Subject: Re: Tearing down DMA transfer setup after DMA client has finished
To: Geert Uytterhoeven <geert@linux-m68k.org>,
        Mans Rullgard <mans@mansr.com>
Cc: Vinod Koul <vinod.koul@intel.com>,
        Russell King <linux@arm.linux.org.uk>, dmaengine@vger.kernel.org,
        Linus Walleij <linus.walleij@linaro.org>,
        Dan Williams <dan.j.williams@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Jon Mason <jdmason@kudzu.us>, Mark Brown <broonie@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>, Lee Jones <lee.jones@linaro.org>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Maxime Ripard <maxime.ripard@free-electrons.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Peter Ujfalusi <peter.ujfalusi@ti.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Sebastian Frias <sf84@laposte.net>,
        Thibaud Cornic <thibaud_cornic@sigmadesigns.com>
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From: Mason <slash.tmp@free.fr>
Message-ID: <964975b8-5c00-9726-20e4-a1e801ce28d6@free.fr>
Date: Thu, 8 Dec 2016 13:17:08 +0100
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On 08/12/2016 13:03, Geert Uytterhoeven wrote:

> Måns Rullgård wrote:
>
>> Geert Uytterhoeven writes:
>>
>>> Can you fall back to PIO if requesting a channel fails?
>>
>> Why are we debating this nonsense?  There is an easy fix that doesn't
>> require changing the semantics of existing functions or falling back to
>> slow pio.
> 
> You still want to fall back to PIO if the DMA engine is not available
> at all (e.g. DMA engine driver not compiled in, or module not loaded).

FWIW, the ECC engine is tied to the DMA engine. So PIO means
not only taking a hit from tying up the CPU for a slooow
transfer, but also a huge hit if ECC must be computed in SW.
(A 100x perf degradation is not unlikely.)

Regards.