LTDC_BPCR
regmap_read(ldev->regmap, LTDC_BPCR, &vactive_start);
regmap_read(ldev->regmap, LTDC_BPCR, &bpcr);
regmap_update_bits(ldev->regmap, LTDC_BPCR, BPCR_AVBP | BPCR_AHBP, val);