Re: [PATCH i-g-t] lib/intel_pat: add hardcoded PAT fallback for older kernels

["Wang, X" <x.wang@intel.com>]

On 3/26/2026 22:22, himanshu.girotra@intel.com wrote:
> From: Himanshu Girotra <himanshu.girotra@intel.com>
>
> When IGT runs on kernels that lack the gt0/pat_sw_config debugfs entry
> (commit "drm/xe: expose PAT software config to debugfs"), the PAT cache
> is NULL and intel_get_pat_idx() hits a fatal assertion.  This breaks
> basic tests like xe_exec_basic on stock distro kernels (e.g. Ubuntu
> 25.10).
>
> Add xe_pat_fallback() with hardcoded PAT indices matching the kernel's
> xe_pat_init_early() for platforms up to Crescent Island (xe3p XPC).
> When the debugfs is unavailable, the fallback provides correct values
> instead of crashing.  Newer platforms (xe3p LPG and beyond) are not
> covered because their kernel support arrived after the debugfs entry
> was already in place.
>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Matt Roper <matthew.d.roper@intel.com>
> Cc: Francois Dugast <francois.dugast@intel.com>
> Signed-off-by: Himanshu Girotra <himanshu.girotra@intel.com>
> ---
>   lib/intel_pat.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 72 insertions(+), 4 deletions(-)
>
> diff --git a/lib/intel_pat.c b/lib/intel_pat.c
> index b202a6241..71633b314 100644
> --- a/lib/intel_pat.c
> +++ b/lib/intel_pat.c
> @@ -97,6 +97,62 @@ int32_t xe_get_pat_sw_config(int drm_fd, struct intel_pat_cache *xe_pat_cache)
>   	return parsed;
>   }
>   
> +/*
> + * Hardcoded PAT indices for Xe platforms, used as a fallback when the
> + * kernel doesn't expose gt0/pat_sw_config in debugfs.
> + *
> + * Covers platforms up to Crescent Island (xe3p XPC) that have Xe driver
> + * support in current stable kernels.  Anything newer must run a kernel
> + * that provides the pat_sw_config debugfs entry.
> + *
> + * TODO: drop this fallback once stable kernels ship with pat_sw_config.
> + */
> +static bool xe_pat_fallback(int fd, struct intel_pat_cache *pat)
> +{
> +	uint16_t dev_id = intel_get_drm_devid(fd);
> +
> +	pat->uc_comp = XE_PAT_IDX_INVALID;
> +
> +	if (intel_graphics_ver(dev_id) == IP_VER(35, 11)) {
> +		/* Xe3p XPC (GFX ver 35.11): no WT, no compression */
> +		pat->uc = 3;
> +		pat->wt = 3;   /* No WT on XPC; use UC */
> +		pat->wb = 2;
> +		pat->max_index = 31;
> +	} else if (intel_get_device_info(dev_id)->graphics_ver == 30 ||
> +		   intel_get_device_info(dev_id)->graphics_ver == 20) {
> +		/* Xe2 / Xe3: GFX ver 20 / 30 */
> +		pat->uc = 3;
> +		pat->wt = 15;
> +		pat->wb = 2;
> +		pat->uc_comp = 12;
> +		pat->max_index = 31;
> +
> +		/* Wa_16023588340: CLOS3 entries at end of table are unusable */
> +		if (intel_graphics_ver(dev_id) == IP_VER(20, 1))
> +			pat->max_index -= 4;
> +	} else if (IS_METEORLAKE(dev_id)) {
> +		pat->uc = 2;
> +		pat->wt = 1;
> +		pat->wb = 3;
> +		pat->max_index = 3;
> +	} else if (IS_PONTEVECCHIO(dev_id)) {
> +		pat->uc = 0;
> +		pat->wt = 2;
> +		pat->wb = 3;
> +		pat->max_index = 7;
> +	} else if (IS_DG2(dev_id) || intel_graphics_ver(dev_id) <= IP_VER(12, 10)) {
> +		pat->uc = 3;
> +		pat->wt = 2;
> +		pat->wb = 0;
> +		pat->max_index = 3;
> +	} else {
> +		return false;
> +	}
> +
> +	return true;
> +}
> +
Since we've already hardcoded some things, essentially repeating
parameters from the KMD, why don't we just copy the complete PAT
table as well? This way, all tests for xe_pat's dependencies on
the complete PAT table will pass smoothly.
>   static void intel_get_pat_idx(int fd, struct intel_pat_cache *pat)
>   {
>   	uint16_t dev_id;
> @@ -105,14 +161,26 @@ static void intel_get_pat_idx(int fd, struct intel_pat_cache *pat)
>   	 * For Xe, use the PAT cache stored in struct xe_device.
>   	 * xe_device_get() populates the cache while still root; forked
>   	 * children that inherit the xe_device can use it post-drop_root().
> +	 *
> +	 * Fall back to hardcoded values when the kernel lacks the
> +	 * pat_sw_config debugfs. Platforms newer than Crescent Island
> +	 * must have the debugfs available.
>   	 */
>   	if (is_xe_device(fd)) {
>   		struct xe_device *xe_dev = xe_device_get(fd);
>   
> -		igt_assert_f(xe_dev->pat_cache,
> -			     "PAT sw_config not available -- "
> -			     "debugfs not accessible (missing root or not mounted?)\n");
> -		*pat = *xe_dev->pat_cache;
> +		if (xe_dev->pat_cache) {
> +			*pat = *xe_dev->pat_cache;
> +		} else if (xe_pat_fallback(fd, pat)) {
> +			igt_info("PAT sw_config debugfs not available, "
> +				 "using hardcoded fallback\n");
> +		} else {
> +			igt_assert_f(false,
> +				     "PAT sw_config not available and no "
> +				     "hardcoded fallback for this platform -- "
> +				     "kernel with 'drm/xe: expose PAT software "
> +				     "config to debugfs' required\n");
> +		}
>   		return;
>   	}
>