Date:   Wed, 13 Feb 2019 12:00:42 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To:     Stanislaw Gruszka <sgruszka@redhat.com>
Cc:     Felix Fietkau <nbd@nbd.name>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        lorenzo@kernel.org
Subject: Re: [PATCH 0/4] do not use sg if not properly supported by usb
 controller
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On Feb 13, Stanislaw Gruszka wrote:
> On Tue, Feb 12, 2019 at 11:09:03PM +0100, Lorenzo Bianconi wrote:
> > > > On Tue, Feb 12, 2019 at 04:08:37PM +0100, Lorenzo Bianconi wrote:
> > > > > > > > What unnecessary operation ?
> > > > > > > 
> > > > > > > the ones in mt76u_fill_bulk_urb()
> > > > > > 
> > > > > > Your patches also add extra operations on hotpath due to urb->num_sgs and
> > > > > > dev->usb.sg_en checks.
> > > > > 
> > > > > here I guess you should use mt76u_check_sg() instead of
> > > > > udev->bus->sg_tablesize > 0 so I think you need to precompute it since it is in
> > > > > the hotpath
> > > > 
> > > > It's not necessary. If udev->bus->sg_tablesize > 0 is true and rest
> > > > of mt76u_check_sg() is not, we will submit urb with urb->num_sgs = 1 to
> > > > usb host driver, what is fine. 
> > > > 
> > > > > Moreover the RFC series has been tested by multiple users and on multiple
> > > > > devices
> > > > 
> > > > I know. Perhaps you could test my patch on rpi ?
> > > 
> > > sure, I will do it later
> > 
> > I confirm that even with Stanislaw's patch the usb dongle is properly working
> > on rpi3+
> 
> Thanks for testing. Would be ok to you to post my patch against
> wireless-drivers tree and cc stable as fix for non-SG usb hosts,
> drop your set for -next and work for fix for SG issue on AMD IOMMU?

I agree that your patch works (since it does not use SG I/O :)) but
I think it is more clear (and manageable) to have two separated
routines for memory allocation.
Moreover I think that this check has to be done in the control plane instead
of the data plane, so I would like to spend some more time in order to see if it is
possible to remove some checks in the hot-path

Regards,
Lorenzo

> 
> Stanislaw