Message-ID: <20180409115901.GB29946@ulmo>
Date:   Mon, 9 Apr 2018 13:59:01 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>,
        Patrice CHOTARD <patrice.chotard@...com>,
        Tejun Heo <tj@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH] ata: ahci-platform: add reset control support

On Fri, Apr 06, 2018 at 10:29:37AM +0200, Hans de Goede wrote:
> 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@...hat.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@...ionext.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.

On a side-note, I also think the symbolic flags make the code easier to
read. Having something like:

	ahci_platform_get_resources(pdev, true, false, true);

becomes really difficult to understand.

Thierry

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