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

[Ramon Fried]

On Mon, Mar 23, 2020 at 2:22 PM Marek Vasut <marex at denx.de> wrote:
>
> On 3/23/20 1:19 PM, Ramon Fried wrote:
> > 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 :)
>
> I mean, I am asking about the need for those barriers here, that's my
> question here.
Better safe than sorry. unless you have the means to test it to see if
it doesn't break.

Thanks,
Ramon.