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

[Lucas De Marchi]

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