Re: [RFT][PATCH] generic device DMA implementation

[Manfred Spraul]

>
>
>+
>+Consistent memory is memory for which a write by either the device or
>+the processor can immediately be read by the processor or device
>+without having to worry about caching effects.
>
This is not entirely correct:
The driver must use the normal memory barrier instructions even in 
coherent memory. Could you copy the section about wmb() from DMA-mapping 
into your new documentation?

+
+Warnings:  Memory coherency operates at a granularity called the cache
+line width.  In order for memory mapped by this API to operate
+correctly, the mapped region must begin exactly on a cache line
+boundary and end exactly on one (to prevent two separately mapped
+regions from sharing a single cache line).  Since the cache line size
+may not be known at compile time, the API will not enforce this
+requirement.  Therefore, it is recommended that driver writers who
+don't take special care to determine the cache line size at run time
+only map virtual regions that begin and end on page boundaries (which
+are guaranteed also to be cache line boundaries).
+

Noone obeys that rule, and it's not trivial to fix it.

- kmalloc (32,GFP_KERNEL) returns a 32-byte object, even if the cache line size is 128 bytes. The 4 objects in the cache line could be used by four different users.
- sendfile() with an odd offset.

Is it really impossible to work around that in the platform specific code?
In the worst case, the arch code could memcopy to/from a cacheline aligned buffer.

--
    Manfred