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

Thomas Zimmermann
2022-01-28T08:32:12Z

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Hi

Am 27.01.22 um 17:34 schrieb Lucas De Marchi:
> On Thu, Jan 27, 2022 at 03:26:43PM +0100, Thomas Zimmermann wrote:
>> Hi
>>
>> 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@linaro.org>
>>> Cc: Christian König <christian.koenig@amd.com>
>>> Cc: linux-media@vger.kernel.org
>>> Cc: dri-devel@lists.freedesktop.org
>>> Cc: linaro-mm-sig@lists.linaro.org
>>> Cc: linux-kernel@vger.kernel.org
>>> Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
>>> ---
>>>  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);
>>> +}
>>
>> Please don't add a new function. Rather please add the offset
>> parameter to dma_buf_map_memcpy_to() and update the callers. There are
>> only two calls to dma_buf_map_memcpy_to() within the kernel. To make
>> it clear what the offset applies to, I'd call the parameter 'dst_offset'.
>>
>>> +
>>> +/**
>>> + * 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);
>>> +}
>>> +
>>
>> With the dma_buf_map_memcpy_to() changes, please just call this
>> function dma_buf_map_memcpy_from().
>>
>>>  /**
>>>   * 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__));            \
>>> +})
>>
>> As the original author of this file, I feel like this shouldn't be
>> here. At least not until we have another driver using that pattern.
>
> Let me try to clear out the confusion. Then maybe I can extend
> the documentation of this function in v2 if I'm able to convince this is
> useful here.
>
> This is not about importer/exporter, having this to work cross-driver.
> This is about using dma_buf_map (which we are talking about on renaming
> to iosys_map or something else) for inner driver
> allocations/abstractions. The abstraction added by iosys_map helps on
> sharing the same functions we had before.  And this macro here is very
> useful when the buffer is described by a struct layout. Example:
>
>     struct bla {
>         struct inner inner1;
>         struct inner inner2;
>         u32 x, y ,z;
>     };
>
> Functions that would previously do:
>
>     struct bla *bla = ...;
>
>     bla->x = 100;
>     bla->y = 200;
>     bla->inner1.inner_inner_field = 30;
>
> Can do the below, having the system/IO memory abstracted away
> (calling it iosys_map here instead of dma_buf_map, hopeful it helps):
>
>     struct iosys_map *map = ...;

Please don't start renaming anything here. If we want to do this, let's
have a separate mail thread for coloring the bike shed.

>
>     iosys_map_write_field(map, struct bla, x, 100);
>     iosys_map_write_field(map, struct bla, y, 200);
>     iosys_map_write_field(map, struct bla,
>                   inner1.inner_inner_field, 30);

I don't have strong feelings about these macros. They just seemed not
needed in general. But I we want to add them here, I 'd like to propose
a few small changes.

Again, please add an offset parameter for the map's pointer.

Then I'd call them either dma_buf_map_rd/dma_buf_map_wr for read/write
OR dma_buf_map_ld/dma_buf_map_st for load/store. They should take a C
type. Something like this

dma_buf_map_wr(map, offset, int32, 0x01234);
val = dam_buf_map_rd(map, offset, int32);

Hopefully, that's flexible enough for all users. On top of that, you can
build additional helpers like dma_buf_map_rd_field() and
dma_buf_map_wr_field().

Ok?

Best regards
Thomas

>
> When we are using mostly the same map, the individual drivers can add
> quick helpers on top. See the ads_blob_write() added in this series,
> which guarantees the map it's working on is always the guc->ads_map,
> while reducing verbosity to use the API. From patch
> "drm/i915/guc: Add read/write helpers for ADS blob":
>
> #define ads_blob_read(guc_, field_)                                    \
>        dma_buf_map_read_field(&(guc_)->ads_map, struct __guc_ads_blob, \
>                               field_)
>
> #define ads_blob_write(guc_, field_, val_)                             \
>        dma_buf_map_write_field(&(guc_)->ads_map, struct __guc_ads_blob,\
>                                field_, val_)
>
> So in intel_guc_ads, we can have a lot of:
>
> -    bla->x = 100;
> +    ads_blob_write(guc, x, 10);
>
> thanks
> Lucas De Marchi

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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