On Wed, 13 Feb 2008 16:55:54 -0700 "Dan Williams" <[EMAIL PROTECTED]> wrote:
> Well, the other two possibilities are: > > 1/ Spin/sleep until a descriptor shows up Won't work since the transfer hasn't been started yet, so it will spin indefinitely. I guess we could return, send the command and use a waitqueue to wait for the callback. But this will make error handling seriously yucky. > 2/ Fall back to PIO for a few transfers Which means killing performance for large transfers. Not really an option. It's ok to use PIO for small and/or odd transfers like "read 2 bytes from this SDIO register", something that the mmc-block driver would never ask us to do. Using PIO for huge block data transfers will really hurt. > Descriptor availability is improved if the code interleaves allocation > and submission. Currently it looks like we wait until all descriptors > for the scatterlist are allocated before we start submitting. No, none of the descriptors will appear before the command has been sent, the card has responded, and a full block of data has been transferred. I suppose we could send the command earlier, but I don't think it will help a lot and it will complicate error handling. There may be room for improvement though. The current scheme of splitting DMA preparation and submission was initially used because older versions of the controller would instantly fail with an overrun or underrun if the data wasn't available immediately when the command had been sent. Sending the command earlier and doing interleaved allocation and submission might improve performance a bit. I think I'll try triggering the mmc tasklet from the DMA callback for now. Scheduling a tasklet from another tasklet should work just fine, right? Haavard -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
