linux-kernel - Re: [PATCH] usb: gadget: f_fs: report error if excess data received

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Date:	Thu, 19 May 2016 09:34:33 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	"Du\, Changbin" <changbin.du@...el.com>,
	Felipe Balbi <felipe.balbi@...ux.intel.com>,
	Alan Stern <stern@...land.harvard.edu>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	"gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
	"rui.silva\@linaro.org" <rui.silva@...aro.org>,
	"k.opasiak\@samsung.com" <k.opasiak@...sung.com>,
	"lars\@metafoo.de" <lars@...afoo.de>,
	"linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: gadget: f_fs: report error if excess data received

On Thu, May 19 2016, Changbin Du wrote:
>> On Wed, May 18 2016, Felipe Balbi wrote:
>> > we've been through this before. This needs to be done at the gadget
>> > layer. Gadget driver can over-allocate ahead of time if
>> > gadget->quirk_ep_out_aligned_size is true, then we avoid memcpy() at
>> > the UDC driver level.
>> 
>> Right, all right, so let’s look at it from a regular USB function point
>> of view.  If a USB function allocates a request which is not aligned,
>> UDC will align the buffer and *drop* excess data.  Seeing how ugly

> Do you mean UDC driver align the buffer? I searched the code, currently
> only DWC3 needs buffer size to be aligned to MaxPacketSize on ep out.
> And the align is done in f_fs driver.

I thought that was what was happening based on Felipe’s comment about
avoiding memcpy.  I looked at the code now and dunno what actually
happens.

>> f_fs’s code is becoming, I’m now leaning to letting to f_fs do the same
>> thing: if user space makes an unaligned read, f_fs aligns the buffer and
>> then drops excess data.
>> 
>> Any arguments for f_fs to not drop the data apply to UDC, so they should
>> behave identically.
>> 
> I'd prefer fail the request at all, and it is better done in HW.
> Because per the USB Spec that device can return NAK if a function was
> unable to accept data From the host.  The DWC3 has not been design as
> this, if software fail the transfer, it is a little weird for host.
>
> So, now we have 3 choices:
> 1) buffer the excess data
> 2) fail the transfer

You mean fail when more data has been sent (i.e. drop the whole packet)
or fail at entry to read() if the buffer is not aligned?

> 3) drop the excess data, then print an warning message
>
> Which one do you prefer?

I think f_fs should mimic whatever happens if unaligned request is
queued on dwc3.  As far as I understand, this is not 1.

I’ll be travelling again on Friday so I’ll finish up the patch doing 1
so we will have a choice between 1 (my patch) and 3 (your patch).

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»