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

Daniel Vetter
2022-01-27T09:02:28Z

On Thu, Jan 27, 2022 at 08:59:36AM +0100, Christian König wrote:
> 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.

See my other reply.

This is about struct dma_buf_map, which is just a tagged pointer.

Which happens to be used by the dma_buf cross-driver interface, but it's
also used plenty internally in buffer allocation helpers, fbdev,
everything else. And it was _meant_ to be used like that - this thing is
my idea, I know :-)

I guess we could move/rename it, but like I said I really don't have any
good ideas. Got some?
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation