Re: [PATCH net-next v3 1/4] net: stmmac: Switch to zero-copy in non-XDP RX path

Jon Hunter
2025-02-07T13:42:47Z

On 07/02/2025 09:07, Furong Xu wrote:
> Hi Jon,
>
> On Wed, 29 Jan 2025 14:51:35 +0000, Jon Hunter <jonathanh@nvidia.com> wrote:
>> Hi Furong,
>>
>> On 27/01/2025 13:28, Thierry Reding wrote:
>>> On Sat, Jan 25, 2025 at 11:03:47PM +0800, Furong Xu wrote:
>>>> Hi Thierry
>>>>
>>>> On Sat, 25 Jan 2025 12:20:38 +0200, Ido Schimmel wrote:
>>>>
>>>>> On Fri, Jan 24, 2025 at 10:42:56AM +0800, Furong Xu wrote:
>>>>>> On Thu, 23 Jan 2025 22:48:42 +0100, Andrew Lunn <andrew@lunn.ch>
>>>>>> wrote:
>>>>>>>> Just to clarify, the patch that you had us try was not intended
>>>>>>>> as an actual fix, correct? It was only for diagnostic purposes,
>>>>>>>> i.e. to see if there is some kind of cache coherence issue,
>>>>>>>> which seems to be the case? So perhaps the only fix needed is
>>>>>>>> to add dma-coherent to our device tree?
>>>>>>>
>>>>>>> That sounds quite error prone. How many other DT blobs are
>>>>>>> missing the property? If the memory should be coherent, i would
>>>>>>> expect the driver to allocate coherent memory. Or the driver
>>>>>>> needs to handle non-coherent memory and add the necessary
>>>>>>> flush/invalidates etc.
>>>>>>
>>>>>> stmmac driver does the necessary cache flush/invalidates to
>>>>>> maintain cache lines explicitly.
>>>>>
>>>>> Given the problem happens when the kernel performs syncing, is it
>>>>> possible that there is a problem with how the syncing is performed?
>>>>>
>>>>> I am not familiar with this driver, but it seems to allocate multiple
>>>>> buffers per packet when split header is enabled and these buffers are
>>>>> allocated from the same page pool (see stmmac_init_rx_buffers()).
>>>>> Despite that, the driver is creating the page pool with a non-zero
>>>>> offset (see __alloc_dma_rx_desc_resources()) to avoid syncing the
>>>>> headroom, which is only present in the head buffer.
>>>>>
>>>>> I asked Thierry to test the following patch [1] and initial testing
>>>>> seems OK. He also confirmed that "SPH feature enabled" shows up in the
>>>>> kernel log.
>>>>
>>>> It is recommended to disable the "SPH feature" by default unless some
>>>> certain cases depend on it. Like Ido said, two large buffers being
>>>> allocated from the same page pool for each packet, this is a huge waste
>>>> of memory, and brings performance drops for most of general cases.
>>>>
>>>> Our downstream driver and two mainline drivers disable SPH by default:
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/ethernet/stmicro/stmmac/dwmac-dwc-qos-eth.c#n357
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/ethernet/stmicro/stmmac/dwmac-intel.c#n471
>>>
>>> Okay, that's something we can look into changing. What would be an
>>> example of a use-case depending on SPH? Also, isn't this something
>>> that should be a policy that users can configure?
>>>
>>> Irrespective of that we should fix the problems we are seeing with
>>> SPH enabled.
>>
>>
>> Any update on this?
>
> Sorry for my late response, I was on Chinese New Year holiday.

No problem! Happy new year!

> The fix is sent, and it will be so nice to have your Tested-by: tag there:
> https://lore.kernel.org/all/20250207085639.13580-1-0x1207@gmail.com/

Thanks, I have tested and responded. All looking good!

Jon

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