On Thu, 7 Apr 2016, Felipe Balbi wrote:
> Just so we have some sort of documentation as to why
> we limit our Mass Storage transfers to 240 sectors,
> let's update the comment to make clearer that
> devices were found that would choke with larger
> transfers.
>
> While at that, also make sure to clarify that other
> operating systems have similar, albeit different,
> limits on mass storage transfers.
>
> Signed-off-by: Felipe Balbi <felipe.balbi@xxxxxxxxxxxxxxx>
> ---
> drivers/usb/storage/scsiglue.c | 19 ++++++++++++++++++-
> 1 file changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/usb/storage/scsiglue.c b/drivers/usb/storage/scsiglue.c
> index dba51362d2e2..2a8c203a67dc 100644
> --- a/drivers/usb/storage/scsiglue.c
> +++ b/drivers/usb/storage/scsiglue.c
> @@ -565,7 +565,24 @@ static const struct scsi_host_template usb_stor_host_template = {
> /* lots of sg segments can be handled */
> .sg_tablesize = SCSI_MAX_SG_CHAIN_SEGMENTS,
>
> - /* limit the total size of a transfer to 120 KB */
> +
> + /*
> + * Limit the total size of a transfer to 120 KB.
> + *
> + * Some devices are known to choke with anything larger. It seems like
> + * the problem stems from the fact that original IDE controllers had
> + * only an 8-bit register to hold the number of sectors in one transfer
> + * and even those couldn't handle a full 256 sectors.
> + *
> + * Because we want to make sure we interoperate with as many devices as
> + * possible, we will maintain a 240 sector transfer size limit for USB
> + * Mass Storage devices.
> + *
> + * Tests show that other operating have similar limits with Microsoft
> + * Windows™ 7 limitting transfers to 128 sectors for both USB2 and USB3
> + * and Apple Mac OS X™ 10.11 limitting transfers to 256 sectors for USB2
> + * and 2048 for USB3 devices.
> + */
Minor nit: "limiting" is misspelled twice.
Alan Stern
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