This thread seems to have fallen through the cracks. Maybe now would be a good time to address the problem (since we originally planned to fix it in 5.11!). The questions listed below are pretty self-contained, although the rest of the discussion isn't. But I never received any answers. Alan Stern On Mon, Nov 30, 2020 at 03:36:18PM -0500, Alan Stern wrote: > [Added linux-scsi to CC: list. When discussing code in a particular > subsystem, it's a good idea to include that subsystem's mailing list in > the CC:.] > > On Tue, Dec 01, 2020 at 03:01:56AM +0800, Tom Yan wrote: > > For the record, > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/scsi/scsi_host.h?h=v5.10-rc6#n753 > > > > On Tue, 1 Dec 2020 at 02:57, Tom Yan <tom.ty89@xxxxxxxxx> wrote: > > > > > > This maybe? https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/scsi/scsi_lib.c?h=v5.10-rc6#n1816 > > > > > > UAS: > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/usb/storage/uas.c?h=v5.10-rc6#n918 > > > BOT (AFAICT): > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/scsi/hosts.c?h=v5.10-rc6#n466 > > > > > > It would explain why the issue is only triggered with UAS drives. > > In brief, a recent change -- calling scsi_add_host_with_dma rather than > scsi_add_host -- in the USB uas driver has caused a regression in > performance. (Note that the shost->dma_dev value is set differently as > a result of this change.) Hans has determined that the problem seems > to be related to permanent changes in the dma_dev's settings caused by > scsi_add_host_with_dma. > > Tom pointed out that __scsi_init_queue contains a couple of questionable > assignments: > > dma_set_seg_boundary(dev, shost->dma_boundary); > > and > > dma_set_max_seg_size(dev, queue_max_segment_size(q)); > > where dev = shost->dma_dev -- in this case, a USB host controller. > > So an important question is why decisions related to a particular SCSI > host should affect the DMA settings of a device somewhere else in the > heirarchy? Sure, the properties of the USB controller should constrain > the settings available to the SCSI host, but there doesn't seem to be > any good reason for restrictions to go in the other direction. > > Doesn't the way we handle DMA permit a child device to impose additional > restrictions (such as a smaller max segment size) beyond those of the > parent device which actually performs the DMA transfer? > > > > The questions (from me) are: > > > 1. From the scsi layer POV (as per what __scsi_init_queue() does), > > > what/which should we use as dma_dev? > > We should be using the USB host controller, because it is the device > which actually performs the DMA transfers. > > > > 2. Do we really need to set dma_boundary in the UAS host template (to > > > PAGE_SIZE - 1)? > > I don't know. But in theory it should be possible to have settings > (like this one) which affect only the transfers carried out by the SCSI > host, not the transfers carred out by other drivers which might use the > same USB controller. > > > > 3. Kind of the same question as #1: when we clamp hw_max_sectors to > > > dma max mapping size, should the size actually be "the smaller one > > > among dev and sysdev"? Or is one of the two sizes *always* the smaller > > > one? > > I assume you're referring to code in the uas driver. There the value of > dev is meaningless as far as DMA is concerned. Only sysdev matters. > > Alan Stern > > > > On Tue, 1 Dec 2020 at 02:19, Hans de Goede <hdegoede@xxxxxxxxxx> wrote: > > > > > > > > Hi, > > > > > > > > On 11/30/20 6:20 PM, Alan Stern wrote: > > > > > On Mon, Nov 30, 2020 at 02:36:38PM +0100, Hans de Goede wrote: > > > > >> Hi, > > > > >> > > > > >> On 11/30/20 2:30 PM, Greg KH wrote: > > > > >>> On Mon, Nov 30, 2020 at 02:23:48PM +0100, Hans de Goede wrote: > > > > >>>> Hi, > > > > >>>> > > > > >>>> On 11/30/20 1:58 PM, Tom Yan wrote: > > > > >>>>> It's merely a moving of comment moving for/and a no-behavioral-change > > > > >>>>> adaptation for the reversion.> > > > > >>>> > > > > >>>> IMHO the revert of the troublesome commit and the other/new changes really > > > > >>>> should be 2 separate commits. But I will let Alan and Greg have the final > > > > >>>> verdict on this. > > > > >>> > > > > >>> I would prefer to just revert the commits and not do anything > > > > >>> different/special here so late in the release cycle. > > > > >>> > > > > >>> So, if Alan agrees, I'll be glad to do them on my end, I just need the > > > > >>> commit ids for them. > > > > >> > > > > >> The troublesome commit are (in reverse, so revert, order): > > > > >> > > > > >> 5df7ef7d32fe ("uas: bump hw_max_sectors to 2048 blocks for SS or faster drives") > > > > >> 558033c2828f ("uas: fix sdev->host->dma_dev") > > > > >> 0154012f8018 ("usb-storage: fix sdev->host->dma_dev") > > > > >> > > > > >> Alan, the reason for reverting these is that using scsi_add_host_with_dma() as the > > > > >> last 2 patches do, with the dmadev argument of that call pointing to the device > > > > >> for the XHCI controller is causing changes to the DMA settings of the XHCI controller > > > > >> itself which is causing regressions in 5.10, see this email thread: > > > > >> > > > > >> https://lore.kernel.org/linux-usb/fde7e11f-5dfc-8348-c134-a21cb1116285@xxxxxxxxxx/T/#t > > > > > > > > > > It's hard to go wrong with reverting, so it's okay with me. > > > > > > > > > > Still, Hans, have you checked out the difference between the > > > > > scsi_add_host() and scsi_add_host_with_dma() calls? It's just a matter > > > > > of using dev vs. sysdev. In particular, have you checked to see what > > > > > those two devices are on your system? > > > > > > > > Its not just dev vs sysdev, its iface->dev vs bus->sysdev, and I assume > > > > that the latter is actually the XHCI controller. > > > > > > > > my vote goes to reverting to avoid the regression for 5.10, esp. since > > > > this is a clean revert of 3 patches with nothing depending / building > > > > on top of the reverted commits. > > > > > > > > Then for 5.11 we can retry to introduce similar changes. I would be happy > > > > to try a new patch-set for 5.11. > > > > > > > > > It seems likely that if one of those calls messes up some DMA settings, > > > > > the other one does too -- just maybe not settings that matter much. > > > > > > > > I'm not very familiar with all the DMA mapping / mask code, but AFAIK making > > > > changes to the DMA settings of a child will not influence the parent. > > > > > > > > Where as when passing bus->sysdev, then changes are made to a device > > > > which is shared with other devices on the bus, which is why we see > > > > a regression in an USB NIC driver being triggered by the UAS driver > > > > binding to a device (on the same bus). > > > > > > > > At least that is my interpretation of this. I bisected the regression > > > > and that pointed at the UAS DMA change and reverting it fixes things, > > > > confirming that I did not make any mistakes during the bisect. > > > > > > > > Regards, > > > > > > > > Hans > > > >
