Date:   Tue, 12 Feb 2019 15:25:30 +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
Message-ID: <20190212142528.GB24197@localhost.localdomain>
References: <cover.1549977282.git.lorenzo@kernel.org>
 <20190212134550.GA12256@redhat.com>
 <20190212135100.GA15292@redhat.com>
 <CAJ0CqmVhH61P1V1fmhicPXwA7vHjv+Hh7h2DYbci1uDRJCyENA@mail.gmail.com>
 <20190212141725.GD12906@redhat.com>
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> On Tue, Feb 12, 2019 at 03:09:53PM +0100, Lorenzo Bianconi wrote:
> > >
> > > (repost with corrected Lorenzo email)
> > >
> > > On Tue, Feb 12, 2019 at 02:45:50PM +0100, Stanislaw Gruszka wrote:
> > > > On Tue, Feb 12, 2019 at 02:24:47PM +0100, LorenzoBianconilorenzo@kernel.org wrote:
> > > > > From: Lorenzo Bianconi <lorenzo@kernel.org>
> > > > >
> > > > > Use linear fragment and not a single usb scatter-gather buffer in mt76u
> > > > > {tx,rx} datapath if the usb controller has sg data length constraints.
> > > > > Moreover add disable_usb_sg module parameter in order to explicitly
> > > > > disable scatter-gather. SG I/O is not supported by all host drivers and
> > > > > some users have reported sg issues on AMD IOMMU.
> > > >
> > > > Again. This is not right approach. SG issues should be fixed
> > > > not workarounded.
> > 
> > Hi Stanislaw,
> > 
> > here we do not use SG, so num_sg is 0 and we use transfer_buffer. I do
> > not see how I am working around the issue.
> 
> By avoiding SG buffer allocation and configuration which most likely
> need to be fixed.

In my series I:
1- set num_sg to 0
2- use transfer_buffer

please correct me if I am wrong but in your solution you did the same since AFAIK
PageHighMem is always 0 so you end up setting num_sg to 0 and using
transfer_buffer as well. Is my understanding correct?

> 
> > Moreover with this approach we avoid some unnecessary operation in the hotpath
> 
> What unnecessary operation ?

the ones in mt76u_fill_bulk_urb()

Regards,
Lorenzo

> 
> Stanislaw