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From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Date:   Tue, 12 Feb 2019 01:06:00 +0100
Message-ID: <CAJ0CqmVFVBXi5E07-ZsYojC7mP4ogpwbcDkDTeebHwX+ayz2DQ@mail.gmail.com>
Subject: Re: [BUG] mt76x0u: Probing issues on Raspberry Pi 3 B+
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Stanislaw Gruszka <sgruszka@redhat.com>,
        Stefan Wahren <stefan.wahren@i2se.com>,
        Felix Fietkau <nbd@nbd.name>,
        Doug Anderson <dianders@chromium.org>,
        Minas Harutyunyan <hminas@synopsys.com>,
        USB list <linux-usb@vger.kernel.org>,
        linux-wireless <linux-wireless@vger.kernel.org>
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>
> On Mon, 11 Feb 2019, Stanislaw Gruszka wrote:
>
> > On Mon, Feb 11, 2019 at 10:12:57AM -0500, Alan Stern wrote:
> > > On Mon, 11 Feb 2019, Lorenzo Bianconi wrote:
> > >
> > > > Here it is a different issue respect to the AMD IOMMU one, dwc2 host driver
> > > > does not implement SG I/O so probing fails. I guess it is still useful to
> > > > implement a 'legacy' mode that enable mt76 on host controllers that do not implement
> > > > SG I/O (rpi is a very common device so it will be cool to have mt76 working on
> > > > it). Moreover we are not removing functionalities, user experience will remain
> > > > the same
> > >
> > > Has anyone considered adding SG support to dwc2?  It shouldn't be very
> > > difficult.  The corresponding change for ehci-hcd required adding no
> > > more than about 30 lines of code.
> >
> > That would be cool. Perhaps somebody with dwc2 hardware could do this.
> >
> > However in mt76x02u we possibly would like to support other usb host
> > drivers with sg_tablesize = 0 . I would like to clarify what is correct
> > to do with such drivers.
> >
> > Is ok to pass buffer via urb->sg with urb->num_sgs = 1 ? Or maybe
> > urb->num_sgs should be 0 to pass buffer via urb->sg on such drivers ?
> > Or maybe non of above is correct and the only option that will work
> > in 100% is pass buffer via urb->transfer_buffer ?
>
> See the kerneldoc for usb_sg_init(), usb_sg_wait(), and usb_sg_cancel()
> in drivers/usb/core/message.c.  These routines will always do the right
> thing -- however usb_sg_wait() must be called in process context.
>
> Alan Stern
>

Hi Alan,

I actually used usb_sg_init()/usb_sg_wait() as reference to implement
mt76u {tx/rx} datapath, but I will double-check.
I guess we should even consider if there are other usb host drivers
that do not implement SG I/O and it is worth to support.
I am wondering if the right approach is to add SG to the controller
one by one or have legacy I/O in mt76 (not sure what is the 'best'
approach)
What do you think?

Regards,
Lorenzo