_sriov_mode_to_string(xe_device_sriov_mode(xe)));
> -		return -EOPNOTSUPP;
> -	}
> -
>  	ret = kstrtou32(buff, 0, &num_engines);
>  	if (ret)
>  		return ret;
> @@ -139,21 +164,27 @@ ccs_mode_store(struct device *kdev, struct device_attribute *attr,
>  	}
>  
>  	/* CCS mode can only be updated when there are no drm clients */
> -	mutex_lock(&xe->drm.filelist_mutex);
> +	guard(mutex)(&xe->drm.filelist_mutex);
>  	if (!list_empty(&xe->drm.filelist)) {
> -		mutex_unlock(&xe->drm.filelist_mutex);
>  		xe_gt_dbg(gt, "Rejecting compute mode change as there are active drm clients\n");
>  		return -EBUSY;
>  	}
>  
> -	if (gt->ccs_mode != num_engines) {
> -		xe_gt_info(gt, "Setting compute mode to %d\n", num_engines);
> -		gt->ccs_mode = num_engines;
> -		xe_gt_record_user_engines(gt);
> -		xe_gt_reset(gt);
> +	if (gt->ccs_mode == num_engines)> +		return count;
> +
> +	ret = gt_prepare_ccs_mode_enabling(gt);
> +	if (ret) {
> +		xe_gt_dbg(gt, "Rejecting compute mode change as VFs are enabled\n");

maybe this should be closer to the xe_sriov_pf_lockdown() call ?
or 'prepare' helper should be named differently ?
as here it is hard to match 'prepare_ccs_mode_enabling' name with VFs ...

> +		return ret;
>  	}

or make coding more straight:

	/*
	 * Changing default CCS mode is only allowed when there
	 * are no VFs. Try to lockdown PF to find out.
	 */
	if (xe_gt_ccs_mode_default(gt) && IS_SRIOV_PF(xe)) {
		ret = xe_sriov_pf_lockdown(xe);
		if (ret) {
			xe_gt_dbg(gt, "Can't change CCS mode: VFs are enabled\n");
			return ret;
		}
	}

>  
> -	mutex_unlock(&xe->drm.filelist_mutex);
> +	xe_gt_info(gt, "Setting compute mode to %d\n", num_engines);
> +	gt->ccs_mode = num_engines;
> +	xe_gt_record_user_engines(gt);
> +	xe_gt_reset(gt);
> +
> +	gt_finish_ccs_mode_enabling(gt);

and here:

	/* We may end PF lockdown once CCS mode is default again */
	if (xe_gt_ccs_mode_default(gt) && IS_SRIOV_PF(xe))
		xe_sriov_pf_end_lockdown(xe);

>  
>  	return count;
>  }
> diff --git a/drivers/gpu/drm/xe/xe_gt_ccs_mode.h b/drivers/gpu/drm/xe/xe_gt_ccs_mode.h
> index f8779852cf0d..53a595b0882c 100644
> --- a/drivers/gpu/drm/xe/xe_gt_ccs_mode.h
> +++ b/drivers/gpu/drm/xe/xe_gt_ccs_mode.h
> @@ -20,5 +20,17 @@ static inline bool xe_gt_ccs_mode_enabled(const struct xe_gt *gt)
>  	return hweight32(CCS_MASK(gt)) > 1;
>  }
>  
> +/**
> + * xe_gt_ccs_mode_default() - Check if CCS mode is default (single CCS mode)
> + * @gt: GT structure
> + *
> + * Return: %true if actual CCS mode is single mode, or
> + *         %false otherwise (CCS in alternate/multi mode)
> + */
> +static inline bool xe_gt_ccs_mode_default(struct xe_gt *gt)

if you don't plan to use this helper in other places than ccs_mode.c,
then just make it as a simple static helper in ccs_mode.c

> +{
> +	return gt->ccs_mode == 1;
> +}
> +
>  #endif
>  

otherwise LGTM