Date:   Wed, 16 Jan 2019 12:19:46 +0100
From:   Stanislaw Gruszka <sgruszka@redhat.com>
To:     Lorenzo Bianconi <lorenzo.bianconi@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: <20190116111946.GB5234@redhat.com>
References: <cover.1547549771.git.lorenzo.bianconi@redhat.com>
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 <20190115154746.GA30564@localhost.localdomain>
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On Tue, Jan 15, 2019 at 04:47:47PM +0100, Lorenzo Bianconi wrote:
> Hi Stanislaw,

Hi :-)

> > 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.

So there if diffrence for USB host driver when we have one usb->sg
sengment and if we just pass the buffer via urb->transfer_buf . I think
most USB host drivers behave the same in such cases. For what USB
hardware/driver this is needed ? Perhaps simpler fix could be done
in USB host driver?

Also I'm not sure for what this new module parameter is needed ?
 
> @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.

Ok, having separe URB for each fragment make no sense since we have
embedded sg structure in URB.

Regards
Stanislaw