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: <20171108213909.mm4mnzga3vic3sbc@sasha-lappy>
Date:   Wed, 8 Nov 2017 21:39:11 +0000
From:   "Levin, Alexander (Sasha Levin)" <alexander.levin@....verizon.com>
To:     Mark Brown <broonie@...nel.org>
CC:     "Levin, Alexander (Sasha Levin)" <alexander.levin@....verizon.com>,
        "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 08:57:52PM +0000, Mark Brown wrote:
>On Wed, Nov 08, 2017 at 08:49:55PM +0000, Levin, Alexander (Sasha Levin) wrote:
>> From: Jiada Wang <jiada_wang@...tor.com>
>>
>> [ Upstream commit 66459c5a50a787c474b734b586328f7111ab6df0 ]
>>
>> Previously DMA watermark level is configured to fifosize/2,
>> DMA mode can be used only when transfer length can be divided
>> by 'watermark level * bpw', which makes DMA mode not pratical.
>>
>> This patch adjusts watermark level to largest number (no bigger
>> than fifosize/2) which can divide 'tranfer length / bpw' for
>> each transfer.
>
>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, the impact is "just" performance, but doesn't it result in quite
a significat impact?

-- 

Thanks,
Sasha