On Thu, Sep 2, 2010 at 6:32 PM, David Brownell <david-b@xxxxxxxxxxx> wrote: > > > --- On Thu, 9/2/10, Oliver Neukum <oneukum@xxxxxxx> wrote: > >> > >> > >> > Passing a pointer to a few bytes inside of a >> > kmalloc'd block has always been legit, even if >> > that pointer isn't aligned to N!=1 bytes. >> >> Is that also true with respect to usb_alloc_coherent() ? > > Why wouldn't it be? The buffer pointer is valid... > > The memory returned from usb_alloc_coherent will always be aligned to at least 32 bytes (it uses a set of buffer pools with 32/128/512/ PAGE_SIZE/2 and uses the smallest suitable buffer. [drivers/usb/core/buffer.c] So provided the HCD doesn't have alignment requirements stricter then 32 byte that should always work. I don't think it is possible for drivers to add an offset to the result of usb_alloc_coherent() since that would also require adjusting the dma_addr_t which is supposed to be an opaque handle not an address right? However, when a driver maps its own buffers and specifies URB_NO_TRANSFER_DMA_MAP what are the requirements? Does it have to use usb_alloc_coherent()? Does it have to use a coherent mapping at all or can it use the streaming dma api and sync the buffers manually? If the buffer is not aligned to the _hardware_ requirements fixing it (whether in the HCD or usb core and by whatever technique - bounce buffer, PIO,..) will require CPU access to the buffer. If the buffer is coherently mapped that will work but if not it will need to be unmapped or synced first (I believe this applies even if the device hardware never touches the buffer?) How is the HCD or usb core to know how a driver mapped buffer was obtained to know if and how to sync it? Maybe it's possible to just dma_sync_single_for_cpu() in all cases where cpu access is required (even if the buffer was not obtained from dma_map_single). I don't know and suspect the answer is likely to be architecture dependant so "try it and see" doesn't seem a good idea. Regards, Martin -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
