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Date:   Fri, 11 Aug 2023 15:54:35 +0200
Message-ID: <87350psmzo.wl-tiwai@suse.de>
From:   Takashi Iwai <tiwai@suse.de>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christoph Hellwig <hch@lst.de>, linux-kernel@vger.kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Mark Brown <broonie@kernel.org>, netdev@vger.kernel.org
Subject: Re: [PATCH RFC] Introduce uniptr_t as a generic "universal" pointer
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On Wed, 09 Aug 2023 20:08:30 +0200,
Takashi Iwai wrote:
> 
> On Wed, 09 Aug 2023 19:01:50 +0200,
> Linus Torvalds wrote:
> > 
> > On Wed, 9 Aug 2023 at 09:05, Takashi Iwai <tiwai@suse.de> wrote:
> > >
> > > OTOH, it simplifies the code well for us; as of now, we have two
> > > callbacks for copying PCM memory from/to the device, distinct for
> > > kernel and user pointers.  It's basically either copy_from_user() or
> > > memcpy() of the given size depending on the caller.  The sockptr_t or
> > > its variant would allow us to unify those to a single callback.
> > 
> > I didn't see the follow-up patches that use this, but...
> > 
> > > (And yeah, iov_iter is there, but it's definitely overkill for the
> > > purpose.)
> > 
> > You can actually use a "simplified form" of iov_iter, and it's not all that bad.
> > 
> > If the actual copying operation is just a memcpy, you're all set: just
> > do copy_to/from_iter(), and it's a really nice interface, and you
> > don't have to carry "ptr+size" things around.
> > 
> > And we now have a simple way to generate simple iov_iter's, so
> > *creating* the iter is trivial too:
> > 
> >         struct iov_iter iter;
> >         int ret = import_ubuf(ITER_SRC/DEST, uptr, len, &iter);
> > 
> >         if (unlikely(ret < 0))
> >                 return ret;
> > 
> > and you're all done. You can now pass '&iter' around, and it has a
> > nice user pointer and a range in it, and copying that thing is easy.
> > 
> > Perhaps somewhat strangely (*) we don't have the same for a simple
> > kernel buffer, but adding that wouldn't be hard. You either end up
> > using a 'kvec', or we could even add something like ITER_KBUF if it
> > really matters.
> > 
> > Right now the kernel buffer init is a *bit* more involved than the
> > above ubuf case:
> > 
> >         struct iov_iter iter;
> >         struct kvec kvec = { kptr, len};
> > 
> >         iov_iter_kvec(&iter, ITER_SRC/DEST, &kvec, 1, len);
> > 
> > and that's maybe a *bit* annoying, but we could maybe simplify this
> > with some helper macros even without ITER_KBUF.
> > 
> > So yes, iov_iter does have some abstraction overhead, but it really
> > isn't that bad. And it *does* allow you to do a lot of things, and can
> > actually simplify the users quite a bit, exactly because it allows you
> > to just pass that single iter pointer around, and you automatically
> > have not just the user/kernel distinction, you have the buffer size,
> > and you have a lot of helper functions to use it.
> > 
> > I really think that if you want a user-or-kernel buffer interface, you
> > should use these things.
> > 
> > Please? At least look into it.
> 
> All sounds convincing, I'll take a look tomorrow.  Thanks!

FYI, I rewrote and tested patches, and it looks promising.

The only missing piece in the upstream side was the export of
import_ubuf().


Takashi