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

[Marek Vasut]

On 3/23/20 1:50 PM, Ramon Fried wrote:
> 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.

On STM32MP1, yes. Not on the Tegra platform.