linux-kernel - Re: [PATCH 0/3] uio/dma-buf: Give UIO users access to DMA addresses.

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Message-ID: <20250414135228.600c13e6@windsurf>
Date: Mon, 14 Apr 2025 13:52:28 +0200
From: Thomas Petazzoni <thomas.petazzoni@...tlin.com>
To: Christian König <christian.koenig@....com>
Cc: Christoph Hellwig <hch@...radead.org>, Bastien Curutchet
 <bastien.curutchet@...tlin.com>, Sumit Semwal <sumit.semwal@...aro.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] uio/dma-buf: Give UIO users access to DMA
 addresses.

Hello Christian,

On Mon, 14 Apr 2025 11:11:48 +0200
Christian König <christian.koenig@....com> wrote:

> Maybe I should try to better explain the concern here. The question
> is "Where is the source code of your FPGA driver?".
> 
> I mean that you are trying to replace the out-of-tree solution is
> rather welcomed, but the out-of-tree solutions are out-of-tree
> because they don't fit with requirements to be added to the core
> Linux tree.
> 
> And one of those requirements is that you need to provide the source
> code of the userspace user of this interface, in this case here that
> is your FPGA driver. An MIT/X11 license is usually sufficient, GPL is
> of course seen as better and it must not be a toy application, but
> rather the real thing.

Where is this requirement for UIO? The UIO subsystem does not have such
a requirement, unlike indeed some other kernel subsystems such as DRM.

But the practical situation is different: for DRM it makes a lot of
sense to enforce having open-source code in user-space, as we want to
force GPU vendors to open their OpenGL/Vulkan implementations. However,
for UIO it makes little sense, because UIO is typically used to control
some super-specific FPGA IP blocks that are totally irrelevant outside
of the very specific product they are included in. Most likely if those
drivers were open-sourced and tried to be upstream they would be
rejected because their usefulness in the upstream kernel is basically
zero (but they would have an on-going maintenance effort for the whole
community).

> And that is what people usually don't want and that's why no in-tree
> solution exists for this.

And that doesn't make sense because UIO already exists, and allows to
achieve 95% of what people already need, to the exception of this DMA
issue.

Thomas
-- 
Thomas Petazzoni, co-owner and CEO, Bootlin
Embedded Linux and Kernel engineering and training
https://bootlin.com