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

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Message-ID: <20220127093332.wnkd2qy4tvwg5i5l@ldmartin-desk2>
Date:   Thu, 27 Jan 2022 01:33:32 -0800
From:   Lucas De Marchi <lucas.demarchi@...el.com>
To:     Daniel Vetter <daniel@...ll.ch>,
        Christian König <christian.koenig@....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

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(..));

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
>http://blog.ffwll.ch