Hi Alan, On Monday 01 August 2011 21:15:01 Alan Cooper wrote: > On Mon, Aug 1, 2011 at 7:47 AM, Sebastian Andrzej Siewior wrote: > > * Laurent Pinchart | 2011-08-01 13:13:15 [+0200]: > >>> What about using kmalloc() + dma_map_single() + dma_unmap_single() > >>> instead of arch dependend code in drivers? Those are nops x86 and > >>> perform the required syncs on other architectures. > >> > >>Do you mean creating and tearing down the mapping for each transform ? > >>Isn't that very costly on many non-x86 platforms ? > >> > > You need consistent memory between hardware and cpu and this leaves you > > with two options: > > - use uncached memory > > - use cached memory and flush it > > > > If the architecture is cache coherent then you don't have to worry, the > > memory controller will snoop the cache line and invalidate the cache > > line or update the memory. If this is not the case then you have to do > > this on your own. > > Using uncached memory has the benefit that reads/writes are performed > > directly to main memory but since you bypass the cache, every access > > goes across the bus. So uncached memory is probably a good if you need > > to access the memory just once or twice. > > The dma ops usually flush the cache operations. So once the cache line > > for this memory address is clean then the CPU can fill its cache by > > fetching a bigger memory block instead of 4 bytes only (or 1 if the > > memcpy works on bytes). > > > > Is it possible to align the buffers somehow differrently and avoid the > > memcpy in the first place? > > > > Sebastian > > Thanks very much for the feedback. > I don't think it's possible to remove the need for the memcpy because the > ISOC USB transfer data in the buffer has holes and must be coalesced back > into a contiguous stream. > > If I understand correctly, there are two primary ways for a driver > like UVC to get buffers that will be used to hold USB transfer data. > 1. Use kmalloc() and pass the buffer address in the URB. In this case the > USB driver will handle all DMA mapping and cache coherency issues on each > transfer. 2. Use usb_alloc_coherent(), pass the address in the URB and set > the URB_NO_TRANSFER_DMA_MAP in the URB"s "transfer_flag". This will map > the buffer once when allocated and the system will not do any per transfer > mapping or cache management. > > Currently I see that sound, video, HID and network USB drivers all use > usb_alloc_coherent. To solve the performance issues on systems without > cache coherent DMA, it looks like I'll have to change all these drivers > (with the possible exception of HID) to use kmalloc. > > It seems like the reason to choose between the two methods is for > platform specific performance considerations. Ideally this kind of choice > would be made once in the USB driver instead of by every driver that sits on > top of the USB driver. Could you try kmalloc() on your MIPS platform ? How much does it help ? -- Regards, Laurent Pinchart -- 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
