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

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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@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);
+}

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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