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Date:   Mon, 30 Nov 2020 12:20:04 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Hans de Goede <hdegoede@redhat.com>
Cc:     Greg KH <gregkh@linuxfoundation.org>, Tom Yan <tom.ty89@gmail.com>,
        Christoph Hellwig <hch@lst.de>,
        linux-usb <linux-usb@vger.kernel.org>,
        Mathias Nyman <mathias.nyman@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-pci@vger.kernel.org, Lu Baolu <baolu.lu@linux.intel.com>
Subject: Re: [PATCH 2/2] usb-storage: revert from scsi_add_host_with_dma() to
 scsi_add_host()
Message-ID: <20201130172004.GA966032@rowland.harvard.edu>
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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?

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.

Alan Stern