Date:   Wed, 20 Feb 2019 17:22:18 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To:     Stanislaw Gruszka <sgruszka@redhat.com>
Cc:     Felix Fietkau <nbd@nbd.name>,
        Stefan Wahren <stefan.wahren@i2se.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Doug Anderson <dianders@chromium.org>,
        Minas Harutyunyan <hminas@synopsys.com>,
        USB list <linux-usb@vger.kernel.org>,
        linux-wireless <linux-wireless@vger.kernel.org>
Subject: Re: [BUG] mt76x0u: Probing issues on Raspberry Pi 3 B+
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> On Wed, Feb 20, 2019 at 02:22:08PM +0100, Lorenzo Bianconi wrote:
> > > On Tue, Feb 19, 2019 at 01:19:26PM +0100, Felix Fietkau wrote:
> > > > >> >> The way I see it, we have two choices.
> > > > >> >> 1. Fix dwc2 to do its alignment quirk for the urb->sg != NULL case
> > > > >> >> 2. Rely on urb->transfer_buffer and keep urb->sg NULL
> > > > >> > 
> > > > >> > I agree, if this is only needed for dwc2. Though I would investigate
> > > > >> > if this is not a bug on other platforms as well.
> > > > >> >From what I can see, using Lorenzo's patches seems to be the better
> > > > >> solution, since they avoid these corner cases in dwc2 (and maybe other
> > > > >> drivers as well). I will apply them and then we'll see if we need to do
> > > > >> any further improvements later on.
> > > > > 
> > > > > They work on rpi dwc2, but they do not address root of the problem.
> > > > > There is clearly something wrong how mt76usb handle SG, what is not
> > > > > fixed. And adding disable_usb_sg module parameter for hcd's supporting
> > > > > SG should be red flag.
> > > > I think we're simply dealing with multiple issues here, only some of
> > > > which are fixed by Lorenzo's patches.
> > > > I'm pretty sure it's still wrong for mt76 to try to align its buffers,
> > > > since the Linux USB API supports non-aligned transfer buffers and it
> > > > should be up to the controller driver to deal with that.
> > > 
> > > Agree.
> > > 
> > > > dwc2 tries to do that, but that has limitations which I already pointed
> > > > out and which are properly dealt with by Lorenzo's patches.
> > > 
> > > I planed to just accept current solution, but I started to work on patch
> > > that remove len, sglen arguments from mt76u_buf_alloc() and use
> > > q->buf_size and SKB_WITH_OVERHEAD(q->buf_size) directly and realized how
> > > related code is now tangled.
> > > 
> > > Would be ok to send this patch with proper changelog as fix for RPI
> > > against wireless-drivers and cc:stable (assuming it works and really
> > > fix things on RPI) and revert Lorenzo's patches in -next ?
> > 
> > Hi Stanislaw,
> > 
> > what is the advantage of doing so?
> To have small fix proper for -stable to fix the problem in 4.20 and 4.19,
> and have simpler code.

merging the series I sent we will have a pretty simple approach, just a
single routine that allocates the rx buffers in the control path according to
the operating mode.

> 
> > You have duplicated most of the fields that are
> > already in the urb data structure and you use transfer_buffer (no SG I/O).
> URB has plenty of fields, I duplicated 2. If size of mt76u_buf is a concern
> this can be optimized by packing num_sgs, len, done into fields variable.
> 
> > Moreover I have ready a series that removes 99% of the dual allocation code and
> > maintain it in the control path (instead of the datapath one).
> > I need just to rebase it ontop of your series. I will post it soon.
> So I would ask what is the point to adding bunch of code and remove it in very
> next patch?

In the first series I fixed the issue, in this one I improved the code, I have
no added any new feature

Regards,
Lorenzo

> 
> Stanislaw