linux-kernel - Re: [PATCH 0/1] swiotlb: Make IO_TLB

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Message-ID: <dc6f299b18f7870c7bffecca25cee9e436a32c7b.camel@siemens.com>
Date: Thu, 24 Apr 2025 05:12:39 +0000
From: "Li, Hua Qian" <HuaQian.Li@...mens.com>
To: "robin.murphy@....com" <robin.murphy@....com>, "m.szyprowski@...sung.com"
	<m.szyprowski@...sung.com>
CC: "Kiszka, Jan" <jan.kiszka@...mens.com>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "Su, Bao Cheng" <baocheng.su@...mens.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/1] swiotlb: Make IO_TLB_SEGSIZE Configurable

On Tue, 2025-04-22 at 15:36 +0200, Marek Szyprowski wrote:
> On 22.04.2025 08:37, huaqian.li@...mens.com wrote:
> > From: Li Hua Qian <huaqian.li@...mens.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.

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

-- 
Hua Qian Li
Siemens AG
www.siemens.com