Re: [PATCH for-next v2 1/4] RDMA/erdma: Support non-contiguous kernel QP buffers

[Cheng Xu <chengyou@linux.alibaba.com>]

On 9/3/26 8:38 PM, Cheng Xu wrote:
> 
> 
> On 9/3/26 5:10 PM, Leon Romanovsky wrote:
>> On Thu, Aug 27, 2026 at 04:25:20PM +0800, Cheng Xu wrote:
>>> A single coherent allocation for kernel QP queues can fail for large
>>> queues when memory is fragmented.
>>>
>>> Allocate page-sized coherent buffers and describe them with the existing
>>> MTT. Keep the userspace QP path unchanged.
>>>
>>> Signed-off-by: Cheng Xu <chengyou@linux.alibaba.com>
>>> ---
>>>  drivers/infiniband/hw/erdma/erdma_cq.c    |   4 +-
>>>  drivers/infiniband/hw/erdma/erdma_qp.c    |  38 +++--
>>>  drivers/infiniband/hw/erdma/erdma_verbs.c | 195 +++++++++++++---------
>>>  drivers/infiniband/hw/erdma/erdma_verbs.h |  44 ++++-
>>>  4 files changed, 179 insertions(+), 102 deletions(-)
>>
>> <...>
>>

<...>

>>
>>> +struct erdma_buf_list {
>>> +	void *buf;
>>> +	dma_addr_t dma_addr;
>>> +};
>>
>> This struct is very similar to scatter-gather list, why don't you use it
>> directly?
> 
> Good idea. I will use struct scatterlist in the next revision.

Hi Leon,

I switched to scatterlist in v3, but Sashiko pointed out an issue with
using sg_set_buf() and sg_virt() on dma_alloc_coherent() memory [1].

To handle this correctly, the driver would still need to retain the
original CPU addresses returned by dma_alloc_coherent(). Using scatterlist
does not simplify this implementation: we still need separate storage for
the CPU addresses, and the only scatterlist field we actually need is the
DMA address. A small structure holding both addresses would therefore be
simpler.

I would like to switch back to a small structure holding the CPU and DMA
addresses in v4. This would keep the implementation simpler while
preserving the original addresses returned by dma_alloc_coherent().

[1] https://sashiko.dev/#/patchset/20260908033709.89898-1-chengyou%40linux.alibaba.com

Thanks,
Cheng Xu


> Thanks,
> Cheng Xu
> 
>>
>> Thanks