linux-kernel - Re: [PATCH 1/8] net: qualcomm: rmnet: fix struct rmnet_map

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Date:   Mon, 20 May 2019 19:32:29 -0600
From:   Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>
To:     Alex Elder <elder@...aro.org>
Cc:     arnd@...db.de, david.brown@...aro.org, agross@...nel.org,
        davem@...emloft.net, bjorn.andersson@...aro.org,
        ilias.apalodimas@...aro.org, cpratapa@...eaurora.org,
        syadagir@...eaurora.org, evgreen@...omium.org, benchan@...gle.com,
        ejcaruso@...gle.com, netdev@...r.kernel.org,
        linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] net: qualcomm: rmnet: fix struct rmnet_map_header

>> 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.
> 

Higher order bit is Command / Data.

>> 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

Can't this bitfields just be used similar to how struct tcphdr &
iphdr are currently defined. That way, you dont have to make
these many changes.

diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map.h 
b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map.h
index 884f1f5..302d1db 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_map.h
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_map.h
@@ -40,9 +40,17 @@ enum rmnet_map_commands {
  };

  struct rmnet_map_header {
+#if defined(__LITTLE_ENDIAN_BITFIELD)
         u8  pad_len:6;
         u8  reserved_bit:1;
         u8  cd_bit:1;
+#elif defined (__BIG_ENDIAN_BITFIELD)
+       u8  cd_bit:1;
+       u8  reserved_bit:1;
+       u8  pad_len:6;
+#else
+#error "Please fix <asm/byteorder.h>"
+#endif
         u8  mux_id;
         __be16 pkt_len;
  }  __aligned(1);


-- 
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