> From: Hans de Goede
>
> Copy the sg alignment trick from the usb-storage driver, without this I'm
> seeing intermittent errors when using uas devices with an ehci controller.
>
> Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
> ---
> drivers/usb/storage/uas.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/drivers/usb/storage/uas.c b/drivers/usb/storage/uas.c
> index 3c4cf1e..2b946dd 100644
> --- a/drivers/usb/storage/uas.c
> +++ b/drivers/usb/storage/uas.c
> @@ -928,6 +928,24 @@ static int uas_eh_bus_reset_handler(struct scsi_cmnd *cmnd)
> static int uas_slave_alloc(struct scsi_device *sdev)
> {
> sdev->hostdata = (void *)sdev->host->hostdata;
> +
> + /* USB has unusual DMA-alignment requirements: Although the
> + * starting address of each scatter-gather element doesn't matter,
> + * the length of each element except the last must be divisible
> + * by the Bulk maxpacket value. There's currently no way to
> + * express this by block-layer constraints, so we'll cop out
> + * and simply require addresses to be aligned at 512-byte
> + * boundaries. This is okay since most block I/O involves
> + * hardware sectors that are multiples of 512 bytes in length,
> + * and since host controllers up through USB 2.0 have maxpacket
> + * values no larger than 512.
> + *
> + * But it doesn't suffice for Wireless USB, where Bulk maxpacket
> + * values can be as large as 2048. To make that work properly
> + * will require changes to the block layer.
> + */
> + blk_queue_update_dma_alignment(sdev->request_queue, (512 - 1));
The comment isn't true for anything connected to xhci.
Pragmatically the solution might be reasonable though.
(IIRC the usenet code knows whether it can do 'random' SG.)
David
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