linux-kernel - Re: [PATCH] USB: gadget: dummy

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Message-ID: <Y1Vv0tJZoSeqX003@rowland.harvard.edu>
Date:   Sun, 23 Oct 2022 12:46:10 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Felipe Balbi <balbi@...nel.org>,
        Jakob Koschel <jakobkoschel@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] USB: gadget: dummy_hcd: switch char * to u8 *

On Sun, Oct 23, 2022 at 05:53:19PM +0200, Greg Kroah-Hartman wrote:
> On Fri, Oct 21, 2022 at 10:30:37AM -0700, Linus Torvalds wrote:
> > On Thu, Oct 20, 2022 at 11:44 PM Greg Kroah-Hartman
> > <gregkh@...uxfoundation.org> wrote:
> > >
> > > The function handle_control_request() casts the urb buffer to a char *,
> > > and then treats it like a unsigned char buffer when assigning data to
> > > it.  On some architectures, "char" is really signed, so let's just
> > > properly set this pointer to a u8 to take away any potential problems as
> > > that's what is really wanted here.
> > 
> > I think you might as well also remove the cast that was always a bit odd:
> > 
> >                 buf[0] = (u8)dum->devstatus;
> > 
> > although maybe it's intentional ("look, ma, I'm truncating this
> > value") because 'devstatus' is a 'u16' type?
> 
> (adding Alan as he's the owner of this file now)
> 
> Yes, devstatus is a u16 as that's what the USB spec says it should be,
> but so far only 7 of the lower bits have been used.  I guess to do this
> properly we should also copy the upper 8 bits in to buf[1], eventhough
> in reality it's only ever going to be 0x00 for now.

I count only 5 of the bits being used, not 7.  (See Figure 9-4 in 
section 9.4.5 of the USB-3.1 spec.)  Maybe you're thinking of Feature 
flags rather than Status bits?  They do have a lot of overlap.

Dave Brownell wrote that code initially, so we'll never know why he 
included the cast.  My guess is the same as Linus's: It's there to alert 
the reader that a 16-bit value is being shortened to squeeze into an 
8-bit slot.

> Although if we ever do get another 2 status bits defined, this code will
> break so we probably should do that too.
> 
> I'll go do that for a v2 of this and properly comment it.

Note that there's an out-of-date comment line just above this part of 
the code:

		 * device: remote wakeup, selfpowered

Thanks to USB-3 support, the device recipient now defines three more 
bits in devstatus: LTM_ENABLED, U1_ENABLED, and U2_ENABLED.

Alan Stern