On Tue, Sep 01, 2020 at 08:55:07AM +0800, Tom Yan wrote: > Hi, > > So a recent commit that disable 64 bit dma addressing for ASM1142: > https://github.com/torvalds/linux/commit/ec37198acca7b4c17b96247697406e47aafe0605 > > I notice that it also causes (hw) max_sectors of uas drives connected > to the controller to drop from 1024 blocks (SCSI_DEFAULT_MAX_SECTORS) > to 512 blocks, because we clamp it against > dma_max_mapping_size($some_device). > > For uas drives, $some_device is `sdev->host->dma_dev`: > https://github.com/torvalds/linux/blob/v5.8/drivers/scsi/scsi_lib.c#L1778 > (`shost` is `sdev->host`: > https://github.com/torvalds/linux/blob/v5.8/drivers/scsi/scsi_lib.c#L1873) > > But for bot/msc drives: $some_device is some other device: > https://github.com/torvalds/linux/blob/v5.8/drivers/usb/storage/scsiglue.c#L92 > > In that case, max_sectors remains to be 2048 blocks for SS BOT drives > connected to the ASM1142 controller. > > I tried to change it to `sdev->host->dma_dev`. It lowered their > max_sectors to 512 blocks. > > So my questions are: > 1. Do we really need to do any clamping to max_sectors in either (uas/bot) case? > 2. If so, should we change the $some_device in either case? > > For the record, in the original commit (for BOT drives): > https://github.com/torvalds/linux/commit/d74ffae8b8dd17eaa8b82fc163e6aa2076dc8fb1 > The commit message states that the clamping is needed because of some > other change, which is supposed to cause max_segment_size to raise > from 0x10000 to 0xffffffff, but when I check the sysfs files for both > kinds of drives, all of the values are 0x10000. > > Also see https://github.com/virtio-win/kvm-guest-drivers-windows/issues/498 You should take a look at Arnd Bergmann's commit a8c06e407ef9 ("usb: separate out sysdev pointer from usb_bus"), which is what introduced this difference originally. As far as I know, both drivers (usb-storage and uas) should be using the bus->sysdev device for the purpose of clamping max_sectors. If uas doesn't use that device then it should be fixed. And yes, the clamping is necessary. Alan Stern
