drm/i915/ttm: implement access_memory

It looks like we need this for local-memory, if we want to use ptrace.
Something more is still needed if we want to handle non-mappable memory,
which looks quite annoying.

v2:
  - ttm_bo_kmap doesn't seem to work well here, and seems to expect
    contiguous resource.
v3(Andi):
  - s/PAGE_SIZE/bytes/ when passing in the size of the mapping.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/6989Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221003172819.99245-1-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_ttm.c

@@ -699,6 +699,50 @@ static unsigned long i915_ttm_io_mem_pfn(struct ttm_buffer_object *bo,
 	return ((base + sg_dma_address(sg)) >> PAGE_SHIFT) + ofs;
 }
 
+static int i915_ttm_access_memory(struct ttm_buffer_object *bo,
+				  unsigned long offset, void *buf,
+				  int len, int write)
+{
+	struct drm_i915_gem_object *obj = i915_ttm_to_gem(bo);
+	resource_size_t iomap = obj->mm.region->iomap.base -
+		obj->mm.region->region.start;
+	unsigned long page = offset >> PAGE_SHIFT;
+	unsigned long bytes_left = len;
+
+	/*
+	 * TODO: For now just let it fail if the resource is non-mappable,
+	 * otherwise we need to perform the memcpy from the gpu here, without
+	 * interfering with the object (like moving the entire thing).
+	 */
+	if (!i915_ttm_resource_mappable(bo->resource))
+		return -EIO;
+
+	offset -= page << PAGE_SHIFT;
+	do {
+		unsigned long bytes = min(bytes_left, PAGE_SIZE - offset);
+		void __iomem *ptr;
+		dma_addr_t daddr;
+
+		daddr = i915_gem_object_get_dma_address(obj, page);
+		ptr = ioremap_wc(iomap + daddr + offset, bytes);
+		if (!ptr)
+			return -EIO;
+
+		if (write)
+			memcpy_toio(ptr, buf, bytes);
+		else
+			memcpy_fromio(buf, ptr, bytes);
+		iounmap(ptr);
+
+		page++;
+		buf += bytes;
+		bytes_left -= bytes;
+		offset = 0;
+	} while (bytes_left);
+
+	return len;
+}
+
 /*
  * All callbacks need to take care not to downcast a struct ttm_buffer_object
  * without checking its subclass, since it might be a TTM ghost object.
@@ -715,6 +759,7 @@ static struct ttm_device_funcs i915_ttm_bo_driver = {
 	.delete_mem_notify = i915_ttm_delete_mem_notify,
 	.io_mem_reserve = i915_ttm_io_mem_reserve,
 	.io_mem_pfn = i915_ttm_io_mem_pfn,
+	.access_memory = i915_ttm_access_memory,
 };
 
 /**