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

[Lucas De Marchi]

On Thu, Jan 27, 2022 at 03:33:12PM +0100, Thomas Zimmermann 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.
> >
> > Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> > Cc: Christian König <christian.koenig@xxxxxxx>
> > Cc: linux-media@xxxxxxxxxxxxxxx
> > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> > Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
> > ---
> >  include/linux/dma-buf-map.h | 29 +++++++++++++++++++++++++++++
> >  1 file changed, 29 insertions(+)
> >
> > diff --git a/include/linux/dma-buf-map.h b/include/linux/dma-buf-map.h
> > index 65e927d9ce33..3514a859f628 100644
> > --- a/include/linux/dma-buf-map.h
> > +++ b/include/linux/dma-buf-map.h
> > @@ -131,6 +131,35 @@ struct dma_buf_map {
> >  		.is_iomem = false, \
> >  	}
> > +/**
> > + * DMA_BUF_MAP_INIT_OFFSET - Initializes struct dma_buf_map from another dma_buf_map
> > + * @map_:	The dma-buf mapping structure to copy from
> > + * @offset:	Offset to add to the other mapping
> > + *
> > + * Initializes a new dma_buf_struct based on another. This is the equivalent of doing:
> > + *
> > + * .. code-block: c
> > + *
> > + *	dma_buf_map map = other_map;
> > + *	dma_buf_map_incr(&map, &offset);
> > + *
> > + * Example usage:
> > + *
> > + * .. code-block: c
> > + *
> > + *	void foo(struct device *dev, struct dma_buf_map *base_map)
> > + *	{
> > + *		...
> > + *		struct dma_buf_map = DMA_BUF_MAP_INIT_OFFSET(base_map, FIELD_OFFSET);
> > + *		...
> > + *	}
> > + */
> > +#define DMA_BUF_MAP_INIT_OFFSET(map_, offset_)	(struct dma_buf_map)	\
> > +	{								\
> > +		.vaddr = (map_)->vaddr + (offset_),			\
> > +		.is_iomem = (map_)->is_iomem,				\
> > +	}
> > +
>
> It's illegal to access .vaddr  with raw pointer. Always use a 
> dma_buf_memcpy_() interface. So why would you need this macro when you 
> have dma_buf_memcpy_*() with an offset parameter?

I did a better job with an example in 20220127093332.wnkd2qy4tvwg5i5l@ldmartin-desk2

While doing this series I had code like this when using the API in a function to
parse/update part of the struct mapped:

	int bla_parse_foo(struct dma_buf_map *bla_map)
	{
		struct dma_buf_map foo_map = *bla_map;
		...

		dma_buf_map_incr(&foo_map, offsetof(struct bla, foo));

		...
	}

Pasting the rest of the reply here:

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(..));


This is useful when you have several small functions in charge of
updating/reading inner struct members.

> I've also been very careful to distinguish between .vaddr and 
> .vaddr_iomem, even in places where I wouldn't have to. This macro 
> breaks the assumption.

That's one reason I think if we have this macro, it should be in the
dma_buf_map.h header (or whatever we rename these APIs to). It's the
only place where we can safely add code that relies on the implementation
of the "private" fields in struct dma_buf_map.

Lucas De Marchi

> Best regards
> Thomas
>
> >  /**
> >   * dma_buf_map_set_vaddr - Sets a dma-buf mapping structure to an address in system memory
> >   * @map:	The dma-buf mapping structure
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Ivo Totev