Re: [PATCH 01/19] dma-buf-map: Add read/write helpers

[Christian König]

Am 27.01.22 um 08:36 schrieb Matthew Brost:
> [SNIP]
> > >    /**
> > >     * dma_buf_map_memcpy_to - Memcpy into dma-buf mapping
> > >     * @dst:	The dma-buf mapping structure
> > > @@ -263,4 +304,44 @@ static inline void dma_buf_map_incr(struct dma_buf_map *map, size_t incr)
> > >    		map->vaddr += incr;
> > >    }
> > > +/**
> > > + * dma_buf_map_read_field - Read struct member from dma-buf mapping with
> > > + * arbitrary size and handling un-aligned accesses
> > > + *
> > > + * @map__:	The dma-buf mapping structure
> > > + * @type__:	The struct to be used containing the field to read
> > > + * @field__:	Member from struct we want to read
> > > + *
> > > + * Read a value from dma-buf mapping calculating the offset and size: this assumes
> > > + * the dma-buf mapping is aligned with a a struct type__. A single u8, u16, u32
> > > + * or u64 can be read, based on the offset and size of type__.field__.
> > > + */
> > > +#define dma_buf_map_read_field(map__, type__, field__) ({				\
> > > +	type__ *t__;									\
> > > +	typeof(t__->field__) val__;							\
> > > +	dma_buf_map_memcpy_from_offset(&val__, map__, offsetof(type__, field__),	\
> > > +				       sizeof(t__->field__));				\
> > > +	val__;										\
> > > +})
> > > +
> > > +/**
> > > + * dma_buf_map_write_field - Write struct member to the dma-buf mapping with
> > > + * arbitrary size and handling un-aligned accesses
> > > + *
> > > + * @map__:	The dma-buf mapping structure
> > > + * @type__:	The struct to be used containing the field to write
> > > + * @field__:	Member from struct we want to write
> > > + * @val__:	Value to be written
> > > + *
> > > + * Write a value to the dma-buf mapping calculating the offset and size.
> > > + * A single u8, u16, u32 or u64 can be written based on the offset and size of
> > > + * type__.field__.
> > > + */
> > > +#define dma_buf_map_write_field(map__, type__, field__, val__) ({			\
> > > +	type__ *t__;									\
> > > +	typeof(t__->field__) val____ = val__;						\
> > > +	dma_buf_map_memcpy_to_offset(map__, offsetof(type__, field__),			\
> > > +				     &val____, sizeof(t__->field__));			\
> > > +})
> > > +
> > Uff well that absolutely looks like overkill to me.
> Hold on...
>
> > That's a rather special use case as far as I can see and I think we should
> > only have this in the common framework if more than one driver is using it.
> I disagree, this is rather elegant.
>
> The i915 can't be the *only* driver that defines a struct which
> describes the layout of a dma_buf object.

That's not the problem, amdgpu as well as nouveau are doing that as 
well. The problem is DMA-buf is a buffer sharing framework between drivers.

In other words which importer is supposed to use this with a DMA-buf 
exported by another device?

> IMO this base macro allows *all* other drivers to build on this write
> directly to fields in structures those drivers have defined.

Exactly that's the point. This is something drivers should absolutely 
*NOT* do.

That are driver internals and it is extremely questionable to move this 
into the common framework.

Regards,
Christian.

>   Patches
> later in this series do this for the GuC ads.
>
> Matt
>   
> > Regards,
> > Christian.
> >
> > >    #endif /* __DMA_BUF_MAP_H__ */