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

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Date:   Thu, 27 Jan 2022 10:02:18 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König <christian.koenig@....com>
Cc:     Matthew Brost <matthew.brost@...el.com>,
        intel-gfx@...ts.freedesktop.org,
        Lucas De Marchi <lucas.demarchi@...el.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org, linux-media@...r.kernel.org
Subject: Re: [Intel-gfx] [PATCH 01/19] dma-buf-map: Add read/write helpers

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
http://blog.ffwll.ch