linux-kernel - Re: [PATCH] USB: g_serial: Allow to override the default VID/PID

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Date:	Fri, 03 Dec 2010 23:09:33 +0100
From:	Jef Driesen <jefdriesen@...enet.be>
To:	Greg KH <greg@...ah.com>
CC:	Robert Lukassen <Robert.Lukassen@...tom.com>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	Greg Kroah-Hartman <gregkh@...e.de>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: g_serial: Allow to override the default VID/PID

On 30/11/10 18:46, Greg KH wrote:
> On Mon, Nov 22, 2010 at 11:20:55AM +0100, Robert Lukassen wrote:
>> Hi,
>>
>> I just revisited the patch I submitted and I can see that it needs
>> fixing. The intention of the patch was to provide the idVendor&
>> idProduct kernel module parameter overrides to the individual
>> functions at 'bind()' time. Prior to the patch, the overrides were
>> done after the bind, making it hard for individual functions to
>> determine the correct value for idVendor (some functions may need
>> to use that value, e.g. USB audio when it registers itself
>> as an ALSA device; it would want to use the same Vendor id).
>>
>> The patch makes sure that that happens, however, right after the bind()
>> the patched values are lost due to the copying of the gadget specific
>> device descriptor. Here's the snippet from 2.6.34:
>>
>>   999        usb_gadget_set_selfpowered(gadget);
>> 1000
>>
>> 1001        /* interface and string IDs start at zero via kzalloc.
>> 1002         * we force endpoints to start unassigned; few controller
>> 1003         * drivers will zero ep->driver_data.
>> 1004         */
>> 1005        usb_ep_autoconfig_reset(cdev->gadget);
>> 1006
>> 1007        /* composite gadget needs to assign strings for whole device (like
>> 1008         * serial number), register function drivers, potentially update
>> 1009         * power state and consumption, etc
>> 1010         */
>> 1011        status = composite->bind(cdev);
>> 1012        if (status<  0)
>> 1013                goto fail;
>> 1014
>> 1015        cdev->desc = *composite->dev;
>> 1016        cdev->desc.bMaxPacketSize0 = gadget->ep0->maxpacket;
>> 1017
>> 1018        /* standardized runtime overrides for device ID data */
>> 1019        if (idVendor)
>> 1020                cdev->desc.idVendor = cpu_to_le16(idVendor);
>> 1021        if (idProduct)
>> 1022                cdev->desc.idProduct = cpu_to_le16(idProduct);
>> 1023        if (bcdDevice)
>> 1024                cdev->desc.bcdDevice = cpu_to_le16(bcdDevice);
>> 1025
>>
>> In line 1015, the composite device descriptor is copied to the cdev->desc
>> field. Then the values get patched.
>>
>> My patch just moved lines 1018-1024, so that the functions could access
>> the patched values. The ill side effect is that these patched values
>> are lost when the *composite->dev (== device descriptor) is copied
>> in line 1015.
>>
>> It would seem that the patch should also have moved lines 1015-1016 in front
>> of the composite->bind(cdev) call.
>>
>> It would require all composite gadgets (such as serial.c) and all functions
>> (such as f_acm.c) to do any modifications to the device descriptor as passed
>> to them in the 'bind' call. For functions that is natural, as they should be
>> unaware of the gadget that uses them.
>>
>> The composite gadgets such as 'serial' seem to deal with it differently.
>> The serial.c gadget refers to the device descriptor as 'device_desc', and it
>> is a static struct. It references it in its 'init' function (which is ok, it is
>> prior to the registration point) and in the gs_bind() call. At that point it should
>> probably not be using 'device_desc', but cdev->desc.
>>
>> I think we can go one of two ways. I'd be happy to go over the composite gadgets and
>> change their use of their device descriptors, such that they use cdev->desc at
>> bind() time. Combining that with the moving the copying of the *composite->dev
>> to before calling 'composite->bind()' would make things consistent.
>>
>> The other way is reverting my original patch. For those functions that need access
>> to the overridden idVendor/idProduct code, we'll need to work out another
>> (generic) solution.
>
> So, do you want me to revert your original patch and wait for you to
> come up with a "better" solution?
>
> That seems like the correct thing to do at the moment.

If the original commit is reverted, the idVendor and idProduct module parameters 
can still be accessed through the global variables directly (like I did in my 
own pathc). Since access to those values appears to be the main goal of the 
patch, I think no functionality will be lost by reverting the commit.
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