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Subject: Re: [PATCH] ata: ahci-platform: add reset control support
To:     Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Cc:     Thierry Reding <thierry.reding@gmail.com>,
        Patrice CHOTARD <patrice.chotard@st.com>,
        Tejun Heo <tj@kernel.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        "linux-ide@vger.kernel.org" <linux-ide@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mediatek@lists.infradead.org" 
        <linux-mediatek@lists.infradead.org>
References: <6ce60a0b-0409-016a-d29c-91b8b9e2ad07@redhat.com>
 <cd3bc161-9a38-1e58-b66f-3ef81e3ed220@redhat.com>
 <20180406134838.2ADC.4A936039@socionext.com>
From:   Hans de Goede <hdegoede@redhat.com>
Message-ID: <22a2bcd7-eff5-c20f-e056-f5946e579d5e@redhat.com>
Date:   Fri, 6 Apr 2018 10:29:37 +0200
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Hi,

On 06-04-18 06:48, Kunihiko Hayashi wrote:
> Hi Hans,
> 
> On Thu, 5 Apr 2018 16:08:24 +0200
> Hans de Goede <hdegoede@redhat.com> wrote:
> 
>> Hi,
>>
>> On 05-04-18 16:00, Hans de Goede wrote:
>>> Hi,
>>>> On 05-04-18 15:54, Thierry Reding wrote:
>>>> On Thu, Apr 05, 2018 at 03:27:03PM +0200, Hans de Goede wrote:
>>>>> Hi,
>>>>>
>>>>> On 05-04-18 15:17, Patrice CHOTARD wrote:
>>>>>> Hi Thierry
>>>>>>
>>>>>> On 04/05/2018 11:54 AM, Thierry Reding wrote:
>>>>>>> On Fri, Mar 23, 2018 at 10:30:53AM +0900, Kunihiko Hayashi wrote:
>>>>>>>> Add support to get and control a list of resets for the device
>>>>>>>> as optional and shared. These resets must be kept de-asserted until
>>>>>>>> the device is enabled.
>>>>>>>>
>>>>>>>> This is specified as shared because some SoCs like UniPhier series
>>>>>>>> have common reset controls with all ahci controller instances.
>>>>>>>>
>>>>>>>> Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
>>>>>>>> ---
>>>>>>>> ??? .../devicetree/bindings/ata/ahci-platform.txt????? |? 1 +
>>>>>>>> ??? drivers/ata/ahci.h???????????????????????????????? |? 1 +
>>>>>>>> ??? drivers/ata/libahci_platform.c???????????????????? | 24 +++++++++++++++++++---
>>>>>>>> ??? 3 files changed, 23 insertions(+), 3 deletions(-)
>>>>>>>
>>>>>>> This causes a regression on Tegra because we explicitly request the
>>>>>>> resets after the call to ahci_platform_get_resources().
>>>>>>
>>>>>> I confirm, we got exactly the same behavior on STi platform.
>>>>>>
>>>>>>>
>>>>>>> ?? From a quick look, ahci_mtk and ahci_st are in the same boat, adding the
>>>>>>> corresponding maintainers to Cc.
>>>>>>>
>>>>>>> Patrice, Matthias: does SATA still work for you after this patch? This
>>>>>>> has been in linux-next since next-20180327.
>>>>>>
>>>>>> SATA is still working after this patch, but a kernel warning is
>>>>>> triggered due to the fact that resets are both requested by
>>>>>> libahci_platform and by ahci_st driver.
>>>>>
>>>>> So in your case you might be able to remove the reset handling
>>>>> from the ahci_st driver and rely on the new libahci_platform
>>>>> handling instead? If that works that seems like a win to me.
>>>>>
>>>>> As said elsewhere in this thread I think it makes sense to keep (or re-add
>>>>> after a revert) the libahci_platform reset code, but make it conditional
>>>>> on a flag passed to ahci_platform_get_resources(). This way we get
>>>>> the shared code for most cases and platforms which need special handling
>>>>> can opt-out.
>>>>
>>>> Agreed, although I prefer such helpers to be opt-in, rather than
>>>> opt-out. In my experience that tends make the helpers more resilient to
>>>> this kind of regression. It also simplifies things because instead of
>>>> drivers saying "I want all the helpers except this one and that one",
>>>> they can simply say "I want these helpers and that one". In the former
>>>> case whenever you add some new (opt-out) feature, you have to update all
>>>> drivers and add the exception. In the latter you only need to extend the
>>>> drivers that want to make use of the new helper.
>>
>> Erm, the idea never was to make this opt-out but rather opt in, so
>> we add a flags parameter to ahci_platform_get_resources() and all
>> current users pass in 0 for that to keep the current behavior.
>>
>> And only the generic drivers/ata/ahci_platform.c driver will pass
>> in a the new AHCI_PLATFORM_GET_RESETS flag, which makes
>> ahci_platform_get_resources() (and the other functions) also deal
>> with resets.
>>
>>>> With that in mind, rather than adding a flag to the
>>>> ahci_platform_get_resources() function, it might be more flexible to
>>>> split the helpers into finer-grained functions. That way drivers can
>>>> pick whatever functionality they want from the helpers.
>>>> Good point, so lets:
>>>> 1) Revert the patch for now
>>> 2) Have a new version of the patch which adds a ahci_platform_get_resets() helper
>>> 3) Modify the generic drivers/ata/ahci_platform.c driver to call the new
>>>   ?? ahci_platform_get_resets() between its ahci_platform_get_resources()
>>>   ?? and ahci_platform_enable_resources() calls.
>>>   ?? I think that ahci_platform_enable_resources() should still automatically
>>>   ?? do the right thing wrt resets if ahci_platform_get_resets() was called
>>>   ?? (otherwise the resets array will be empty and should be skipped)
>>>> This should make the generic driver usable for the UniPhier SoCs and
>>> maybe some other drivers like the ahci_st driver can also switch to the
>>> new ahci_platform_get_resets() functionality to reduce their code a bit.
>>
>> So thinking slightly longer about this, with the opt-in variant
>> (which is what I intended all along) I do think that a flags parameter
>> is better, because the whole idea behind lib_ahci_platform is to avoid
>> having to do err = get_resource_a(), if (err) bail, err = get_resource_b()
>> if (err) bail, etc. in all the ahci (platform) drivers. And having fine
>> grained helpers re-introduces that.
> 
> In case of adding a flag instead of get_resource_a(),
> for example, we add the flag for use of resets,
> 
> -struct ahci_host_priv *ahci_platform_get_resources(struct platform_device *pdev)
> +struct ahci_host_priv *ahci_platform_get_resources(struct platform_device *pdev,
> +                                                  bool use_reset)
> 
> and for now all the drivers using this function need to add the argument as false
> to the caller.
> 
> -       hpriv = ahci_platform_get_resources(pdev);
> +       hpriv = ahci_platform_get_resources(pdev, false);
> 
> Surely this can avoid adding functions such get_resource_a(). If we apply another
> feature later, we add its flag as one of the arguments instead. Is it right?

Yes, that is right, but instead of adding a "bool use_reset" please add
an "unsigned int flags" parameter instead and a:

#define AHCI_PLATFORM_GET_RESETS	0x01

And update all callers of ahci_platform_get_resources to pass 0 for flags
except for drivers/ata/ahci_platform.c. This way we only need to modify
all callers once, and if we want to add another optional resource in
the future we can add a:

#define AHCI_PLATFORM_GET_FOO		0x02

Without needing to change all callers again.

Regards,

Hans