QMI and MBIM: Do you want a "Max Command Size" ioctl?

[Aleksander Morgado]

On 03/13/2013 10:49 AM, Bjørn Mork wrote:
> Hello,
> 
> this question is relevant for both QMI and MBIM support. You do need to
> know the exact message size used by the device (and driver) when using
> /dev/cdc-wdmX devices.  This is particularily important for MBIM, where
> different devices used varying size and will fail if you use the wrong
> value.  Currently you have to parse the MBIM descriptors to retrieve
> it.  But it is also relevant for QMI, where you should use whatever the
> driver uses and have no other way to find out than reading the driver
> source and copy the value from there.
> 
> The obvious simpler alternative is to make the driver export the value
> to userspace.
> 
> We're currently discussing if and how such an userspace API should be
> added to cdc-wdm.c, effectively implementing it for all the three
> drivers qmi_wwan, cdc_mbim and cdc-wdm.
> 
> The current proposal [1] is adding a new ioctl.
> 
> As an userspace API is carved in diamond when it is added, we need to be
> absolutely sure this is the right thing to do before going forward.
> Greg KH has requested [2] acks from the userspace authors:
> 
>  " Oh, also get the
>    userspace authors to agree and sign-off on this, I want their agreement
>    that this is something they can support.
>  "
> 
> As for the discussion of sysfs vs ioctl, I believe Oliver summarized [3]
> the advantages of the ioctl quite nicely.
> 
> There are more details in the rest of the thread if anyone is
> interested, including an alternative sysfs implementation.
> 
> Sample usage will of course be quite simple from any C application:
> 
> #include <linux/usb/cdc-wdm.h>
>         __u16 msgsize;
>         int fd = open("/dev/cdc-wdm0", O_RDWR);
> 	if (ioctl(fd, IOCTL_WDM_MAX_COMMAND, &msgsize) < 0) {
>            /* then fall back to whatever you do today, using
>             * a fixed value or parsing USB descriptors
>             */
>         }
> 
> The questions for you as userspace authors are:
>  a) Should these drivers export "max message size" to userspace?
>  b) Is an ioctl the best solution?
>  c) Will you use the ioctl in your userspace implementation?
> 
> I really appreciate any feedback, including "We do not want this at
> all".  Thanks.
> 

We do really need max message size for MBIM; and as you say, it may be
good to have the max message size info also for QMI, so the new ioctl
seems a good addition. So +1 from my side, for what it's worth.

-- 
Aleksander