netdev - Re: [PATCH] qmi_wwan: support modify usbnet's rx_urb

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Date:   Mon, 03 Aug 2020 12:32:46 +0200
From:   Bjørn Mork <bjorn@...k.no>
To:     Daniele Palmas <dnlplm@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, yzc666@...ease.com,
        David Miller <davem@...emloft.net>, kuba@...nel.org,
        netdev@...r.kernel.org, linux-usb <linux-usb@...r.kernel.org>,
        carl <carl.yin@...ctel.com>
Subject: Re: [PATCH] qmi_wwan: support modify usbnet's rx_urb_size

Daniele Palmas <dnlplm@...il.com> writes:
> Il giorno lun 3 ago 2020 alle ore 10:18 Greg KH
> <gregkh@...uxfoundation.org> ha scritto:
>
>> Actually, no, this all should be done "automatically", do not change the
>> urb size on the fly.  Change it at probe time based on the device you
>> are using, do not force userspace to "know" what to do here, as it will
>> not know that at all.
>>
>
> the problem with doing at probe time is that rx_urb_size is not fixed,
> but depends on the configuration done at the userspace level with
> QMI_WDA_SET_DATA_FORMAT, so the userspace knows that.

Yes, but the driver "will know" (or "may assume") this based on the
QMI_WWAN_FLAG_MUX flag, as long as we are using the driver internal
(de)muxing.  We should be able to automatically set a sane rx_urb_size
value based on this?

Not sure if the rmnet driver currently can be used on top of qmi_wwan?
That will obviously need some other workaround.

> Currently there's a workaround for setting rx_urb_size i.e. changing
> the network interface MTU: this is fine for most uses with qmap, but
> there's the limitation that certain values (multiple of the endpoint
> size) are not allowed.

And this also requires an additional setup step for user/userspace,
which we should try to avoid if possible.

I'm all for a fully automatic solution.  I don't think rx_urb_size
should be directly configurable. And it it were, then it should be
implemented in the usbnet framework. It is not a qmi_wwan specific
attribute.

Bjørn