Re: [PATCH v2] drm/i915/bdb: Fix version check

["Souza, Jose" <jose.souza@intel.com>]

On Thu, 2021-09-23 at 18:49 +0200, Lukasz Majczak wrote:
> With patch "drm/i915/vbt: Fix backlight parsing for VBT 234+"
> the size of bdb_lfp_backlight_data structure has been increased,
> causing if-statement in the parse_lfp_backlight function
> that comapres this structure size to the one retrieved from BDB,
> always to fail for older revisions.
> This patch calculates expected size of the structure for a given
> BDB version and compares it with the value gathered from BDB.
> Tested on Chromebook Pixelbook (Nocturne) (reports bdb->version = 221)

Fixes: d381baad29b4 ("drm/i915/vbt: Fix backlight parsing for VBT 234+")

> 
> Tested-by: Lukasz Majczak <lma@semihalf.com>
> Signed-off-by: Lukasz Majczak <lma@semihalf.com>
> ---
>  drivers/gpu/drm/i915/display/intel_bios.c     | 11 +++++++++--
>  drivers/gpu/drm/i915/display/intel_vbt_defs.h |  5 +++++
>  2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_bios.c b/drivers/gpu/drm/i915/display/intel_bios.c
> index 3c25926092de..90eae6da12e0 100644
> --- a/drivers/gpu/drm/i915/display/intel_bios.c
> +++ b/drivers/gpu/drm/i915/display/intel_bios.c
> @@ -428,6 +428,7 @@ parse_lfp_backlight(struct drm_i915_private *i915,
>  	const struct lfp_backlight_data_entry *entry;
>  	int panel_type = i915->vbt.panel_type;
>  	u16 level;
> +	size_t exp_size;
>  
>  	backlight_data = find_section(bdb, BDB_LVDS_BACKLIGHT);
>  	if (!backlight_data)
> @@ -450,9 +451,15 @@ parse_lfp_backlight(struct drm_i915_private *i915,
>  		return;
>  	}
>  
> +	if (bdb->version <= 234)
> +		exp_size = EXP_BDB_LFP_BL_DATA_SIZE_REV_234;
> +	else if (bdb->version > 234 && bdb->version <= 236)
> +		exp_size = EXP_BDB_LFP_BL_DATA_SIZE_REV_236;
> +	else
> +		exp_size = sizeof(struct bdb_lfp_backlight_data);

Usually we go by the newest(IP version, platform...) to the oldest:


if (bdb->version >= 236)
	exp_size = sizeof(struct bdb_lfp_backlight_data);
else if (bdb->version >= 234)
	exp_size = offsetof(struct bdb_lfp_backlight_data, brightness_precision_bits);
else
	exp_size = offsetof(struct bdb_lfp_backlight_data, brightness_level);


backlight_control was added in version 191 so no need to set exp_size for older versions.

> +
>  	i915->vbt.backlight.type = INTEL_BACKLIGHT_DISPLAY_DDI;
> -	if (bdb->version >= 191 &&
> -	    get_blocksize(backlight_data) >= sizeof(*backlight_data)) {
> +	if (bdb->version >= 191  && get_blocksize(backlight_data) >= exp_size) {
>  		const struct lfp_backlight_control_method *method;
>  
>  		method = &backlight_data->backlight_control[panel_type];
> diff --git a/drivers/gpu/drm/i915/display/intel_vbt_defs.h b/drivers/gpu/drm/i915/display/intel_vbt_defs.h
> index 330077c2e588..ba9990e5983c 100644
> --- a/drivers/gpu/drm/i915/display/intel_vbt_defs.h
> +++ b/drivers/gpu/drm/i915/display/intel_vbt_defs.h
> @@ -814,6 +814,11 @@ struct lfp_brightness_level {
>  	u16 reserved;
>  } __packed;
>  
> +#define EXP_BDB_LFP_BL_DATA_SIZE_REV_234 \
> +	offsetof(struct bdb_lfp_backlight_data, brightness_level)

version 234 starts at brightness_level but the size of 234 data must be included to the size, so it should be:
offsetof(struct bdb_lfp_backlight_data, brightness_precision_bits).

> +#define EXP_BDB_LFP_BL_DATA_SIZE_REV_236 \
> +	offsetof(struct bdb_lfp_backlight_data, brightness_precision_bits)

> +
>  struct bdb_lfp_backlight_data {
>  	u8 entry_size;
>  	struct lfp_backlight_data_entry data[16];