net: mvpp2: store physical address of buffer in rx_desc->buf_cookie

net: mvpp2: store physical address of buffer in rx_desc->buf_cookie

The RX descriptors of the PPv2 hardware allow to store several
information, amongst which:

 - the DMA address of the buffer in which the data has been received
 - a "cookie" field, left to the use of the driver, and not used by the
   hardware

In the current implementation, the "cookie" field is used to store the
virtual address of the buffer, so that in the receive completion path,
we can easily get the virtual address of the buffer that corresponds to
a completed RX descriptors.

On PPv2.1, used on 32-bit platforms, those two fields are 32-bit wide,
which is enough to store a DMA address in the first field, and a virtual
address in the second field.

On PPv2.2, used on 64-bit platforms, these two fields have been extended
to 40 bits. While 40 bits is enough to store a DMA address (as long as
the DMA mask is 40 bits or lower), it is not enough to store a virtual
address. Therefore, the "cookie" field can no longer be used to store
the virtual address of the buffer.

However, as Russell King pointed out, the RX buffers are always
allocated in the kernel linear mapping, and therefore using
phys_to_virt() on the physical address of the RX buffer is possible and
correct.

Therefore, this commit changes the driver to use the "cookie" field to
store the physical address instead of the virtual
address. phys_to_virt() is used in the receive completion path to
retrieve the virtual address from the physical address.

It is obviously important to realize that the DMA address and physical
address are two different things, which is why we store both in the RX
descriptors. While those addresses may be identical in some situations,
it remains two distinct concepts, and both addresses should be handled
separately.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>