linux-kernel - RE: DMA direct mapping fix for 5.4 and earlier stable branches

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Date:   Tue, 9 Feb 2021 10:19:07 +0000
From:   <obayashi.yoshimasa@...ionext.com>
To:     <gregkh@...uxfoundation.org>
CC:     <sumit.garg@...aro.org>, <hch@....de>, <m.szyprowski@...sung.com>,
        <robin.murphy@....com>, <iommu@...ts.linux-foundation.org>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
        <daniel.thompson@...aro.org>
Subject: RE: DMA direct mapping fix for 5.4 and earlier stable branches

> How do you judge "mature"?

  My basic criteria are
* Function is exist, but bug fix is necessary: "mature"
* Function extension is necessary: "immature"

> And again, if a feature isn't present in a specific kernel version, why would you think that it would be
> a viable solution for you to use?

  So, "a feature isn't present in a specific kernel version", 
also means "immature" according to my criteria.

Regards.

> -----Original Message-----
> From: Greg KH <gregkh@...uxfoundation.org>
> Sent: Tuesday, February 9, 2021 6:23 PM
> To: Obayashi, Yoshimasa/尾林 善正 <obayashi.yoshimasa@...ionext.com>
> Cc: sumit.garg@...aro.org; hch@....de; m.szyprowski@...sung.com; robin.murphy@....com;
> iommu@...ts.linux-foundation.org; linux-kernel@...r.kernel.org; stable@...r.kernel.org;
> daniel.thompson@...aro.org
> Subject: Re: DMA direct mapping fix for 5.4 and earlier stable branches
> 
> On Tue, Feb 09, 2021 at 09:05:40AM +0000, obayashi.yoshimasa@...ionext.com wrote:
> > > > As the drivers are currently under development and Socionext has
> > > > chosen 5.4 stable kernel for their development. So I will let
> > > > Obayashi-san answer this if it's possible for them to migrate to
> > > > 5.10 instead?
> >
> >   We have started this development project from last August, so we
> > have selected 5.4 as most recent and longest lifetime LTS version at
> > that time.
> >
> >   And we have already finished to develop other device drivers, and
> > Video converter and CODEC drivers are now in development.
> >
> > > Why pick a kernel that doesn not support the features they require?
> > > That seems very odd and unwise.
> >
> >   From the view point of ZeroCopy using DMABUF, is 5.4 not mature
> > enough, and is 5.10 enough mature ?
> >   This is the most important point for judging migration.
> 
> How do you judge "mature"?
> 
> And again, if a feature isn't present in a specific kernel version, why would you think that it would be
> a viable solution for you to use?
> 
> good luck!
> 
> greg k-h