linux-kernel - Re: [PATCH 2/2] usb-storage: revert from scsi_add_host_with_dma() to scsi_add

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Date:   Mon, 1 Mar 2021 10:59:27 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Christoph Hellwig <hch@....de>, Hans de Goede <hdegoede@...hat.com>
Cc:     Tom Yan <tom.ty89@...il.com>,
        linux-usb <linux-usb@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        SCSI development list <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH 2/2] usb-storage: revert from scsi_add_host_with_dma() to
 scsi_add_host()

On Fri, Feb 26, 2021 at 06:53:52AM +0100, Christoph Hellwig wrote:
> On Thu, Feb 25, 2021 at 11:35:57AM -0500, Alan Stern wrote:
> > 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.
> 
> usb-storage must use scsi_add_host_with_dma to use the right device
> for DMA mapping and parameters.  The calls to set the DMA options
> on the device are needed so that IOMMU merging doesn't change the
> imposed requirements.  If these requirements slow you down you need
> to relax them, as apparently the hardware is able to handle bigger
> limits.

Hans, don't you have the right equipment to test this approach?

Alan Stern