linux-kernel - Re: [PATCH 2/5] drivers: mmc: sunxi: limit A64 MMC2 to 8K DMA buffer

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Date:   Thu, 5 Jan 2017 23:33:28 +0000
From:   André Przywara <andre.przywara@....com>
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Rob Herring <robh@...nel.org>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>, Chen-Yu Tsai <wens@...e.org>,
        Hans De Goede <hdegoede@...hat.com>,
        Icenowy Zheng <icenowy@...c.xyz>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, linux-mmc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-sunxi@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] drivers: mmc: sunxi: limit A64 MMC2 to 8K DMA buffer

On 05/01/17 17:57, Maxime Ripard wrote:
> Hi Rob,
> 
> On Wed, Jan 04, 2017 at 08:07:50AM -0600, Rob Herring wrote:
>> On Mon, Jan 02, 2017 at 11:03:43PM +0000, Andre Przywara wrote:
>>> From: Maxime Ripard <maxime.ripard@...e-electrons.com>
>>>
>>> Unlike the A64 user manual reports, the third MMC controller on the
>>> A64 (and the only one capable of 8-bit HS400 eMMC transfers) has a
>>> DMA buffer size limit of 8KB (much like the very old Allwinner SoCs).
>>> This does not affect the other two controllers, so introduce a new
>>> DT compatible string to let the driver use different settings for that
>>> particular device. This will also help to enable the high-speed transfer
>>> modes of that controller later.
>>>
>>> Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
>>> Signed-off-by: Andre Przywara <andre.przywara@....com>
>>> ---
>>>  Documentation/devicetree/bindings/mmc/sunxi-mmc.txt | 1 +
>>>  drivers/mmc/host/sunxi-mmc.c                        | 7 +++++++
>>>  2 files changed, 8 insertions(+)
>>
>> Acked-by: Rob Herring <robh@...nel.org>
> 
> Some kind of a digression on this: we have three MMC controllers on
> this SoC. Like this patch shows, the third one is clearly different,
> and supports both more modes, a wider bus, and specific quirks. We
> need a new compatible for this one, everything's perfect.
> 
> However, the other two are mostly the same, but seems to need
> different tuning parameters to get more performances out of the
> controller (but this is unclear yet). How do we usually deal with
> that?

I guess you wanted to hear Rob's opinion ;-), but "get more performance"
sounds like we add one (or more) properties to tune those values.
If I get this right, it works with default values, but is sub-optimal?

Cheers,
Andre.