Re: [Intel-gfx] [PATCH 02/19] dma-buf-map: Add helper to initialize second map

Lucas De Marchi
2022-01-27T15:59:17Z

On Thu, Jan 27, 2022 at 03:33:12PM +0100, Thomas Zimmermann wrote:
>
>
>Am 26.01.22 um 21:36 schrieb Lucas De Marchi:
>>When dma_buf_map struct is passed around, it's useful to be able to
>>initialize a second map that takes care of reading/writing to an offset
>>of the original map.
>>
>>Add a helper that copies the struct and add the offset to the proper
>>address.
>>
>>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 | 29 +++++++++++++++++++++++++++++
>> 1 file changed, 29 insertions(+)
>>
>>diff --git a/include/linux/dma-buf-map.h b/include/linux/dma-buf-map.h
>>index 65e927d9ce33..3514a859f628 100644
>>--- a/include/linux/dma-buf-map.h
>>+++ b/include/linux/dma-buf-map.h
>>@@ -131,6 +131,35 @@ struct dma_buf_map {
>> .is_iomem = false, \
>> }
>>+/**
>>+ * DMA_BUF_MAP_INIT_OFFSET - Initializes struct dma_buf_map from another dma_buf_map
>>+ * @map_: The dma-buf mapping structure to copy from
>>+ * @offset: Offset to add to the other mapping
>>+ *
>>+ * Initializes a new dma_buf_struct based on another. This is the equivalent of doing:
>>+ *
>>+ * .. code-block: c
>>+ *
>>+ * dma_buf_map map = other_map;
>>+ * dma_buf_map_incr(&map, &offset);
>>+ *
>>+ * Example usage:
>>+ *
>>+ * .. code-block: c
>>+ *
>>+ * void foo(struct device *dev, struct dma_buf_map *base_map)
>>+ * {
>>+ * ...
>>+ * struct dma_buf_map = DMA_BUF_MAP_INIT_OFFSET(base_map, FIELD_OFFSET);
>>+ * ...
>>+ * }
>>+ */
>>+#define DMA_BUF_MAP_INIT_OFFSET(map_, offset_) (struct dma_buf_map) \
>>+ { \
>>+ .vaddr = (map_)->vaddr + (offset_), \
>>+ .is_iomem = (map_)->is_iomem, \
>>+ }
>>+
>
>It's illegal to access .vaddr with raw pointer. Always use a
>dma_buf_memcpy_() interface. So why would you need this macro when you
>have dma_buf_memcpy_*() with an offset parameter?

I did a better job with an example in 20220127093332.wnkd2qy4tvwg5i5l@ldmartin-desk2

While doing this series I had code like this when using the API in a function to
parse/update part of the struct mapped:

int bla_parse_foo(struct dma_buf_map *bla_map)
{
struct dma_buf_map foo_map = *bla_map;
...

dma_buf_map_incr(&foo_map, offsetof(struct bla, foo));

...
}

Pasting the rest of the reply here:

I had exactly this code above, but after writting quite a few patches
using it, particularly with functions that have to write to 2 maps (see
patch 6 for example), it felt much better to have something to
initialize correctly from the start

struct dma_buf_map other_map = *bla_map;
/* poor Lucas forgetting dma_buf_map_incr(map, offsetof(...)); */

is error prone and hard to debug since you will be reading/writting
from/to another location rather than exploding

While with the construct below

other_map;
...
other_map = INITIALIZER()

I can rely on the compiler complaining about uninitialized var. And
in most of the cases I can just have this single line in the beggining of the
function when the offset is constant:

struct dma_buf_map other_map = INITIALIZER(bla_map, offsetof(..));

This is useful when you have several small functions in charge of
updating/reading inner struct members.

>
>I've also been very careful to distinguish between .vaddr and
>.vaddr_iomem, even in places where I wouldn't have to. This macro
>breaks the assumption.

That's one reason I think if we have this macro, it should be in the
dma_buf_map.h header (or whatever we rename these APIs to). It's the
only place where we can safely add code that relies on the implementation
of the "private" fields in struct dma_buf_map.

Lucas De Marchi

>
>Best regards
>Thomas
>
>> /**
>> * dma_buf_map_set_vaddr - Sets a dma-buf mapping structure to an address in system memory
>> * @map: The dma-buf mapping structure
>
>--
>Thomas Zimmermann
>Graphics Driver Developer
>SUSE Software Solutions Germany GmbH
>Maxfeldstr. 5, 90409 Nürnberg, Germany
>(HRB 36809, AG Nürnberg)
>Geschäftsführer: Ivo Totev