Re: [PATCH v4] dma-buf: Add a capabilities directory

[Greg KH <greg@kroah.com>]

On Thu, Jun 02, 2022 at 06:17:31AM +0000, Simon Ser wrote:
> On Thursday, June 2nd, 2022 at 07:40, Greg KH <greg@kroah.com> wrote:
> 
> > On Wed, Jun 01, 2022 at 04:13:14PM +0000, Simon Ser wrote:
> >
> > > To discover support for new DMA-BUF IOCTLs, user-space has no
> > > choice but to try to perform the IOCTL on an existing DMA-BUF.
> >
> > Which is correct and how all kernel features work (sorry I missed the
> > main goal of this patch earlier and focused only on the sysfs stuff).
> >
> > > However, user-space may want to figure out whether or not the
> > > IOCTL is available before it has a DMA-BUF at hand, e.g. at
> > > initialization time in a Wayland compositor.
> >
> > Why not just do the ioctl in a test way? That's how we determine kernel
> > features, we do not poke around in sysfs to determine what is, or is
> > not, present at runtime.
> >
> > > Add a /sys/kernel/dmabuf/caps directory which allows the DMA-BUF
> > > subsystem to advertise supported features. Add a
> > > sync_file_import_export entry which indicates that importing and
> > > exporting sync_files from/to DMA-BUFs is supported.
> >
> > No, sorry, this is not a sustainable thing to do for all kernel features
> > over time. Please just do the ioctl and go from there. sysfs is not
> > for advertising what is and is not enabled/present in a kernel with
> > regards to functionality or capabilities of the system.
> >
> > If sysfs were to export this type of thing, it would have to do it for
> > everything, not just some random tiny thing of one kernel driver.
> 
> I'd argue that DMA-BUF is a special case here.

So this is special and unique just like everything else?  :)

> To check whether the import/export IOCTLs are available, user-space
> needs a DMA-BUF to try to perform the IOCTL. To get a DMA-BUF,
> user-space needs to enumerate GPUs, pick one at random, load GBM or
> Vulkan, use that heavy-weight API to allocate a "fake" buffer on the
> GPU, export that buffer into a DMA-BUF, try the IOCTL, then teardown
> all of this. There is no other way.
> 
> This sounds like a roundabout way to answer the simple question "is the
> IOCTL available?". Do you have another suggestion to address this
> problem?

What does userspace do differently if the ioctl is present or not?

And why is this somehow more special than of the tens of thousands of
other ioctl calls where you have to do exactly the same thing you list
above to determine if it is present or not?

And how have you specifically tied this sysfs to the ioctl so that if it
changes or is ported elsewhere, that sysfs attribute will also know to
be added?

You already have shipping kernels today without this attribute, you
can't go back in time and add the attribute to those kernels just to
reflect the ioctl being present or not, so you have to handle this case
in userspace today, making this not needed at all.  Do you want to have
two test cases in your userspace code, one that does "is the sysfs file
there?  No, ok, let's see if we are on an older kernel without it, yet
the ioctl is present..."  When really you can just do "let's see if the
ioctl is present" logic as you already do that today.

DMA bufs are not special, they are merely one of tens of thousands of
ioctls in the kernel.  Think of the overall picture here please, that's
the only way to create a maintainable system over long periods of time,
like the kernel needs to be.

thanks,

greg k-h