Re: [PATCH v2 1/1] usb: gadget: f_fs: Add support for SuperSpeed Mode

[Manu Gautam <mgautam@codeaurora.org>]

On 9/26/2013 2:10 AM, Felipe Balbi wrote:
> Hi,
>
> (please avoid top-posting)
>
> On Tue, Sep 24, 2013 at 03:00:20PM +0530, Manu Gautam wrote:
>> Hi Felipe,
>>
>> I wanted to mention one point with respect to this patch: Below
>> changes in the functionfs.h to add ss_count (super speed descriptors
>> count) in desc_header (which is passed from userspace) make the driver
>> incompatible with existing userspace applications compiled against old
>> header file. Let me know if that is acceptable.  We are using this
>> driver with Android for adbd (android debug bridge) and these changes
>> are required to support adb over Super Speed controllers e.g. DWC3
>> along with changed in adbd to pass SS EP and companion descriptors.
>
> Good you mentioned, it saves me the trouble of reviewing this patch :-)
>
> It's not acceptable to break userspace ABI at all. If you want
> SuperSpeed support on function fs, we need to figure out a way to do so
> without breaking userspace.
>
> This might mean adding a separate userspace interface to be used with
> superspeed. While at that, we might want to add a few bytes of reserved,
> unused space in our structures for situations where we need to add more
> data into it, just to make it slightly future proof.
>

Thanks for your reply.
As you suggested we can have a different interface for super speed
which would be optional to workaround ABI compatibility issue.
Let me know if below interface looks fine to you, I will then implement
accordingly:

diff --git a/include/uapi/linux/usb/functionfs.h 
b/include/uapi/linux/usb/functionfs.h
index d6b0128..b8cb740 100644
--- a/include/uapi/linux/usb/functionfs.h
+++ b/include/uapi/linux/usb/functionfs.h
@@ -9,8 +9,9 @@


  enum {
-       FUNCTIONFS_DESCRIPTORS_MAGIC = 1,
-       FUNCTIONFS_STRINGS_MAGIC     = 2
+       FUNCTIONFS_DESCRIPTORS_MAGIC     = 1,
+       FUNCTIONFS_STRINGS_MAGIC         = 2,
+       FUNCTIONFS_SS_DESCRIPTORS_MAGIC  = 3
  };


@@ -60,6 +61,25 @@ struct usb_functionfs_descs_head {
   * |   2 | payload         |      | descriptor's payload     |
   */

+struct usb_functionfs_ss_descs_head {
+       __le32 magic;
+       __le32 length;
+       __le32 reserved;
+       __le32 ss_count;
+} __attribute__((packed));
+
+/*
+ * SS Descriptors format:
+ *
+ * | off | name      | type         | description 
     |
+ * 
|-----+-----------+--------------+--------------------------------------|
+ * |   0 | magic     | LE32         | FUNCTIONFS_SS_DESCRIPTORS_MAGIC 
     |
+ * |   4 | length    | LE32         | length of the whole data chunk 
     |
+ * |   8 | ss_count  | LE32         | number of super-speed descriptors 
    |
+ * |  12 | reserved field 
     |
+ * |  16 | ss_descrs | Descriptor[] | list of super-speed descriptors 
     |
+ */
+
  struct usb_functionfs_strings_head {
         __le32 magic;
         __le32 length;