On Fri, 11 Oct 2019, Andrey Konovalov wrote: > On Fri, Oct 11, 2019 at 4:08 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > Now yes, it's true that defining status as an array on the stack is > > also a bug, since USB transfer buffers are not allowed to be stack > > variables. > > Hi Alan, > > I'm curious, what is the reason for disallowing that? Should we try to > somehow detect such cases automatically? Transfer buffers are read and written by DMA. On systems that don't have cache-coherent DMA controllers, it is essential that the CPU does not access any cache line involved in a DMA transfer while the transfer is in progress. Otherwise the data in the cache would be different from the data in the buffer, leading to corruption. (In theory it would be okay for the CPU to read (not write!) a cache line assigned to a buffer for a DMA write (not read!) transfer. But even doing that isn't really a good idea.) (Also, this isn't an issue for x86 architectures, because x86 has cache-coherent DMA. But it is an issue on other architectures.) In practice, this means transfer buffers have to be allocated by something like kmalloc, so that they occupies their own separate set of cache lines. Buffers on the stack obviously don't satisfy this requirement. At some point there was a discussion about automatically detecting when on-stack (or otherwise invalid) buffers are used for DMA transfers. I don't recall what the outcome was. Alan Stern
