Message-ID: <YfKIgLKyLbCDtl7B@phenom.ffwll.local>
Date:   Thu, 27 Jan 2022 12:56:48 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König 
        <ckoenig.leichtzumerken@...il.com>
Cc:     Christian König <christian.koenig@....com>,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        linaro-mm-sig@...ts.linaro.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-media@...r.kernel.org
Subject: Re: [Linaro-mm-sig] Re: [Intel-gfx] [PATCH 02/19] dma-buf-map: Add
 helper to initialize second map

On Thu, Jan 27, 2022 at 12:44:21PM +0100, Christian König wrote:
> Am 27.01.22 um 12:16 schrieb Daniel Vetter:
> > On Thu, Jan 27, 2022 at 11:21:20AM +0100, Christian König wrote:
> > > Am 27.01.22 um 11:00 schrieb Daniel Vetter:
> > > > On Thu, Jan 27, 2022 at 01:33:32AM -0800, Lucas De Marchi wrote:
> > > > > On Thu, Jan 27, 2022 at 09:57:25AM +0100, Daniel Vetter wrote:
> > > > > > On Thu, Jan 27, 2022 at 09:02:54AM +0100, Christian König wrote:
> > > > > > > Am 27.01.22 um 08:57 schrieb Lucas De Marchi:
> > > > > > > > On Thu, Jan 27, 2022 at 08:27:11AM +0100, Christian König wrote:
> > > > > > > > > Am 26.01.22 um 21:36 schrieb Lucas De Marchi:
> > > > > > > > > > When dma_buf_map struct is passed around, it's useful to be able to
> > > > > > > > > > initialize a second map that takes care of reading/writing to an offset
> > > > > > > > > > of the original map.
> > > > > > > > > > 
> > > > > > > > > > Add a helper that copies the struct and add the offset to the proper
> > > > > > > > > > address.
> > > > > > > > > Well what you propose here can lead to all kind of problems and is
> > > > > > > > > rather bad design as far as I can see.
> > > > > > > > > 
> > > > > > > > > The struct dma_buf_map is only to be filled in by the exporter and
> > > > > > > > > should not be modified in this way by the importer.
> > > > > > > > humn... not sure if I was  clear. There is no importer and exporter here.
> > > > > > > Yeah, and exactly that's what I'm pointing out as problem here.
> > > > > > > 
> > > > > > > You are using the inter driver framework for something internal to the
> > > > > > > driver. That is an absolutely clear NAK!
> > > > > > > 
> > > > > > > We could discuss that, but you guys are just sending around patches to do
> > > > > > > this without any consensus that this is a good idea.
> > > > > > Uh I suggested this, also we're already using dma_buf_map all over the
> > > > > > place as a convenient abstraction. So imo that's all fine, it should allow
> > > > > > drivers to simplify some code where on igpu it's in normal kernel memory
> > > > > > and on dgpu it's behind some pci bar.
> > > > > > 
> > > > > > Maybe we should have a better name for that struct (and maybe also a
> > > > > > better place), but way back when we discussed that bikeshed I didn't come
> > > > > > up with anything better really.
> > > > > I suggest iosys_map since it abstracts access to IO and system memory.
> > > > > 
> > > > > > > > There is a role delegation on filling out and reading a buffer when
> > > > > > > > that buffer represents a struct layout.
> > > > > > > > 
> > > > > > > > struct bla {
> > > > > > > >       int a;
> > > > > > > >       int b;
> > > > > > > >       int c;
> > > > > > > >       struct foo foo;
> > > > > > > >       struct bar bar;
> > > > > > > >       int d;
> > > > > > > > }
> > > > > > > > 
> > > > > > > > 
> > > > > > > > This implementation allows you to have:
> > > > > > > > 
> > > > > > > >       fill_foo(struct dma_buf_map *bla_map) { ... }
> > > > > > > >       fill_bar(struct dma_buf_map *bla_map) { ... }
> > > > > > > > 
> > > > > > > > and the first thing these do is to make sure the map it's pointing to
> > > > > > > > is relative to the struct it's supposed to write/read. Otherwise you're
> > > > > > > > suggesting everything to be relative to struct bla, or to do the same
> > > > > > > > I'm doing it, but IMO more prone to error:
> > > > > > > > 
> > > > > > > >       struct dma_buf_map map = *bla_map;
> > > > > > > >       dma_buf_map_incr(map, offsetof(...));
> > > > > > Wrt the issue at hand I think the above is perfectly fine code. The idea
> > > > > > with dma_buf_map is really that it's just a special pointer, so writing
> > > > > > the code exactly as pointer code feels best. Unfortunately you cannot make
> > > > > > them typesafe (because of C), so the code sometimes looks a bit ugly.
> > > > > > Otherwise we could do stuff like container_of and all that with
> > > > > > typechecking in the macros.
> > > > > I had exactly this code above, but after writting quite a few patches
> > > > > using it, particularly with functions that have to write to 2 maps (see
> > > > > patch 6 for example), it felt much better to have something to
> > > > > initialize correctly from the start
> > > > > 
> > > > > 	struct dma_buf_map other_map = *bla_map;
> > > > > 	/* poor Lucas forgetting dma_buf_map_incr(map, offsetof(...)); */
> > > > > 
> > > > > is error prone and hard to debug since you will be reading/writting
> > > > > from/to another location rather than exploding
> > > > > 
> > > > > While with the construct below
> > > > > 
> > > > > 	other_map;
> > > > > 	...
> > > > > 	other_map = INITIALIZER()
> > > > > 
> > > > > I can rely on the compiler complaining about uninitialized var. And
> > > > > in most of the cases I can just have this single line in the beggining of the
> > > > > function when the offset is constant:
> > > > > 
> > > > > 	struct dma_buf_map other_map = INITIALIZER(bla_map, offsetof(..));
> > > > Hm yeah that's a good point that this allows us to rely on the compiler to
> > > > check for uninitialized variables.
> > > > 
> > > > Maybe include the above (with editing, but keeping the examples) in the
> > > > kerneldoc to explain why/how to use this? With that the concept at least
> > > > has my
> > > > 
> > > > Acked-by: Daniel Vetter <daniel.vetter@...ll.ch>
> > > > 
> > > > I'll leave it up to you & Christian to find a prettier color choice for
> > > > the naming bikeshed.
> > > There is one major issue remaining with this and that is dma_buf_vunmap():
> > > 
> > > void dma_buf_vunmap(struct dma_buf *dmabuf, struct dma_buf_map *map);
> > > 
> > > Here we expect the original pointer as returned by dma_buf_map(), otherwise
> > > we vunmap() the wrong area!
> > > 
> > > For all TTM based driver this doesn't matter since we keep the vmap base
> > > separately in the BO anyway (IIRC), but we had at least one case where this
> > > made boom last year.
> > Yeah but isn't that the same if it's just a void *?
> > 
> > If you pass the wrong pointer to an unmap function and not exactly what
> > you go from the map function, then things go boom. This is like
> > complaining that the following code wont work
> > 
> > 	u32 *stuff
> > 
> > 	stuff = kmap_local(some_page);
> > 	*stuff++ = 0;
> > 	*stuff = 1;
> > 	kunmap_locak(stuff);
> > 
> > It's just ... don't do that :-) Also since we pass dma_buf_map by value
> > and not by pointer anywhere, the risk of this happening is pretty low
> > since you tend to work on a copy. Same with void * pointers really.
> > 
> > Now if people start to pass around struct dma_buf_map * as pointers for
> > anything else than out parameters, then we're screwed. But that's like
> > passing around void ** for lolz, which is just wrong (except when it's an
> > out parameter or actually an array of pointers ofc).
> > 
> > Or I really don't get your concern and you mean something else?
> 
> No that's pretty much it. It's just that we hide the pointer inside a
> structure and it is absolutely not obvious to a driver dev that you can't
> do:
> 
> dma_buf_vmap(.., &map);
> dma_buf_map_inr(&map, x);
> dma_buf_vunmap(.., &map);
> 
> As bare minimum I strongly suggest that we add some WARN_ONs to the
> framework to check that the pointer given to dma_buf_vunmap() is at least
> page aligned.

Yeah that might be a good idea. But then we also have to add that check to
dma_buf_vmap, just in case a driver does something really funny :-)
-Daniel

> 
> Christian.
> 
> > -Daniel
> > 
> > 
> > > Christian.
> > > 
> > > > -Daniel
> > > > 
> > > > > Lucas De Marchi
> > > > > 
> > > > > > -Daniel
> > > > > > 
> > > > > > > > IMO this construct is worse because at a point in time in the function
> > > > > > > > the map was pointing to the wrong thing the function was supposed to
> > > > > > > > read/write.
> > > > > > > > 
> > > > > > > > It's also useful when the function has double duty, updating a global
> > > > > > > > part of the struct and a table inside it (see example in patch 6)
> > > > > > > > 
> > > > > > > > thanks
> > > > > > > > Lucas De Marchi
> > > > > > -- 
> > > > > > Daniel Vetter
> > > > > > Software Engineer, Intel Corporation
> > > > > > https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fblog.ffwll.ch%2F&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7C0654a16ea3444271d7c308d9e17bd35d%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637788744226808874%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=Q6soluBglaZLhLszdapaWuUVsqMq5qvJOKiJjO%2B9BTg%3D&amp;reserved=0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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