Re: [PATCH v2 04/11] ARM: tegra: Set spi-max-frequency property to flash node

[Tomeu Vizoso <tomeu.vizoso@collabora.com>]

On 27 January 2015 at 17:48, Stephen Warren <swarren@wwwdotorg.org> wrote:
> On 01/27/2015 04:13 AM, Tomeu Vizoso wrote:
>>
>> On 15 January 2015 at 18:26, Stephen Warren <swarren@wwwdotorg.org> wrote:
>>>
>>> On 01/15/2015 09:12 AM, Tomeu Vizoso wrote:
>>>>
>>>>
>>>> To silence a warning on Nyan boards.
>>>>
>>>> Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
>>>> ---
>>>>    arch/arm/boot/dts/tegra124-nyan-big.dts | 1 +
>>>>    1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/arch/arm/boot/dts/tegra124-nyan-big.dts
>>>> b/arch/arm/boot/dts/tegra124-nyan-big.dts
>>>> index 9a9cffe..94c7ba9 100644
>>>> --- a/arch/arm/boot/dts/tegra124-nyan-big.dts
>>>> +++ b/arch/arm/boot/dts/tegra124-nyan-big.dts
>>>> @@ -1660,6 +1660,7 @@
>>>>
>>>>                  flash@0 {
>>>>                          compatible = "winbond,w25q32dw";
>>>> +                       spi-max-frequency = <25000000>;
>>>
>>>
>>>
>>> This property already exists in the SPI controller. Isn't the max
>>> frequency
>>> supposed to inherit from there? If so, shouldn't the code not warn when
>>> such
>>> inheritance happens, i.e. it'd be better to fix the code?
>>
>>
>> I don't think it's supposed to fall back to the controller's max freq,
>> as each device has its own maximum frequency that it can support and
>> it's not related to what the master supports.
>
>
> If the controller-specific frequency property isn't ever used, we should
> remove it. No point carrying cruft that looks like it does something but
> doesn't.
>
> Logically, each device's max frequency certainly should be influenced by all
> of the controller, board, and device max frequency. Those should all impose
> a cap on the speed used. I'd expect this to be expressed in DT as a single
> property in each device where the user has calculated that max for the
> device, with the option to have all the devices fall back to the "default"
> provided at the controller/bus level if the device imposes a cap that's no
> lower.

My understanding is that each device node needs to specify the maximum
frequency it supports, and the same for each controller.

When configuring the transfer parameters, its frequency will be capped
by both the device and controller maximum frequencies.

So we currently need to specify the maximum frequencies in both device
and controller nodes, and it sounds like a good idea to me.

I'm probably missing your point. Is it that it should be possible for
device nodes to leave unspecified their max frequency and that in that
case the one for the controller should be used instead?

Regards,

Tomeu