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SubjectRe: [PATCHv2 2/2] check quirk to pad epout buf size when not aligned to maxpacketsize
Hi Alan, Michal,

On 11/11/2013 01:09 PM, Michal Nazarewicz wrote:
> On Mon, Nov 11 2013, Alan Stern wrote:
>> On Mon, 11 Nov 2013, Michal Nazarewicz 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: Michal Nazarewicz <mina86@mina86.com>
>>
>> I think this is still wrong.
>>
>>> @@ -824,7 +832,7 @@ static ssize_t ffs_epfile_io(struct file *file,
>>> req->context = &done;
>>> req->complete = ffs_epfile_io_complete;
>>> req->buf = data;
>>> - req->length = len;
>>> + req->length = data_len;
>>
>> IIUC, req->length should still be set to len, not to data_len.

I misunderstood the first time I read it:
In order to avoid DWC3 to stall, we need to update req->length (this is
the most important fix). kmalloc() is updated too to prevent USB
controller to overflow buffer boundaries.

>>
>>>
>>> ret = usb_ep_queue(ep->ep, req, GFP_ATOMIC);
>>>
>>> @@ -836,9 +844,16 @@ static ssize_t ffs_epfile_io(struct file *file,
>>> ret = -EINTR;
>>> usb_ep_dequeue(ep->ep, req);
>>> } else {
>>> + /*
>>> + * XXX We may end up silently droping data here.
>>> + * Since data_len (i.e. req->length) may be bigger
>>> + * than len (after being rounded up to maxpacketsize),
>>> + * we may end up with more data then user space has
>>> + * space for.
>>> + */
>>
>> Then this will never come up. If the host sends a packet that's too
>> long, you'll get a -EOVERFLOW error.
>>
>>> ret = ep->status;
>>> if (read && ret > 0 &&
>>> - unlikely(copy_to_user(buf, data, ret)))
>>> + unlikely(copy_to_user(buf, data, min(ret, len))))
>>> ret = -EFAULT;
>>> }
>>> }
>>
>> The reason for the quirk is that the controller may write all the
>> incoming data to the buffer, even if this is more data than the driver
>> requested.
>
> If that's the case, then it indeed solves the problem of silently
> throwing away data. I guess it makes more sense then my understanding
> of the quirk.

The main reason of this quirk it to prevent DWC3 to stall or to
overflow buffer after usb_ep_queue() above. Since req->length has to be
updated, I disagree with Alan in this case and give my ack to this
Michal's approach.

Br, David Cohen


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