Date:   Tue, 15 Jan 2019 16:47:47 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
To:     Stanislaw Gruszka <sgruszka@redhat.com>
Cc:     nbd@nbd.name, linux-wireless@vger.kernel.org, rosenp@gmail.com
Subject: Re: [RFC 0/4] do not use sg if not properly supported by usb
 controller
Message-ID: <20190115154746.GA30564@localhost.localdomain>
References: <cover.1547549771.git.lorenzo.bianconi@redhat.com>
 <20190115153546.GA8387@redhat.com>
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> On Tue, Jan 15, 2019 at 01:33:41PM +0100, Lorenzo Bianconi wrote:
> > 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. Some users have reported sg issues on AMD IOMMU

Hi Stanislaw,

> 
> Not sure what is the problem , but this patch set look like a workaround
> not fix. If this an issue with IOMMU and sg, seems there is something wrong
> in sg page mappings eigher on mt76 dirver or IOMMU driver.

The main point here I guess is we do not need sg if fragment number is one (e.g
usb2.0). Moreover this can fix IOMMU reported issues.

@Rosen: could you please try this series enabling IOMMU?

> 
> If things need to be fixed in mt76 I whould check if page mappings for
> sg are correct. Or remove sg usage from mt76_usb completly, mt76 MMIO
> version do not use sg for framgments, so most likely USB don't need it
> as well.

usb scatter-gather is used to properly support non-linear skbs (A-MSDU,
with usb3.0) since the hw (unlike pci counterpart) does not support it,
so we need it.

Regards,
Lorenzo

> 
> Thanks
> Stanislaw