drm/amd: Add documentation for how to flash a dGPU

The flashing process for dGPUs uses sysfs files in a
non-obvious way, so document it for users.

Signed-off-by: Mario Limonciello <[e-mail]>
Reviewed-by: Alex Deucher <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

Documentation/gpu/amdgpu/flashing.rst

···
+=======================
+ dGPU firmware flashing
+=======================
+
+IFWI
+----
+Flashing the dGPU integrated firmware image (IFWI) is supported by GPUs that
+use the PSP to orchestrate the update (Navi3x or newer GPUs).
+For supported GPUs, `amdgpu` will export a series of sysfs files that can be
+used for the flash process.
+
+The IFWI flash process is:
+
+1. Ensure the IFWI image is intended for the dGPU on the system.
+2. "Write" the IFWI image to the sysfs file `psp_vbflash`. This will stage the IFWI in memory.
+3. "Read" from the `psp_vbflash` sysfs file to initiate the flash process.
+4. Poll the `psp_vbflash_status` sysfs file to determine when the flash process completes.
+
+USB-C PD F/W
+------------
+On GPUs that support flashing an updated USB-C PD firmware image, the process
+is done using the `usbc_pd_fw` sysfs file.
+
+* Reading the file will provide the current firmware version.
+* Writing the name of a firmware payload stored in `/lib/firmware/amdgpu` to the sysfs file will initiate the flash process.
+
+The firmware payload stored in `/lib/firmware/amdgpu` can be named any name
+as long as it doesn't conflict with other existing binaries that are used by
+`amdgpu`.
+
+sysfs files
+-----------
+.. kernel-doc:: drivers/gpu/drm/amd/amdgpu/amdgpu_psp.c

Documentation/gpu/amdgpu/index.rst

···
    module-parameters
    driver-core
    display/index
+   flashing
    xgmi
    ras
    thermal

drivers/gpu/drm/amd/amdgpu/amdgpu_psp.c

···
 	drm_dev_exit(idx);
 }
 
+/**
+ * DOC: usbc_pd_fw
+ * Reading from this file will retrieve the USB-C PD firmware version. Writing to
+ * this file will trigger the update process.
+ */
 static DEVICE_ATTR(usbc_pd_fw, 0644,
 		   psp_usbc_pd_fw_sysfs_read,
 		   psp_usbc_pd_fw_sysfs_write);
···
 	return 0;
 }
 
+/**
+ * DOC: psp_vbflash
+ * Writing to this file will stage an IFWI for update. Reading from this file
+ * will trigger the update process.
+ */
 static struct bin_attribute psp_vbflash_bin_attr = {
 	.attr = {.name = "psp_vbflash", .mode = 0660},
 	.size = 0,
···
 	.read = amdgpu_psp_vbflash_read,
 };
 
+/**
+ * DOC: psp_vbflash_status
+ * The status of the flash process.
+ * 0: IFWI flash not complete.
+ * 1: IFWI flash complete.
+ */
 static ssize_t amdgpu_psp_vbflash_status(struct device *dev,
 					 struct device_attribute *attr,
 					 char *buf)