lx_kit: get buffer dataspace from virt addr

Introduce a method to access the dataspace capability of the underlying
backing store for a memory allocation. This is necessary for drivers
where the memory is managed manually and the capability needs to be
given to a client.

Issue #4325.

repos/dde_linux/src/include/lx_kit/memory.h

@@ -105,6 +105,7 @@ class Lx_kit::Mem_allocator
 		              Cache                  cache_attr);
 
 		Attached_dataspace & alloc_dataspace(size_t size);
+		Dataspace_capability attached_dataspace_cap(void *addr);
 
 		void * alloc(size_t size, size_t align);
 		addr_t dma_addr(void * addr);

repos/dde_linux/src/lib/lx_kit/memory.cc

@@ -49,6 +49,19 @@ Genode::Attached_dataspace & Lx_kit::Mem_allocator::alloc_dataspace(size_t size)
 }
 
 
+Genode::Dataspace_capability Lx_kit::Mem_allocator::attached_dataspace_cap(void * addr)
+{
+	Genode::Dataspace_capability ret { };
+
+	_virt_to_dma.apply(Buffer_info::Query_addr(addr),
+	                   [&] (Buffer_info const & info) {
+		ret = info.buffer.ds().cap();
+	});
+
+	return ret;
+}
+
+
 void * Lx_kit::Mem_allocator::alloc(size_t size, size_t align)
 {
 	if (!size)