Date:   Wed, 16 Jan 2019 13:21:46 +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: <20190116122145.GA7472@redhat.com>
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On Wed, Jan 16, 2019 at 12:44:33PM +0100, Lorenzo Bianconi wrote:
> > 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?
> 
> According to https://github.com/torvalds/linux/blob/master/drivers/usb/core/hcd.c#L1557
> single sg urb and urb with a configured transfer_buf are managed in a different way.

But this should not make any difference for underlying low level USB
host driver, since we map 1 buffer of the same size, just by using
different routines for that.

> Please not I have not received any confirm that this series fixes the reported issue
> yet :)

What is reported issue ?

Thanks
Stanislaw