linux-kernel - Re: [PATCH 3/4] phy: rockchip-emmc: configure default output tap delay

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Date:	Fri, 13 May 2016 15:25:30 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Brian Norris <briannorris@...omium.org>
Cc:	Kishon Vijay Abraham I <kishon@...com>,
	Heiko Stuebner <heiko@...ech.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	Shawn Lin <shawn.lin@...k-chips.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Brian Norris <computersforpeace@...il.com>
Subject: Re: [PATCH 3/4] phy: rockchip-emmc: configure default output tap delay

Hi,

On Thu, May 12, 2016 at 3:43 PM, Brian Norris <briannorris@...omium.org> wrote:
> The output tap delay controls helps maintain the hold requirements for
> eMMC. The exact value is dependent on the SoC and other factors, though
> it isn't really an exact science. But the default of 0 is not very good,
> as it doesn't give the eMMC much hold time, so let's bump up to 4
> (approx 90 degree phase?). If we need to configure this any further
> (e.g., based on board or speed factors), we may need to consider a
> device tree representation.

As I understand it, this solves much the same problem as my patch in
<https://patchwork.kernel.org/patch/9085581/>, but for the eMMC port
on rk3399 (which doesn't use dw_mmc).  As argued in that patch and
also in the discussion from
<https://patchwork.kernel.org/patch/9030621/>, if we eventually end up
needing to put something in the device tree we need to be really
careful.  Specifically to get the exact right value here I think you
need to consider the input clock, speed mode, and any SoC-specific
delays differences between the clock and the data lines.  That would
imply that, if anything, the device tree data would only contain
information about the SoC-specific delay differences and all other
work to set this value would involve coordination between the PHY and
the SDHCI controller.

However, as also discussed previously, we don't appear to need to be
very exact about the value here.  It seems like setting this to 4 (~90
degrees?) is a much better starting point than leaving it at the
default of 0.

...so I'd be all for landing this patch.  Perhaps Shawn can chime in
and confirm that our understanding is correct and possibly we can
update the commit message.  Then presumably someone at Rockchip can
keep working to find a better way to set this long term.

Sound good?

-Doug