netdev - Re: [MAINTAINERS SUMMIT] Device Passthrough Considered Harmful?

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Message-ID: <CA+Ln22EzL7M+BLXS6dFi0n80XXkQu1CuoUad0EtjZ2ZEnNX=Kg@mail.gmail.com>
Date: Tue, 13 Aug 2024 19:33:59 +0900
From: Tomasz Figa <tomasz.figa@...il.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Daniel Vetter <daniel.vetter@...ll.ch>, Sakari Ailus <sakari.ailus@....fi>, 
	Ricardo Ribalda Delgado <ricardo.ribalda@...il.com>, Dan Williams <dan.j.williams@...el.com>, 
	James Bottomley <James.Bottomley@...senpartnership.com>, ksummit@...ts.linux.dev, 
	linux-cxl@...r.kernel.org, linux-rdma@...r.kernel.org, netdev@...r.kernel.org, 
	jgg@...dia.com
Subject: Re: [MAINTAINERS SUMMIT] Device Passthrough Considered Harmful?

2024年8月13日(火) 19:27 Laurent Pinchart <laurent.pinchart@...asonboard.com>:
>
> Hi Tomasz,
>
> On Tue, Aug 13, 2024 at 07:17:07PM +0900, Tomasz Figa wrote:
> > 2024年7月31日(水) 22:16 Daniel Vetter <daniel.vetter@...ll.ch>:
> > >
> > > On Wed, 31 Jul 2024 at 13:55, Sakari Ailus <sakari.ailus@....fi> wrote:
> > > > This is also very different from GPUs or accel devices that are built to be
> > > > user-programmable. If I'd compare ISPs to different devices, then the
> > > > closest match would probably be video codecs -- which also use V4L2.
> > >
> > > Really just aside, but I figured I should correct this. DRM supports
> > > plenty of video codecs. They're all tied to gpus, but the real reason
> > > really is that the hw has decent command submission support so that
> > > running the entire codec in userspace except the basic memory and
> > > batch execution and synchronization handling in the kernel is a
> > > feasible design.
> >
> > FWIW, V4L2 also has an interface for video decoders that require
> > bitstream processing in software, it's called the V4L2 Stateless
> > Decoder interface [1]. It defines low level data structures that map
> > directly to the particular codec specification, so the kernel
> > interface is generic and the userspace doesn't need to have
> > hardware-specific components. Hardware that consumes command buffers
> > can be supported simply by having the kernel driver fill the command
> > buffers as needed (as opposed to writing the registers directly).
> > On the other hand, DRM also has the fixed function (i.e. V4L2-alike)
> > KMS interface for display controllers, rather than a command buffer
> > passthrough, even though some display controllers actually are driven
> > by command buffers.
> > So arguably it's possible and practical to do both command
> > buffer-based and fixed interfaces for both display controllers and
> > video codecs. Do you happen to know some background behind why one or
> > the other was chosen for each of them in DRM?
> >
> > For how it applies to ISPs, there are both types of ISPs out in the
> > wild, some support command buffers, while some are programmed directly
> > via registers.
>
> Could you provide examples of ISPs that use command buffers ? The
> discussion has remained fairly vague so far, which I think hinders
> progress.
>
> > For the former, I can see some loss of flexibility if
> > the command buffers are hidden behind a fixed function API, because
> > the userspace would only be able to do what the kernel driver supports
> > internally, which could make some use case-specific optimizations very
> > challenging if not impossible.
>
> Let's try to discuss this with specific examples.

AFAIK Intel IPU6 and newer, Qualcomm and MediaTek ISPs use command
buffers natively.

>
> > [1] https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/dev-stateless-decoder.html
> >
> > > And actually good, because your kernel wont ever blow
> > > up trying to parse complex media formats because it just doesn't.
>
> --
> Regards,
>
> Laurent Pinchart