Re: [PATCH 0/1] swiotlb: Make IO_TLB_SEGSIZE Configurable

[Robin Murphy <robin.murphy@arm.com>]

On 24/04/2025 6:12 am, Li, Hua Qian wrote:
> On Tue, 2025-04-22 at 15:36 +0200, Marek Szyprowski wrote:
>> On 22.04.2025 08:37, huaqian.li@siemens.com wrote:
>>> From: Li Hua Qian <huaqian.li@siemens.com>
>>>
>>> This patchset introduces a change to make the IO_TLB_SEGSIZE
>>> parameter
>>> configurable via a new kernel configuration option
>>> (CONFIG_SWIOTLB_SEGSIZE).
>>>
>>> In certain applications, the default value of IO_TLB_SEGSIZE (128)
>>> may
>>> not be sufficient for memory allocation, leading to runtime errors.
>>> By
>>> making this parameter configurable, users can adjust the segment
>>> size to
>>> better suit their specific use cases, improving flexibility and
>>> system
>>> stability.
>>
>> Could You elaborate a bit more what are those certain applications
>> that
>> require increasing IO_TLB_SEGSIZE? I'm not against it, but such
>> change
>> should be well justified and described, while the above cover-letter
>> doesn't provide anything more than is written in the patch
>> description.
> Thank you for your feedback, Marek.
> 
> To provide more context, one specific application that requires
> increasing IO_TLB_SEGSIZE is the Hailo 8 PCIe AI card. This card uses
> dma_alloc_coherent to allocate descriptor lists, as seen in the Hailo
> driver implementation here:
> https://github.com/hailo-ai/hailort-drivers/blob/7161f9ee5918029bd4497f590003c2f87ec32507/linux/vdma/memory.c#L322
> The maximum size (nslots) for these allocations can reach 160, which
> exceeds the current default value of IO_TLB_SEGSIZE (128).
> 
> Since IO_TLB_SEGSIZE is defined as a constant in the kernel:
> 
> `#define IO_TLB_SEGSIZE 128`
> 
> 
> this limitation causes swiotlb_search_pool_area,
> https://github.com/torvalds/linux/blame/v6.15-rc2/kernel/dma/swiotlb.c#L1085,
> (or swiotlb_do_find_slots in older kernels) to fail when attempting to
> allocate contiguous physical memory (CMA). This results in runtime
> errors and prevents the Hailo 8 card from functioning correctly in
> certain configurations.

Hmm, dma_alloc_coherent() should really not be trying to allocate from 
SWIOTLB in the first place - how is that happening?

If you're using restricted DMA for a device which wants significant 
coherent allocations, then it wants to have it's own shared-dma-pool for 
those *as well* as the restricted-dma-pool for bouncing streaming DMA.

Thanks,
Robin.

> By making IO_TLB_SEGSIZE configurable via a kernel configuration option
> (CONFIG_SWIOTLB_SEGSIZE), users can adjust the segment size to
> accommodate such use cases. This change improves flexibility and
> ensures that systems can be tailored to meet the requirements of
> specific hardware, such as the Hailo 8 PCIe AI card, without requiring
> kernel source modifications.
> 
> I hope this example clarifies the need for this change. Please let me
> know if further details or additional examples are required.
> 
> Best Regards,
> Li Hua Qian
>>
>>
>>> Li Hua Qian (1):
>>>     swiotlb: Make IO_TLB_SEGSIZE configurable
>>>
>>>    include/linux/swiotlb.h | 2 +-
>>>    kernel/dma/Kconfig      | 7 +++++++
>>>    2 files changed, 8 insertions(+), 1 deletion(-)
>>>
>> Best regards
>