Re: [PATCH 1/2] net: designware: eqos: reset phy

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

On 08.06.21 10:58, Sascha Hauer wrote:
> On Tue, Jun 08, 2021 at 09:31:03AM +0200, Ahmad Fatoum wrote:
>> Hello Sascha,
>>
>> On 08.06.21 00:22, Sascha Hauer wrote:
>>> On Mon, Jun 07, 2021 at 05:59:02PM +0200, Ahmad Fatoum wrote:
>>>> Hello Sascha,
>>>>
>>>> On 07.06.21 16:10, Sascha Hauer wrote:
>>>>> The designware eqos DT binding has support for specifying reset GPIOs.
>>>>> Add support for them.
>>>>>
>>>>> Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
>>>>> ---
>>>>>  drivers/net/designware_eqos.c | 33 +++++++++++++++++++++++++++++++++
>>>>>  drivers/of/of_gpio.c          |  7 +++++++
>>>>>  2 files changed, 40 insertions(+)
>>>>>
>>>>> diff --git a/drivers/net/designware_eqos.c b/drivers/net/designware_eqos.c
>>>>> index d2baaeaf63..0321024169 100644
>>>>> --- a/drivers/net/designware_eqos.c
>>>>> +++ b/drivers/net/designware_eqos.c
>>>>> @@ -8,9 +8,11 @@
>>>>>  
>>>>>  #include <common.h>
>>>>>  #include <init.h>
>>>>> +#include <gpio.h>
>>>>>  #include <dma.h>
>>>>>  #include <net.h>
>>>>>  #include <of_net.h>
>>>>> +#include <of_gpio.h>
>>>>>  #include <linux/iopoll.h>
>>>>>  #include <linux/time.h>
>>>>>  #include <linux/sizes.h>
>>>>> @@ -189,6 +191,33 @@ struct eqos_desc {
>>>>>  
>>>>>  #define MII_BUSY		(1 << 0)
>>>>>  
>>>>> +static int eqos_phy_reset(struct device_d *dev, struct eqos *eqos)
>>>>> +{
>>>>> +	int phy_reset;
>>>>> +	int ret;
>>>>> +	u32 delays[3] = { 0, 0, 0 };
>>>>> +
>>>>> +	phy_reset = of_get_named_gpio(dev->device_node, "snps,reset-gpio", 0);
>>>>> +
>>>>> +        if (!gpio_is_valid(phy_reset))
>>>>> +		return 0;
>>>>
>>>> Whitespace is wrong.
>>>>
>>>>> +
>>>>> +	ret = gpio_request(phy_reset, "phy-reset");
>>>>> +	if (ret)
>>>>> +		return ret;
>>>>
>>>> Can you use gpiod_get instead? This would reduce the boilerplate here.
>>>
>>> Sure. I didn't realize I don't honour the active high/low flags the way
>>> I did it.
>>>
>>>>
>>>>> +
>>>>> +	of_property_read_u32_array(dev->device_node,
>>>>> +				   "snps,reset-delays-us",
>>>>> +				   delays, ARRAY_SIZE(delays));
>>>>> +
>>>>
>>>> Looks strange to read out a delay and not act on it. I'd prefer
>>>> waiting delays[0] here.
>>>
>>> Yes, it looks strange, but that's because the binding doesn't make much
>>> sense. Why should I insert a delay before doing anything?
>>
>>                    .--------.
>>
>>      POR --------->|R  flip |---- Regulator ----> PHY VDD
>>
>>                 .->|S  flop |
>>
>>                 |  `--------'
>>
>>                 |   
>>
>>                 |   
>>
>>                 |
>>
>> RESET GPIO -----'`-------------------------------> PHY Reset
>>
>> (active low)
>>
>> It's stupid, but it works with Linux and wouldn't with barebox
>> (if PHY VDD takes too long to stabilize)... ^^'
> 
> What we are doing is:
> 
> 1) RESET_GPIO has unknown state
> 2) gpio_request()
> 3) RESET_GPIO has the same unknown state

Linux requests with devm_gpiod_get_optional(..., GPIOD_OUT_LOW),

so it's inactive here (== HIGH, which enables flip flop)

> 4) udelay(delays[0])
> 5) RESET_GPIO still has the same unknown state

Voltage has a while to stabilize while RESET GPIO is inactive here.

> 6) gpio_set_active()
> 7) RESET_GPIO puts phy into reset
> 
> Your FLIPFLOP will also only set its output at step 7), so your
> regulator stabilization time begins at 7), you would still have
> to make delays[1] long enough for the regulator to stabilize and
> the phy come up.
> What matters here is the point where we put RESET_GPIO into a known
> state. Whether you do step 4) or leave it out makes no difference.
> 
>>
>>> I can a delay here, it wouldn't make much difference as all users except
>>> one specify zero delay here anyway. For the one exception I would bet
>>> someone has inserted the first delay without a good reason, they are all
>>> 10000.
>>
>> That's probably true. I still think mimicking Linux' interpretation
>> of bindings is a good general rule to follow.
> 
> As long as they make sense, yes.
> 
>>
>>>>
>>>>> +	gpio_direction_active(phy_reset, 0);
>>>>
>>>> This always sets logical zero, because gpio_request above doesn't
>>>> accept a flag telling it otherwise. You'll need of_get_named_gpio_flags
>>>> here, at which point, you'll have basically open-coded gpiod_get(), so
>>>> you could use that.
>>>
>>> Right.
>>>
>>>>
>>>>> +	udelay(delays[1]);
>>>>
>>>> Linux rounds up to 1 msec granularity here. We should do likewise.
>>>
>>> I don't see a good reason for that. The Linux driver used udelay()
>>> initially, that was changed to msleep as the times were too long for
>>> busy waiting. I have no clue why the author didn't use usleep_range
>>> instead.
>>
>> Same reason: Device trees are tested with Linux. They've a better chance
>> of just working when we round up wait times the same way.
> 
> I am generally with you, but in this case the binding is very clear and
> if you find a bug in the dts with this case, then be happy and fix it.
> 
> Sascha
> 
> 

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