Re: [Intel-gfx] [PATCH 02/19] dma-buf-map: Add helper to initialize second map

Lucas De Marchi
2022-01-27T16:13:06Z

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!

yeah... I think the most confusing aspect here is about the name.

void dma_buf_vunmap(struct dma_buf *dmabuf, struct dma_buf_map *map);

this function is the implementation of the dma_buf, not dma_buf_map,
which is another thing entirely. I think the rename will be benefitial
for this to be cleared out, because then it's more obvious the shallow
copy of the map is the equivalent of having

u8 *p = buffer;
...
p += 10;

Etc. You can't kfree(p) and expect it to work.

Lucas De Marchi

>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.
>
>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
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>