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From: Kishon Vijay Abraham I <kishon@ti.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Chunyan Zhang <zhang.chunyan@linaro.org>,
	Ulf Hansson <ulf.hansson@linaro.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	<linux-mmc@vger.kernel.org>, <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: Fri, 9 Nov 2018 10:57:45 +0530	[thread overview]
Message-ID: <44e1a97e-0446-283f-72f7-fd8d39a74b0f@ti.com> (raw)
In-Reply-To: <CAK8P3a2X4tbx_R-5VgfApF-UPmJ_XuzH3hO_uVAfffC6h8WrJw@mail.gmail.com>

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.

Thanks
Kishon

  parent reply	other threads:[~2018-11-09  5:28 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 [this message]
2018-11-19 16:32         ` Ulf Hansson
2018-11-05  3:16 ` [PATCH RFC 3/3] dt-bindings: sdhci-omap: Add example for using dmaengine Chunyan Zhang

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