linux-kernel - Re: [PATCH 5/6] usb: don't inherity DMA properties for USB devices

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Date:   Mon, 14 Sep 2020 09:51:40 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org,
        Russell King <linux@...linux.org.uk>,
        Santosh Shilimkar <ssantosh@...nel.org>,
        Jim Quinlan <james.quinlan@...adcom.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-remoteproc@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-acpi@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 5/6] usb: don't inherity DMA properties for USB devices

On Mon, Sep 14, 2020 at 09:33:42AM +0200, Christoph Hellwig wrote:
> As the comment in usb_alloc_dev correctly states, drivers can't use
> the DMA API on usb device, and at least calling dma_set_mask on them
> is highly dangerous.  Unlike what the comment states upper level drivers
> also can't really use the presence of a dma mask to check for DMA
> support, as the dma_mask is set by default for most busses.
> 
> Setting the dma_mask comes from "[PATCH] usbcore dma updates (and doc)"
> in BitKeeper times, as it seems like it was primarily for setting the
> NETIF_F_HIGHDMA flag in USB drivers, something that has long been
> fixed up since.
> 
> Setting the dma_pfn_offset comes from commit b44bbc46a8bb
> ("usb: core: setup dma_pfn_offset for USB devices and, interfaces"),
> which worked around the fact that the scsi_calculate_bounce_limits
> functions wasn't going through the proper driver interface to query
> DMA information, but that function was removed in commit 21e07dba9fb1
> ("scsi: reduce use of block bounce buffers") years ago.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>