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Date:   Thu, 25 Feb 2021 11:35:57 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Tom Yan <tom.ty89@gmail.com>, Hans de Goede <hdegoede@redhat.com>,
        Christoph Hellwig <hch@lst.de>
Cc:     linux-usb <linux-usb@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        SCSI development list <linux-scsi@vger.kernel.org>
Subject: Re: [PATCH 2/2] usb-storage: revert from scsi_add_host_with_dma() to
 scsi_add_host()
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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@gmail.com> 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@redhat.com> 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@redhat.com/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
> > > >