linux-kernel - Re: [PATCH AUTOSEL for-4.9 04/53] spi: imx: adjust watermark level according to transfer length

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Message-ID: <20171108221102.utzwtwcg2k4clyka@sirena.co.uk>
Date:   Wed, 8 Nov 2017 22:11:02 +0000
From:   Mark Brown <broonie@...nel.org>
To:     "Levin, Alexander (Sasha Levin)" <alexander.levin@....verizon.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Jiada Wang <jiada_wang@...tor.com>
Subject: Re: [PATCH AUTOSEL for-4.9 04/53] spi: imx: adjust watermark level
 according to transfer length

On Wed, Nov 08, 2017 at 09:39:11PM +0000, Levin, Alexander (Sasha Levin) wrote:
> >On Wed, Nov 08, 2017 at 08:49:55PM +0000, Levin, Alexander (Sasha Levin) wrote:
> >This seems like an optimization not a bug fix...

> Hm, is it? I read it as "DMA is not being used at all even though we
> thought we're using it".

Yes, that's how I read it too.  

> Yes, the impact is "just" performance, but doesn't it result in quite
> a significat impact?

Only about double according to the initial commit adding DMA support
which is frankly a bit disappointing although yeah, it's a big win.  My
worry is that if there's a problem with DMA on some device for which a
fix wasn't backported (or where we're using a fallback) this could
expose problems if we start using it.  If you look at the history of the
driver there's some quirks were added later on for example, and I didn't
check the DMA controller drivers or anything and obviously can't see any
out of tree code users may have.

*Probably* it doesn't break anything but since it's not fixing anything
and the risk is data corruption I'd be much more comfortable with a more
thorough risk analysis.

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