Date:   Thu, 27 Jan 2022 15:52:50 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     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: [Intel-gfx] [PATCH 02/19] dma-buf-map: Add helper to initialize
 second map

Hi

Am 27.01.22 um 11:21 schrieb Christian König:
> 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!

Indeed. It's always been a problem with that API, even when it still 
took raw pointers.

The IMHO correct solution would distinguish between a buffer (struct 
dma_buf_map) and a pointer into that buffer (struct dma_buf_ptr).

I don't feel like typing that.

Best regards
Thomas

> 
> 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
>>>> 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 
>>>>
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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