Re: [PATCH 1/8] net: qualcomm: rmnet: fix struct rmnet_map_header

On 5/20/19 3:11 PM, Subash Abhinov Kasiviswanathan wrote:

On 2019-05-20 07:53, Alex Elder wrote:

quoted

The C bit-fields in the first byte of the rmnet_map_header structure
are defined in the wrong order.  The first byte should be formatted
this way:
                 +------- reserved_bit
                 | +----- cd_bit
                 | |
                 v v
    +-----------+-+-+
    |  pad_len  |R|C|
    +-----------+-+-+
     7 6 5 4 3 2 1 0  <-- bit position

But the C bit-fields that define the first byte are defined this way:
    u8 pad_len:6;
    u8 reserved_bit:1;
    u8 cd_bit:1;

If the above illustration is supposed to be in network byte order,
then it is wrong. The documentation has the definition for the MAP
packet.

Network *bit* order is irrelevant to the host.  Host memory is
byte addressable only, and bit 0 is the low-order bit.

Packet format -

Bit             0             1           2-7      8 - 15           16 - 31
Function   Command / Data   Reserved     Pad   Multiplexer ID    Payload length
Bit            32 - x
Function     Raw  Bytes

I don't know how to interpret this.  Are you saying that the low-
order bit of the first byte is the command/data flag?  Or is it
the high-order bit of the first byte?

I'm saying that what I observed when building the code was that
as originally defined, the cd_bit field was the high-order bit
(bit 7) of the first byte, which I understand to be wrong.

If you are telling me that the command/data flag resides at bit
7 of the first byte, I will update the field masks in a later
patch in this series to reflect that.

The driver was written assuming that the host was running ARM64, so
the structs are little endian. (I should have made it compatible
with big and little endian earlier so that is my fault).

Little endian and big endian have no bearing on the host's
interpretation of bits within a byte.

Please clarify.  I want the patches to be correct, and what
you're explaining doesn't really straighten things out for me.

					-Alex

In any case, this patch on its own will break the data operation on
ARM64, so it needs to be folded with other patches.

quoted

And although this isn't portable, I can state that when I build it
the result puts the bit-fields in the wrong location (e.g., the
cd_bit is in bit position 7, when it should be position 0).

Fix this by reordering the definitions of these struct members.
Upcoming patches will reimplement these definitions portably.

Signed-off-by: Alex Elder <redacted>
---
 drivers/net/ethernet/qualcomm/rmnet/rmnet_map.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map.h

b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map.h
index 884f1f52dcc2..b1ae9499c0b2 100644

--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map.h
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map.h

@@ -40,9 +40,9 @@ enum rmnet_map_commands {

 };

 struct rmnet_map_header {
-    u8  pad_len:6;
-    u8  reserved_bit:1;
     u8  cd_bit:1;
+    u8  reserved_bit:1;
+    u8  pad_len:6;
     u8  mux_id;
     __be16 pkt_len;
 }  __aligned(1);