Atmel SAM driver: let gmac_dev_read() return the proper length

[htibosch]

This PR is now closed and replaced with PR #1000

Description

When receiving data, the driver for SAM will increase the number of bytes:

    /* Read +2 bytes because buffers are aligned at -2 bytes */
    bytesLeft = min( bytesLeft + 2, ( int32_t ) ul_frame_size );

This worked OK until PR #941, which is stricter on checking the received length:

-    if( uxBufferLength < ( size_t ) ( ipSIZE_OF_ETH_HEADER + ipSIZE_OF_IPv6_HEADER + ( size_t ) usPayloadLength ) )
+    if( uxBufferLength != ( size_t ) ( ipSIZE_OF_ETH_HEADER + ipSIZE_OF_IPv6_HEADER + ( size_t ) usPayloadLength ) )

Test Steps

Compile an application for SAM with the latest release ( after PR 941 ). Compare the behaviour before and after this PR.

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

As I am not in my office, I am not able to test the change in real hardware.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[kstribrnAmzn]

/bot run uncrustify

[ActoryOu]

Is it necessary to compare with frame size?
The *p_rcv_size might be different when ul_frame_size is smaller than bytesLeft + 2.

if( bytesLeft + 2 <= ul_frame_size )
{
    *p_rcv_size = bytesLeft;
}
else
{
    *p_rcv_size = ul_frame_size;
}

[AniruddhaKanhere]

Suggested change

	*p_rcv_size = bytesLeft;
	*p_rcv_size = min( bytesLeft + 2, ( int32_t ) ul_frame_size );

[AniruddhaKanhere]

@ActoryOu and @htibosch this should be the fix. Am I correct?

[ActoryOu]

Hi @AniruddhaKanhere,
It can't be bytesLeft + 2 because the stack compares packet length with buffer length.
It's only allowed when buffer length is exactly equal to packet length, which is bytesLeft.
Take #941 as reference.