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From: Suwan Kim <suwan.kim027@gmail.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: shuah <shuah@kernel.org>, Christoph Hellwig <hch@lst.de>,
	Valentina Manea <valentina.manea.m@gmail.com>,
	Oliver Neukum <oneukum@suse.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>,
	Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>
Subject: Re: How to resolve an issue in swiotlb environment?
Date: Sat, 22 Jun 2019 02:43:43 +0900
Message-ID: <20190621174342.GA28335@localhost.localdomain> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1906191649350.1596-100000@iolanthe.rowland.org>

On Wed, Jun 19, 2019 at 05:05:49PM -0400, Alan Stern wrote:
> On Wed, 19 Jun 2019, shuah wrote:
> 
> > I missed a lot of the thread info. and went looking for it and found the
> > following summary of the problem:
> > 
> > ==================
> > The issue which prompted the commit this thread is about arose in a
> > situation where the block layer set up a scatterlist containing buffer
> > sizes something like:
> > 
> > 	4096 4096 1536 1024
> > 
> > and the maximum packet size was 1024.  The situation was a little
> > unusual, because it involved vhci-hcd (a virtual HCD).  This doesn't
> > matter much in normal practice because:
> > 
> > 	Block devices normally have a block size of 512 bytes or more.
> > 	Smaller values are very uncommon.  So scatterlist element sizes
> > 	are always divisible by 512.
> > 
> > 	xHCI is the only USB host controller type with a maximum packet
> > 	size larger than 512, and xHCI hardware can do full
> > 	scatter-gather so it doesn't care what the buffer sizes are.
> > 
> > So another approach would be to fix vhci-hcd and then trust that the
> > problem won't arise again, for the reasons above.  We would be okay so
> > long as nobody tried to use a USB-SCSI device with a block size of 256
> > bytes or less.
> > ===================
> > 
> > Out of the summary, the following gives me pause:
> > 
> > "xHCI hardware can do full scatter-gather so it doesn't care what the
> > buffer sizes are."
> > 
> > vhci-hcd won't be able to count on hardware being able to do full
> > scatter-gather. It has to deal with a variety of hardware with
> > varying speeds.
> 
> Sure.  But you can test whether the server's HCD is able to handle 
> scatter-gather transfers, and if it is then you can say that the 
> client-side vhci-hcd is able to handle them as well.  Then all you 
> would have to do is preserve the scatterlist information describing the 
> transfer when you go between the client and the server.
> 
> The point is to make sure that the client-side vhci-hcd doesn't claim
> to be _less_ capable than the server-side actual HCD.  That's what
> leads to the problem described above.
> 
> > "We would be okay so long as nobody tried to use a USB-SCSI device with
> > a block size of 256 bytes or less."
> > 
> > At least a USB Storage device, I test with says 512 block size. Can we
> > count on not seeing a device with block size <= 256 bytes?
> 
> Yes, we can.  In fact, the SCSI core doesn't handle devices with block 
> size < 512.
> 
> > In any case, I am looking into adding SG support vhci-hci at the moment.
> > 
> > Looks like the following is the repo, I should be working with?
> > 
> > git://git.infradead.org/users/hch/misc.git
> 
> It doesn't matter.  Your work should end up being independent of 
> Christoph's, so you can base it on any repo.

I implemented SG support of vhci. I will send it as a patch.
Please look at it and let me know if you have a feedback.

Regards

Suwan Kim

  reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-03  6:42 Yoshihiro Shimoda
2019-06-07 12:00 ` Yoshihiro Shimoda
2019-06-10  7:31   ` Biju Das
2019-06-10 11:13   ` Yoshihiro Shimoda
2019-06-10 12:32     ` Christoph Hellwig
2019-06-10 18:46       ` Alan Stern
2019-06-11  6:41         ` Christoph Hellwig
2019-06-11 14:51           ` Alan Stern
2019-06-12  7:30             ` Christoph Hellwig
2019-06-12  8:52               ` Yoshihiro Shimoda
2019-06-12 11:31                 ` Christoph Hellwig
2019-06-13  4:52                   ` Yoshihiro Shimoda
2019-06-12 11:46               ` Oliver Neukum
2019-06-12 12:06                 ` Christoph Hellwig
2019-06-12 14:43                   ` Alan Stern
2019-06-13  7:39                     ` Christoph Hellwig
2019-06-13 16:57                       ` Martin K. Petersen
2019-06-13 17:16                       ` Alan Stern
2019-06-13 18:18                         ` Greg KH
2019-06-13 23:01                         ` shuah
2019-06-14 14:44                           ` Alan Stern
2019-06-18 15:28                             ` shuah
2019-06-19 20:23                               ` shuah
2019-06-19 21:05                                 ` Alan Stern
2019-06-21 17:43                                   ` Suwan Kim [this message]
2019-06-11  6:49         ` Yoshihiro Shimoda

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