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Subject: Re: [PATCH] ata: ahci-platform: add reset control support
From:   Hans de Goede <hdegoede@redhat.com>
To:     Thierry Reding <thierry.reding@gmail.com>
Cc:     Patrice CHOTARD <patrice.chotard@st.com>,
        Kunihiko Hayashi <hayashi.kunihiko@socionext.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: <1521768653-11824-1-git-send-email-hayashi.kunihiko@socionext.com>
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 <1f7d0738-1963-21c5-c293-e46fb0214ecf@redhat.com>
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Message-ID: <cd3bc161-9a38-1e58-b66f-3ef81e3ed220@redhat.com>
Date:   Thu, 5 Apr 2018 16:08:24 +0200
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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.

Regards,

Hans