drm/xe: Expose PCIe link downgrade attributes

Expose sysfs attributes for PCIe link downgrade capability and status.

v2: Move from debugfs to sysfs (Lucas, Rodrigo, Badal)
    Rework macros and their naming (Rodrigo)
v3: Use sysfs_create_files() (Riana)
    Fix checkpatch warning (Riana)
v4: s/downspeed/downgrade (Lucas, Rodrigo, Riana)
v5: Use PCIe Gen agnostic naming (Rodrigo)
v6: s/pcie_gen/auto_link (Lucas)

Signed-off-by: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Riana Tauro <riana.tauro@intel.com>
Link: https://lore.kernel.org/r/20250506054835.3395220-3-raag.jadav@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_pcode_api.h

@@ -34,6 +34,7 @@
 #define   DGFX_PCODE_STATUS		0x7E
 #define     DGFX_GET_INIT_STATUS	0x0
 #define     DGFX_INIT_STATUS_COMPLETE	0x1
+#define     DGFX_LINK_DOWNGRADE_STATUS	REG_BIT(31)
 
 #define   PCODE_POWER_SETUP			0x7C
 #define     POWER_SETUP_SUBCOMMAND_READ_I1	0x4
@@ -66,6 +67,10 @@
 /* Auxiliary info bits */
 #define   AUXINFO_HISTORY_OFFSET	REG_GENMASK(31, 29)
 
+#define BMG_PCIE_CAP			XE_REG(0x138340)
+#define   LINK_DOWNGRADE		REG_GENMASK(1, 0)
+#define     DOWNGRADE_CAPABLE		2
+
 struct pcode_err_decode {
 	int errno;
 	const char *str;