Messages in this thread | | | Date | Thu, 27 Jan 2022 08:13:58 -0800 | From | Lucas De Marchi <> | Subject | Re: [Linaro-mm-sig] Re: [Intel-gfx] [PATCH 02/19] dma-buf-map: Add helper to initialize second map |
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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@ffwll.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.
Agreed, that should cover most of the cases. I can add a patch doing that.
thanks Lucas De Marchi
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