Re: [PATCH v3 2/3] usb: ffs: check quirk to pad epout buf size whennot aligned to maxpacketsize

[David Cohen]

Hi Alan,

Appreciate your comments. Please, see my reply.

On 10/30/2013 10:35 AM, Alan Stern wrote:
> On Wed, 30 Oct 2013, David Cohen wrote:
>
> > Check gadget.quirk_ep_out_aligned_size to decide if buffer size requires
> > to be aligned to maxpacketsize of an out endpoint. ffs_epfile_io() needs
> > to pad epout buffer to match above condition if quirk is found.
> >
> > Signed-off-by: David Cohen <david.a.cohen@xxxxxxxxxxxxxxx>
> > ---
> >   drivers/usb/gadget/f_fs.c | 17 +++++++++++++++++
> >   1 file changed, 17 insertions(+)
> >
> > diff --git a/drivers/usb/gadget/f_fs.c b/drivers/usb/gadget/f_fs.c
> > index 75e4b78..b49dd55 100644
> > --- a/drivers/usb/gadget/f_fs.c
> > +++ b/drivers/usb/gadget/f_fs.c
> > @@ -755,10 +755,12 @@ static ssize_t ffs_epfile_io(struct file *file,
> >   			     char __user *buf, size_t len, int read)
> >   {
> >   	struct ffs_epfile *epfile = file->private_data;
> > +	struct usb_gadget *gadget = epfile->ffs->gadget;
> >   	struct ffs_ep *ep;
> >   	char *data = NULL;
> >   	ssize_t ret;
> >   	int halt;
> > +	size_t orig_len = len;
> >
> >   	goto first_try;
> >   	do {
> > @@ -794,6 +796,21 @@ first_try:
> >   			goto error;
> >   		}
> >
> > +		/*
> > +		 * Controller requires buffer size to be aligned to
> > +		 * maxpacketsize of an out endpoint.
> > +		 */
> > +		if (gadget->quirk_ep_out_aligned_size && read &&
> > +		    !IS_ALIGNED(len, ep->ep->desc->wMaxPacketSize)) {
>
> IS_ALIGNED works only when the second argument is a power of 2.
>
> Interrupt endpoints are not required to have maxpacket sizes that are
> powers of 2.  Does this code ever get used for an interrupt endpoint?

That's a good point. It won't use interrupt ep on the case I am working
on, but f_fs allows it. I'll cover both cases in next patchset.

> > +			size_t old_len = len;
>
> Why add old_len here when you added orig_len above?

See below.

> > +			len = roundup(orig_len,
> > +				      (size_t)ep->ep->desc->wMaxPacketSize);
> > +			if (unlikely(data) && len > old_len) {
>
> If the original value wasn't aligned, how can len fail to be > old_len?

This code is inside a loop. There is an unlikely case explained in the
end to loop again if ep got disabled or changed while acquiring mutex.
If that happens, I want to avoid to re-allocate buffer if previous size
was already enough. Since 'len' gets updated when aligning buf size, we
need to keep track of 'orig_len' to do only minimum necessary pad on
new alignment. 'old_len' is used to save previous value to decide for
new allocation or not. But maybe I should change it to something more
accurate like 'buf_size' or 'alloc_size'.

Br, David Cohen
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