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

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Message-ID: <0C18FE92A7765D4EB9EE5D38D86A563A05D30D7A@SHSMSX103.ccr.corp.intel.com>
Date:	Thu, 19 May 2016 02:54:14 +0000
From:	"Du, Changbin" <changbin.du@...el.com>
To:	Michal Nazarewicz <mina86@...a86.com>,
	Felipe Balbi <felipe.balbi@...ux.intel.com>,
	Alan Stern <stern@...land.harvard.edu>
CC:	Al Viro <viro@...iv.linux.org.uk>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"rui.silva@...aro.org" <rui.silva@...aro.org>,
	"k.opasiak@...sung.com" <k.opasiak@...sung.com>,
	"lars@...afoo.de" <lars@...afoo.de>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] usb: gadget: f_fs: report error if excess data received

> 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.

> 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
3) drop the excess data, then print an warning message

Which one do you prefer?

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

Best Regards,
Du, Changbin