[Patch net-next v2 4/4] net: fec: Workaround for imx6sx enet tx hang when enable three queues

[Fabio Estevam]

On Tue, Sep 16, 2014 at 2:58 PM,  <Frank.Li at freescale.com> wrote:
> From: Fugang Duan <B38611 at freescale.com>
>
> When enable three queues on imx6sx enet, and then do tx performance
> test with iperf tool, after some time running, tx hang.
>
> Found that:
>         If uDMA is running, software set TDAR may cause tx hang.
>         If uDMA is in idle, software set TDAR don't cause tx hang.
>
> There is a TDAR race condition for mutliQ when the software sets TDAR
> and the UDMA clears TDAR simultaneously or in a small window (2-4 cycles).
> This will cause the udma_tx and udma_tx_arbiter state machines to hang.
> The issue exist at i.MX6SX enet IP.
>
> So, the Workaround is checking TDAR status four time, if TDAR cleared by
> hardware and then write TDAR, otherwise don't set TDAR.
>
> The patch is only one Workaround for the issue TKT210582.

As Shawn pointed out, it is better to use the ERR007885 instead of an
internal bug numbering.