According to commit f735c40e ("net: axienet: Autodetect 64-bit DMA
capability") and AXI-DMA spec (pg021), on 64-bit capable dma, only
writing MSB part of tail descriptor pointer causes DMA engine to start
fetching descriptors. However, we found that it is true only if dma is in
idle state. In other words, dma would use a tailp even if it only has LSB
updated, when the dma is running.

The non-atomicity of this behavior could be problematic if enough
delay were introduced in between the 2 writes. For example, if an
interrupt comes right after the LSB write and the cpu spends long
enough time in the handler for the dma to get back into idle state by
completing descriptors, then the seconcd write to MSB would treat dma
to start fetching descriptors again. Since the descriptor next to the
one pointed by current tail pointer is not filled by the kernel yet,
fetching a null descriptor here causes a dma internal error and halt
the dma engine down.

We suggest that the dma engine should start process a 64-bit MMIO write
to the descriptor pointer only if ONE 32-bit part of it is written on all
states. Or we should restrict the use of 64-bit addressable dma on 32-bit
platforms, since those devices have no instruction to guarantee the write
to LSB and MSB part of tail pointer occurs atomically to the dma.

initial condition:
curp =  x-3;
tailp = x-2;
LSB = x;
MSB = 0;

cpu:                       |dma:
 iowrite32(LSB, tailp)     |  completes #(x-3) desc, curp = x-3
 ...                       |  tailp updated
 => irq                    |  completes #(x-2) desc, curp = x-2
    ...                    |  completes #(x-1) desc, curp = x-1
    ...                    |  ...
    ...                    |  completes #x desc, curp = tailp = x
 <= irqreturn              |  reaches tailp == curp = x, idle
 iowrite32(MSB, tailp + 4) |  ...
                           |  tailp updated, starts fetching...
                           |  fetches #(x + 1) desc, sees cntrl = 0
                           |  post Tx error, halt
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reported-by: Max Hsu <max.hsu@sifive.com>
Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: David S. Miller <davem@davemloft.net>