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

[Matthew Brost]

On Thu, Jan 27, 2022 at 08:24:04AM +0100, Christian König wrote:
> Am 26.01.22 um 21:36 schrieb Lucas De Marchi:
> > In certain situations it's useful to be able to read or write to an
> > offset that is calculated by having the memory layout given by a struct
> > declaration. Usually we are going to read/write a u8, u16, u32 or u64.
> > 
> > Add a pair of macros dma_buf_map_read_field()/dma_buf_map_write_field()
> > to calculate the offset of a struct member and memcpy the data from/to
> > the dma_buf_map. We could use readb, readw, readl, readq and the write*
> > counterparts, however due to alignment issues this may not work on all
> > architectures. If alignment needs to be checked to call the right
> > function, it's not possible to decide at compile-time which function to
> > call: so just leave the decision to the memcpy function that will do
> > exactly that on IO memory or dereference the pointer.
> > 
> > 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 | 81 +++++++++++++++++++++++++++++++++++++
> >   1 file changed, 81 insertions(+)
> > 
> > diff --git a/include/linux/dma-buf-map.h b/include/linux/dma-buf-map.h
> > index 19fa0b5ae5ec..65e927d9ce33 100644
> > --- a/include/linux/dma-buf-map.h
> > +++ b/include/linux/dma-buf-map.h
> > @@ -6,6 +6,7 @@
> >   #ifndef __DMA_BUF_MAP_H__
> >   #define __DMA_BUF_MAP_H__
> > +#include <linux/kernel.h>
> >   #include <linux/io.h>
> >   #include <linux/string.h>
> > @@ -229,6 +230,46 @@ static inline void dma_buf_map_clear(struct dma_buf_map *map)
> >   	}
> >   }
> > +/**
> > + * dma_buf_map_memcpy_to_offset - Memcpy into offset of dma-buf mapping
> > + * @dst:	The dma-buf mapping structure
> > + * @offset:	The offset from which to copy
> > + * @src:	The source buffer
> > + * @len:	The number of byte in src
> > + *
> > + * Copies data into a dma-buf mapping with an offset. The source buffer is in
> > + * system memory. Depending on the buffer's location, the helper picks the
> > + * correct method of accessing the memory.
> > + */
> > +static inline void dma_buf_map_memcpy_to_offset(struct dma_buf_map *dst, size_t offset,
> > +						const void *src, size_t len)
> > +{
> > +	if (dst->is_iomem)
> > +		memcpy_toio(dst->vaddr_iomem + offset, src, len);
> > +	else
> > +		memcpy(dst->vaddr + offset, src, len);
> > +}
> > +
> > +/**
> > + * dma_buf_map_memcpy_from_offset - Memcpy from offset of dma-buf mapping into system memory
> > + * @dst:	Destination in system memory
> > + * @src:	The dma-buf mapping structure
> > + * @src:	The offset from which to copy
> > + * @len:	The number of byte in src
> > + *
> > + * Copies data from a dma-buf mapping with an offset. The dest buffer is in
> > + * system memory. Depending on the mapping location, the helper picks the
> > + * correct method of accessing the memory.
> > + */
> > +static inline void dma_buf_map_memcpy_from_offset(void *dst, const struct dma_buf_map *src,
> > +						  size_t offset, size_t len)
> > +{
> > +	if (src->is_iomem)
> > +		memcpy_fromio(dst, src->vaddr_iomem + offset, len);
> > +	else
> > +		memcpy(dst, src->vaddr + offset, len);
> > +}
> > +
> 
> Well that's certainly a valid use case, but I suggest to change the
> implementation of the existing functions to call the new ones with offset=0.
> 
> This way we only have one implementation.
> 
Trivial - but agree with Christian that is a good cleanup.

> >   /**
> >    * 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.  

IMO this base macro allows *all* other drivers to build on this write
directly to fields in structures those drivers have defined. Patches
later in this series do this for the GuC ads.

Matt
 
> Regards,
> Christian.
> 
> >   #endif /* __DMA_BUF_MAP_H__ */
>