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

[maxime.ripard@free-electrons.com (Maxime Ripard)]

On Thu, Jan 05, 2017 at 11:33:28PM +0000, Andr? Przywara wrote:
> 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@free-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@free-electrons.com>
> >>> Signed-off-by: Andre Przywara <andre.przywara@arm.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@kernel.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?

That would be my understanding too, at least, it works in a decent way
without fiddling with those parameters.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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