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From: David Laight <David.Laight@ACULAB.COM>
To: 'Oliver Neukum' <oneukum@suse.com>,
	"gregKH@linuxfoundation.org" <gregKH@linuxfoundation.org>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>
Subject: UAS: fix alignment of scatter/gather segments
Date: Mon, 29 Apr 2019 14:19:17 +0000	[thread overview]
Message-ID: <e22543f3d87b4867aefdf9c71e2ec4a2@AcuMS.aculab.com> (raw)

From: Oliver Neukum
> Sent: 29 April 2019 14:38
> On Mo, 2019-04-29 at 13:31 +0000, David Laight wrote:
> > From: Oliver Neukum
> > >
> > > +	 * USB has unusual scatter-gather requirements: the length of each
> > > +	 * scatterlist element except the last must be divisible by the
> > > +	 * Bulk maxpacket value.  Fortunately this value is always a
> > > +	 * power of 2.  Inform the block layer about this requirement.
> > > +	 */
> >
> > That isn't the correct restriction for XHCI.
> > It has its own perverse restrictions.
> > I think they are all handled within the xhci driver.
> 
> Yes, but that does not matter. You just cannot assume that only
> XHCI will be used with UAS. In particular virtual drivers will
> be used.

True, but there is no need to enforce a 2k (IIRC) alignment for XHCI.
Perhaps you need a different property from the controller.

Even if you decide the code is 'good enough' (I don't know what the
cost is of enforcing a 2k alignment instead of 512 bytes)
the comment is just plain wrong.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

WARNING: multiple messages have this Message-ID (diff)
From: David Laight <David.Laight@ACULAB.COM>
To: 'Oliver Neukum' <oneukum@suse.com>,
	"gregKH@linuxfoundation.org" <gregKH@linuxfoundation.org>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>
Subject: RE: [PATCH] UAS: fix alignment of scatter/gather segments
Date: Mon, 29 Apr 2019 14:19:17 +0000	[thread overview]
Message-ID: <e22543f3d87b4867aefdf9c71e2ec4a2@AcuMS.aculab.com> (raw)
Message-ID: <20190429141917._jcVEEy-QQNQLP_wImeyOzPlPL2OJlyr6B0ssuGMFqc@z> (raw)
In-Reply-To: <1556545099.20085.14.camel@suse.com>

From: Oliver Neukum
> Sent: 29 April 2019 14:38
> On Mo, 2019-04-29 at 13:31 +0000, David Laight wrote:
> > From: Oliver Neukum
> > >
> > > +	 * USB has unusual scatter-gather requirements: the length of each
> > > +	 * scatterlist element except the last must be divisible by the
> > > +	 * Bulk maxpacket value.  Fortunately this value is always a
> > > +	 * power of 2.  Inform the block layer about this requirement.
> > > +	 */
> >
> > That isn't the correct restriction for XHCI.
> > It has its own perverse restrictions.
> > I think they are all handled within the xhci driver.
> 
> Yes, but that does not matter. You just cannot assume that only
> XHCI will be used with UAS. In particular virtual drivers will
> be used.

True, but there is no need to enforce a 2k (IIRC) alignment for XHCI.
Perhaps you need a different property from the controller.

Even if you decide the code is 'good enough' (I don't know what the
cost is of enforcing a 2k alignment instead of 512 bytes)
the comment is just plain wrong.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

             reply	other threads:[~2019-04-29 14:19 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-29 14:19 David Laight [this message]
2019-04-29 14:19 ` [PATCH] UAS: fix alignment of scatter/gather segments David Laight
  -- strict thread matches above, loose matches on Subject: below --
2019-04-30 14:39 Alan Stern
2019-04-30 14:39 ` [PATCH] " Alan Stern
2019-04-30  9:16 David Laight
2019-04-30  9:16 ` [PATCH] " David Laight
2019-04-29 19:42 Alan Stern
2019-04-29 19:42 ` [PATCH] " Alan Stern
2019-04-29 18:42 Oliver Neukum
2019-04-29 18:42 ` [PATCH] " Oliver Neukum
2019-04-29 17:55 Alan Stern
2019-04-29 17:55 ` [PATCH] " Alan Stern
2019-04-29 16:58 Oliver Neukum
2019-04-29 16:58 ` [PATCH] " Oliver Neukum
2019-04-29 16:08 Alan Stern
2019-04-29 16:08 ` [PATCH] " Alan Stern
2019-04-29 15:57 Oliver Neukum
2019-04-29 15:57 ` [PATCH] " Oliver Neukum
2019-04-29 15:06 David Laight
2019-04-29 15:06 ` [PATCH] " David Laight
2019-04-29 14:32 Oliver Neukum
2019-04-29 14:32 ` [PATCH] " Oliver Neukum
2019-04-29 13:38 Oliver Neukum
2019-04-29 13:38 ` [PATCH] " Oliver Neukum
2019-04-29 13:31 David Laight
2019-04-29 13:31 ` [PATCH] " David Laight
2019-04-29 12:20 Oliver Neukum
2019-04-29 12:20 ` [PATCH] " Oliver Neukum

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