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@xxxxxxxx>: > > > > On Wed, 31 Jul 2024 at 13:55, Sakari Ailus <sakari.ailus@xxxxxx> 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. > [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
