[PATCH 6/6] net: dwc_eth_qos: Prevent DMA from writing updated RX DMA descriptor

[Ramon Fried]

On Mon, Mar 23, 2020 at 2:00 PM Marek Vasut <marex at denx.de> wrote:
>
> On 3/23/20 8:09 AM, Ramon Fried wrote:
> > On Mon, Mar 23, 2020 at 3:45 AM Marek Vasut <marex at denx.de> wrote:
> >>
> >> The DMA may attempt to write a DMA descriptor in the ring while it is
> >> being updated. By writing the DMA descriptor buffer address to 0, it
> >> is assured the DMA will not use such a buffer and the buffer can be
> >> updated without any interference.
> >>
> >> Signed-off-by: Marek Vasut <marex at denx.de>
> >> Cc: Joe Hershberger <joe.hershberger at ni.com>
> >> Cc: Patrice Chotard <patrice.chotard at st.com>
> >> Cc: Patrick Delaunay <patrick.delaunay at st.com>
> >> Cc: Ramon Fried <rfried.dev at gmail.com>
> >> Cc: Stephen Warren <swarren at nvidia.com>
> >> ---
> >>  drivers/net/dwc_eth_qos.c | 4 +++-
> >>  1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/dwc_eth_qos.c b/drivers/net/dwc_eth_qos.c
> >> index 7dadb10fe7..c86b9d59a5 100644
> >> --- a/drivers/net/dwc_eth_qos.c
> >> +++ b/drivers/net/dwc_eth_qos.c
> >> @@ -1431,8 +1431,10 @@ static int eqos_free_pkt(struct udevice *dev, uchar *packet, int length)
> >>
> >>         rx_desc = &(eqos->rx_descs[eqos->rx_desc_idx]);
> >>
> >> +       rx_desc->des0 = 0;
> >> +       mb();
> > Better use wmb() for better understanding your goal here ?
>
> The driver uses mb() all over the place, so I'm just keeping it
> consistent here.
>
> But I wonder whether we even need all those barriers in this driver at all.
In that case that's fine :)