linux-kernel - Re: [PATCH RFC 2/3] mmc: sdhci-omap: Add using external dma

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Date:   Wed, 7 Nov 2018 11:00:57 +0800
From:   Chunyan Zhang <zhang.chunyan@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Kishon Vijay Abraham I <kishon@...com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-mmc@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>, Sekhar Nori <nsekhar@...com>,
        Lyra Zhang <zhang.lyra@...il.com>
Subject: Re: [PATCH RFC 2/3] mmc: sdhci-omap: Add using external dma

On Tue, 6 Nov 2018 at 20:51, Arnd Bergmann <arnd@...db.de> wrote:
>
> On 11/5/18, Kishon Vijay Abraham I <kishon@...com> wrote:
> > On 05/11/18 8:46 AM, Chunyan Zhang wrote:
> >>
> >> +    sdhci_switch_extdma(host, true);
> >
> > A number of devices using sdhci-omap supports ADMA. So switching to
> > external
> > DMA shouldn't be unconditional.
> >
> > IMHO sdhci.c should see if the device supports ADMA or SDMA. If not it
> > should
> > try switching to external DMA and if external DMA too is not supported, it
> > should use PIO.
>
> What's the reasoning for preferring ADMA/SDMA over external DMA if
> both are supported?
>
> I'd expect that if the external DMA for some reason is worse than
> ADMA, we just wouldn't list it in the DT at all, but if it's listed and
> ADMA also works, then the driver should try to use it before falling
> back to ADMA.

Thanks Arnd and Kishon, let's see what Andrian and Ulf would suggest,
and then I will address in next version of patchset.

Thanks again,
Chunyan

>
>        Arnd