[PATCH] RFC: include/usb/ch9.h: pad usb_endpoint_descriptor with 1 byte

[Lucas Stach]

Am Montag, den 24.08.2015, 17:35 +0300 schrieb Antony Pavlov:
> On Mon, 24 Aug 2015 10:49:10 +0200
> Sascha Hauer <s.hauer at pengutronix.de> wrote:
> 
> > On Thu, Aug 20, 2015 at 06:22:18PM +0300, Peter Mamonov wrote:
> > > This patch fixes "Ooops, address error on load or ifetch!",
> > > caused by "usb" command on MIPS architecture.
> > > 
> > > To reproduce the problem follow the following steps:
> > > 
> > > 1. Build barebox for MIPS target with -O0 optimization flag:
> > > 
> > > diff --git a/Makefile b/Makefile
> > > index 5a7fd5f..da9a8be 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -301,7 +301,7 @@ CPPFLAGS        := -D__KERNEL__ -D__BAREBOX__ $(LINUXINCLUDE) -fno-builtin -ffre
> > > 
> > >  CFLAGS          := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
> > >                     -Werror-implicit-function-declaration \
> > > -                   -fno-strict-aliasing -fno-common -Os -pipe
> > > +                   -fno-strict-aliasing -fno-common -O0 -pipe
> > >  AFLAGS          := -D__ASSEMBLY__
> > > 
> > >  LDFLAGS_barebox        := -Map barebox.map
> > > 
> > > 2. Plug in a usb flash drive.
> > > 
> > > 3. Run the "usb" command:
> > > 
> > > 	barebox at QEMU DUNA:/ usb -s
> > > 	USB: scanning bus for devices...
> > > 	&(dev->config.interface[0].ep_desc[0].wMaxPacketSize) = a042732a
> > > 	Bus 001 Device 001: ID 0000:0000 EHCI Host Controller
> > > 	&(dev->config.interface[0].ep_desc[0].wMaxPacketSize) = a0429f02
> > > 	&(dev->config.interface[0].ep_desc[1].wMaxPacketSize) = a0429f0b
> > > 
> > > 	Ooops, address error on load or ifetch!
> > > 
> > > 	$ 0   : 00000000 00000000 a0429f0b a0429de4
> > > 	$ 4   : 00000000 0000000a 00000000 000687e0
> > > 	$ 8   : 00000000 00000000 00000000 fa000000
> > > 	$12   : 00000001 00000004 00000000 00000000
> > > 	$16   : 00000000 00000000 00000000 00000000
> > > 	$20   : 00000000 00000000 00000000 00000000
> > > 	$24   : 00000000 00000000
> > > 	$28   : 00000000 a03ffb40 a03ffb40 a082e9f4
> > > 	Hi    : 00000000
> > > 	Lo    : 00000000
> > > 	epc   : a082ea38
> > > 	ra    : a082e9f4
> > > 	Status: b4000002
> > > 	Cause : 00008010
> > > 	Config: 88d0c083
> > > 
> > > 	### ERROR ### Please RESET the board ###
> > > 
> > > The source of the error is the following line of code from the usb_parse_config() function:
> > > 
> > > 			le16_to_cpus(&(dev->config.interface[ifno].ep_desc[epno].\
> > > 							       wMaxPacketSize));
> > > 
> > > Which tries to load half-word from an un-aligned address 0xa0429f0b stored in v0:
> > > 
> > > a082ea38:       94420000        lhu     v0,0(v0)
> > > 
> > > 0xa0429f0b is an address of wMaxPacketSize member of a struct usb_endpoint_descriptor at offset 4:
> > > 
> > > struct usb_endpoint_descriptor {
> > > 	__u8  bLength;
> > > 	__u8  bDescriptorType;
> > > 	__u8  bEndpointAddress;
> > > 	__u8  bmAttributes;
> > > 	__le16 wMaxPacketSize;
> > > 	__u8  bInterval;
> > > 	__u8  bRefresh;
> > > 	__u8  bSynchAddress;
> > > } __attribute__ ((packed));
> > > 
> > > The instances of the usb_endpoint_descriptor are stored in a "ep_desc" array:
> > > 
> > > struct usb_interface {
> > > 	...
> > > 	struct usb_endpoint_descriptor ep_desc[USB_MAXENDPOINTS];
> > > };
> > > 
> > > Since the size of the usb_endpoint_descriptor is odd (9 bytes),
> > > the second element of the ep_desc array is un-aligned,
> > > as well as it's member wMaxPacketSize:
> > > 
> > > Bus 001 Device 001: ID 0000:0000 EHCI Host Controller
> > > &(dev->config.interface[0].ep_desc[0].wMaxPacketSize) = a0429f02
> > > &(dev->config.interface[0].ep_desc[1].wMaxPacketSize) = a0429f0b
> > > 
> > > A possible fix to this problem is to make the size of the usb_endpoint_descriptor even.
> > > ---
> > >  include/usb/ch9.h | 1 +
> > >  1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/include/usb/ch9.h b/include/usb/ch9.h
> > > index ab5d531..5b261e6 100644
> > > --- a/include/usb/ch9.h
> > > +++ b/include/usb/ch9.h
> > > @@ -379,6 +379,7 @@ struct usb_endpoint_descriptor {
> > >  	/* use USB_DT_ENDPOINT*_SIZE in bLength, not sizeof. */
> > >  	__u8  bRefresh;
> > >  	__u8  bSynchAddress;
> > > +	__u8  fill;
> > >  } __attribute__ ((packed));
> > 
> > This doesn't look very nice since we modify a ch9 defined struct just for
> > the sake of using it in an array somewhere else. We can fix it like
> > this, but please add a comment why we need this.
> 
> We need some fix here to make EHCI works on MIPS architecture.
> On the other hand the ch9 header file is used in linux kernel and in U-boot.
> I don't know, does U-boot EHCI driver works on mips (though AFAIR there was a USB MIPS-related
> patch several days ago), but linux kernel work with this ch9 header succesfully on MIPS.
> So another way to fix the problem is to port some code from modern U-boot or linux.
> 
The question here is: why isn't the compiler emitting the correct
instruction?

Apparently MIPS has a "ulh" instruction to load (possibly) unaligned
halfwords. The placement of the wMaxPacketSize member is static and the
compiler should be able to figure out that it may be placed at an
unaligned position, as requested per the packed attribute attached to
struct usb_endpoint_descriptor.

This isn't a problem we should fix in the barebox code unless we know
exactly why the compiler is doing the wrong thing.

Regards,
Lucas
-- 
Pengutronix e.K.             | Lucas Stach                 |
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