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From: Ulf Hansson <ulf.hansson@linaro.org>
To: Kishon Vijay Abraham I <kishon@ti.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Chunyan Zhang <zhang.chunyan@linaro.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>,
	"linux-mmc@vger.kernel.org" <linux-mmc@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Mark Brown <broonie@kernel.org>, Sekhar Nori <nsekhar@ti.com>,
	Chunyan Zhang <zhang.lyra@gmail.com>
Subject: Re: [PATCH RFC 2/3] mmc: sdhci-omap: Add using external dma
Date: Mon, 19 Nov 2018 17:32:25 +0100	[thread overview]
Message-ID: <CAPDyKFpomN9KyX5u8C9-gBwBNm-sdfHwxbDVYD0hs73WFJR53A@mail.gmail.com> (raw)
In-Reply-To: <44e1a97e-0446-283f-72f7-fd8d39a74b0f@ti.com>

On 9 November 2018 at 06:27, Kishon Vijay Abraham I <kishon@ti.com> wrote:
> Hi Arnd,
>
> On 06/11/18 6:21 PM, Arnd Bergmann wrote:
>> On 11/5/18, Kishon Vijay Abraham I <kishon@ti.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?
>
> Generally from our experiments we've found ADMA gives better throughput than
> DMA. I have to ascertain the actual reasons for that.
>>
>> 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.
>
> I would agree that for newer dtbs. However if you consider omap_hsmmc, external
> DMA bindings are already added in dt. If we try to switch to sdhci-omap with
> the same bindings, systems with older dtbs will use external DMA and give
> lesser throughput.

I was under the impression that the internal DMA (for old versions) is
rather poor.

However, in the end this is a software policy decision of what to use.
I have no strong opinion, so feel free to pick what you think makes
best sense.

Maybe Adrian also have some thoughts?

Kind regards
Uffe

  reply	other threads:[~2018-11-19 16:33 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-05  3:16 [PATCH RFC 0/3] Add support for using external dma in SDHCI Chunyan Zhang
2018-11-05  3:16 ` [PATCH RFC 1/3] mmc: sdhci: add support for using external DMA devices Chunyan Zhang
2018-11-06 11:16   ` Arnd Bergmann
2018-11-07  2:56     ` Chunyan Zhang
2018-11-07 14:13   ` Adrian Hunter
2018-11-05  3:16 ` [PATCH RFC 2/3] mmc: sdhci-omap: Add using external dma Chunyan Zhang
2018-11-05  4:25   ` Kishon Vijay Abraham I
2018-11-06 12:51     ` Arnd Bergmann
2018-11-07  3:00       ` Chunyan Zhang
2018-11-09  5:27       ` Kishon Vijay Abraham I
2018-11-19 16:32         ` Ulf Hansson [this message]
2018-11-05  3:16 ` [PATCH RFC 3/3] dt-bindings: sdhci-omap: Add example for using dmaengine Chunyan Zhang

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