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

From: David Cohen
Date: Mon Nov 04 2013 - 13:58:04 EST

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
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at