[PATCH v2 4/4] arm64: dts: rockchip: use cs-gpios for cros_ec_spi

[jeffy]

Hi doug,

Thanx for your comments.

On 06/23/2017 06:47 AM, Doug Anderson wrote:
> Hi,
>
> On Mon, Jun 19, 2017 at 5:47 PM, Brian Norris <briannorris at chromium.org> wrote:
>> Hi Mark,
>>
>> Forgot to follow up here:
>>
>> On Tue, Jun 13, 2017 at 07:22:25PM +0100, Mark Brown wrote:
>>> On Tue, Jun 13, 2017 at 10:50:44AM -0700, Brian Norris wrote:
>>>> On Tue, Jun 13, 2017 at 01:25:43PM +0800, Jeffy Chen wrote:
>>>>> The cros_ec requires CS line to be active after last message. But the CS
>>>>> would be toggled when powering off/on rockchip spi, which breaks ec xfer.
>>>>> Use GPIO CS to prevent that.
>>>
>>>> I suppose this change is fine. (At least, I don't have a good reason not
>>>> to do this.)
>>>
>>>> But I still wonder whether this is something that the SPI core can be
>>>> expected to handle. drivers/mfd/cros_ec_spi.c already sets the
>>>> appropriate trans->cs_change bits, to ensure CS remains active in
>>>> between certain messages (all under spi_bus_lock()). But you're
>>>> suggesting that your bus controller may deassert CS if you runtime
>>>> suspend the device (e.g., in between messages).
>>>
>>>> So, is your controller just peculiar? Or should the SPI core avoid
>>>> autosuspending the bus controller when it's been instructed to keep CS
>>>> active? Any thoughts Mark?
>>>
>>> This sounds like the controller being unusual - though frankly the
>>> ChromeOS chip select usage is also odd so it's fairly rare for something
>>> like this to come up.  I'd not expect a runtime suspend to loose the pin
>>> state, though possibly through use of pinctrl rather than the
>>> controller.
>>
>> I haven't personally verified this behavior (it probably wouldn't be too
>> hard to rig up a test driver to hold CS low while allowing the
>> controller to autosuspend? spidev can do this?), but Rockchip folks seem
>> to have concluded this.
>>
>> I suppose I'm fine with relying on cs-gpios as a workaround.
>
> I'm similarly hesitant to rely on cs-gpios as a workaround, though I
> won't directly stand in its way...  ...it seems like it would be
> slightly better to actually add a runtime_suspend() callback and
> adjust the pinmux dynamically (that would allow us to use the hardware
> chip select control if we ever enable that in the driver), but I'm not
> sure all the extra work to do that is worth it.
>
> It feels a little bit to me like the workaround here doesn't belong in
> the board's device tree file, though.  This is a quirk of the SoC's
> SPI controller whenever it's runtime suspended.  Any board using this
> SPI could possibly be affected, right?
hmm, so i should add cs_gpio to all rockchip boards right?
>
>
> Oh wait (!!!!)
>
>
> Let's think about this.  Let me ask a question.  When you runtime
> suspend the SPI part (and turn off the power domain) but don't
> configure pins to be GPIO, what happens?  I'm assuming it's one of
> three things:
>
> 1. The line is driven a certain direction (probably low).  This seems unlikely.
>
> 2. The line is no longer driven by the SPI controller and thus the
> pin's pulls take effect.  This seems _likely_.
>
> 3. The line is no longer driven by the SPI controller and somehow the
> pulls stop taking effect.  This seems unlikely.
>
>
> ...I'll assume that #2 is right (please correct if I'm wrong).
> Thinking about that makes me think that we need to worry not just
> about the chip select line but about the other SPI lines too, right?
> AKA if the SPI controller stops driving the chip select line, it's
> probably also not driving MISO, MOSI, or TXD.
>
> ...so looking at all the SPI lines, they all have pullup configured in
> the "default" mode in rk3399.dtsi.
>
> ...and looking as "cros_ec_spi.c", I see that we appear to be using MODE_0.
>
>
> That means if you runtime suspend while the cros EC code was running
> and none of your patches have landed, all lines will float high.
>
> 1. Chip select will be deasserted (this is the problem you're trying to solve).
>
> 2. Data line and clock line will get pulled high.
>
> Using spi.h, MODE_0 means SPI_CPOL=0 and SPI_CPHA=0.  Using Wikipedia
> (which is never wrong, of course), that means data is captured on the
> clock's rising edge.  Thus we'll actually clock one bit of data here,
> but at the same time that we try to turn off chip select.
>
>
> ...now we look at your proposed solution and we'll leave chip select
> on, but we'll still clock one bit of data (oops).  ...or, I guess, if
> the EC itself has pulls configured we might be in some state where the
> different pulls are fighting, but that still seems non-ideal.
>
> ---
>
> So how do we fix this?  IMHO:
>
> Add 4 new pinctrl states in rk3399.dtsi:
>
>    cs_low_clk_low, cs_low_clk_high, cs_high_clk_low, cs_high_clk_high
>
> These would each look something like this:
>
> spi5_cs_low_data_low: spi5-cs-low-data-low {
>    rockchip,pins = <2 22 RK_FUNC_0 &pcfg_output_low>,
>      <2 23 RK_FUNC_0 &pcfg_output_low>;
> };
>
> Where "pcfg_output_low" would be moved from the existing location in
> "rk3399-gru.dtsi" to the main "rk3399.dtsi" file.
>
>
> ...now, you'd define runtime_suspend and runtime_resume functions
> where you'd look at the current state of the chip select and output
> and select one of these 4 pinmuxes so that things _don't_ change.
>
> If the pinmuxes didn't exist you'd simply deny PM Runtime Transitions.
> That would nicely take care of the backward compatibility problem.
> Old DTS files would work, they just wouldn't be able to Runtime PM
> properly.
so we should use runtime pinmuxes to fix this issue, and also support 
cs_gpio as an option right?
>
> ---
>
> Anyway, maybe that's all crazy.  What do others think?
>
>
> -Doug
>
>
>