linux-kernel - Re: [PATCH] usb: gadget: f_midi: Fixed a bug when buflen was smaller than wMaxPacketSize

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Date:	Fri, 01 Apr 2016 13:22:46 +0300
From:	Felipe Balbi <balbi@...nel.org>
To:	Felipe Ferreri Tonello <eu@...ipetonello.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	linux-usb@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	Robert Baldyga <r.baldyga@...sung.com>
Subject: Re: [PATCH] usb: gadget: f_midi: Fixed a bug when buflen was smaller than wMaxPacketSize


Hi,

Felipe Ferreri Tonello <eu@...ipetonello.com> writes:
> Hi Balbi and Mina,
>
> On 30/03/16 13:33, Michal Nazarewicz wrote:
>> On Wed, Mar 30 2016, Felipe Balbi wrote:
>>> a USB packet, right. that's correct. But a struct usb_request can
>>> point to whatever size buffer it wants and UDC is required to split
>>> that into wMaxPacketSize transfers.
>> 
>> D’oh.  Of course.  Disregard all my comments on the patch (except for
>> Ack).
>> 
>
> I didn't really get it. Does that mean that if buflen is multiple of
> wMaxPacketSize, the UDC driver should fit as many [DATA] packets into
> one usb_request and call complete() or it will always call complete() on
> each [DATA] packet, thus not requiring buflen at all?
>
> Does that mean that we can still use buflen and this patch is still
> valid? (besides the endianess issue that was addressed on v2)

if you have e.g. 2048 bytes of data to transfer and wMaxPacketSize is
e.g. 256 bytes, the UDC controller is required to do whatever it needs
to do to transfer 2048 bytes (or less if there's a short packet).

You don't need to break these 2048 bytes into several requests yourself,
the UDC is required to do that for the gadget.

-- 
balbi

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