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

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Date:   Tue, 9 Feb 2021 10:23:12 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     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