Messages in this thread | | | From | Linus Torvalds <> | Date | Wed, 9 Aug 2023 10:01:50 -0700 | Subject | Re: [PATCH RFC] Introduce uniptr_t as a generic "universal" pointer |
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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.
Linus
(*) Well, not so strange - we've just never needed it.
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