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

Jon Hunter
2025-01-29T14:51:57Z

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?

Thanks
Jon

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