From: Christoph Hellwig <hch@lst.de>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Christoph Hellwig <hch@lst.de>,
linux-usb@vger.kernel.org, usb-storage@lists.one-eyed-alien.net,
linux-scsi@vger.kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: usb-storage: stop using block layer bounce buffers
Date: Wed, 2 May 2018 14:47:07 +0200 [thread overview]
Message-ID: <20180502124707.GA22958@lst.de> (raw)
On Fri, Apr 27, 2018 at 10:09:17AM -0400, Alan Stern wrote:
> On Fri, 27 Apr 2018, Christoph Hellwig wrote:
>
> > On Sun, Apr 15, 2018 at 11:24:11AM -0400, Alan Stern wrote:
> > > On Sun, 15 Apr 2018, Christoph Hellwig wrote:
> > >
> > > > USB host controllers now must handle highmem, so we can get rid of bounce
> > > > buffering highmem pages in the block layer.
> > >
> > > Sorry, I don't quite understand what you are saying. Do you mean that
> > > all USB host controllers now magically _do_ handle highmem? Or do you
> > > mean that if they _don't_ handle highmem, we will not support them any
> > > more?
> >
> > USB controller themselves never cared about highmem, drivers did. For
> > PIO based controllers they'd have to kmap any address no in the kernel
> > drirect mapping.
> >
> > Nothing in drivers/usb/host or the other diretories related to host
> > drivers calls page_address (only used in a single gadget) or sg_virt
> > (only used in a few upper level drivers), which makes me assume
> > semi-confidently that no USB host driver is not highmem aware these
> > days.
>
> sg_virt is called in drivers/usb/core/message.c. (Maybe that's what
> you meant by "upper level drivers".) I'm not sure just how important
> that usage is.
I don't really know either. I'll need some guidance from the usb
maintainers on:
- when drivers can submit urbs with a scatterlist
- if there are any drivers that do not want to take highmem
Unfortunately the way dma mapping works in usb is just so deeply
convoluted that I have a hard time following it, and often wonder
what is intentional and what is accidental in it.
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next reply other threads:[~2018-05-02 12:47 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-02 12:47 Christoph Hellwig [this message]
-- strict thread matches above, loose matches on Subject: below --
2018-05-02 14:48 usb-storage: stop using block layer bounce buffers Alan Stern
2018-04-27 14:09 Alan Stern
2018-04-27 5:27 Christoph Hellwig
2018-04-15 15:24 Alan Stern
2018-04-15 14:53 Christoph Hellwig
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