RE: [PATCH v9 07/16] drivers: acpi: implement acpi_dma_configure

["Sricharan" <sricharan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

Hi Lorenzo,

>
>On Sat, Dec 3, 2016 at 11:39 AM, Lorenzo Pieralisi
><lorenzo.pieralisi-5wv7dgnIgG8@public.gmane.org> wrote:
>> On Sat, Dec 03, 2016 at 03:11:09AM +0100, Rafael J. Wysocki wrote:
>>> On Fri, Dec 2, 2016 at 4:38 PM, Lorenzo Pieralisi
>>> <lorenzo.pieralisi-5wv7dgnIgG8@public.gmane.org> wrote:
>>> > Rafael, Mark, Suravee,
>>> >
>>> > On Mon, Nov 21, 2016 at 10:01:39AM +0000, Lorenzo Pieralisi wrote:
>>> >> On DT based systems, the of_dma_configure() API implements DMA
>>> >> configuration for a given device. On ACPI systems an API equivalent to
>>> >> of_dma_configure() is missing which implies that it is currently not
>>> >> possible to set-up DMA operations for devices through the ACPI generic
>>> >> kernel layer.
>>> >>
>>> >> This patch fills the gap by introducing acpi_dma_configure/deconfigure()
>>> >> calls that for now are just wrappers around arch_setup_dma_ops() and
>>> >> arch_teardown_dma_ops() and also updates ACPI and PCI core code to use
>>> >> the newly introduced acpi_dma_configure/acpi_dma_deconfigure functions.
>>> >>
>>> >> Since acpi_dma_configure() is used to configure DMA operations, the
>>> >> function initializes the dma/coherent_dma masks to sane default values
>>> >> if the current masks are uninitialized (also to keep the default values
>>> >> consistent with DT systems) to make sure the device has a complete
>>> >> default DMA set-up.
>>> >
>>> > I spotted a niggle that unfortunately was hard to spot (and should not
>>> > be a problem per se but better safe than sorry) and I am not comfortable
>>> > with it.
>>> >
>>> > Following commit d0562674838c ("ACPI / scan: Parse _CCA and setup
>>> > device coherency") in acpi_bind_one() we check if the acpi_device
>>> > associated with a device just added supports DMA, first it was
>>> > done with acpi_check_dma() and then commit 1831eff876bd ("device
>>> > property: ACPI: Make use of the new DMA Attribute APIs") changed
>>> > it to acpi_get_dma_attr().
>>> >
>>> > The subsequent check (attr != DEV_DMA_NOT_SUPPORTED) is always true
>>> > on _any_ acpi device we pass to acpi_bind_one() on x86, which was
>>> > fine because we used it to call arch_setup_dma_ops(), which is a nop
>>> > on x86. On ARM64 a _CCA method is required to define if a device
>>> > supports DMA so (attr != DEV_DMA_NOT_SUPPORTED) may well be false.
>>> >
>>> > Now, acpi_bind_one() is used to bind an acpi_device to its physical
>>> > node also for pseudo-devices like cpus and memory nodes. For those
>>> > objects, on x86, attr will always be != DEV_DMA_NOT_SUPPORTED.
>>> >
>>> > So far so good, because on x86 arch_setup_dma_ops() is empty code.
>>> >
>>> > With this patch, I use the (attr != DEV_DMA_NOT_SUPPORTED) check
>>> > to call acpi_dma_configure() which is basically a nop on x86 except
>>> > that it sets up the dma_mask/coherent_dma_mask to a sane default value
>>> > (after all we are setting up DMA for the device so it makes sense to
>>> > initialize the masks there if they were unset since we are configuring
>>> > DMA for the device in question) for the given device.
>>> >
>>> > Problem is, as per the explanation above, we are also setting the
>>> > default dma masks for pseudo-devices (eg CPUs) that were previously
>>> > untouched, it should not be a problem per-se but I am not comfortable
>>> > with that, honestly it does not make much sense.
>>> >
>>> > An easy "fix" would be to move the default dma masks initialization out
>>> > of acpi_dma_configure() (as it was in previous patch versions of this
>>> > series - I moved it to acpi_dma_configure() just a consolidation point
>>> > for initializing the masks instead of scattering them in every
>>> > acpi_dma_configure caller) I can send this as a fix-up patch to Joerg if
>>> > we think that's the right thing to do (or I can send it to Rafael later
>>> > when the code is in the merged depending on the timing) just let me
>>> > know please.
>>>
>>> Why can't arch_setup_dma_ops() set those masks too?
>>
>> Because the dma masks set-up is done by the caller (see
>> of_dma_configure()) according to firmware configuration or
>> platform data knowledge. I wanted to replicate the of_dma_configure()
>> interface on ACPI for obvious reasons (on ARM systems), I stopped
>> short of adding ACPI code to mirror of_dma_get_range() equivalent
>> (through the _DMA object) but I am really really nervous about changing
>> the code path on x86 because in theory all is fine, in practice even
>> just setting the masks to sane values can have unexpected consequences,
>> I just can't know (that's why I wasn't doing it in the first iterations
>> of this series).
>>
>> Side note: DT with of_dma_configure() and ACPI with
>> acpi_create_platform_device() set the default dma mask for all
>> platform devices already _regardless_ of what they really are, though
>> arguably acpi_bind_one() touches ways more devices.
>>
>> I really think that removing the default dma masks settings from
>> acpi_dma_configure() is the safer thing to do for the time being (or
>> moving acpi_dma_configure() to acpi_create_platform_device(), where the
>> DMA masks are set-up by default by core ACPI. Mark, Suravee, what was
>> the rationale behind calling arch_setup_dma_ops() in acpi_bind_one() ?)
>
>Alternatively, you can add one more arch wrapper that will be a no-op
>on x86 and that will set up the default masks and call
>arch_setup_dma_ops() on ARM.  Then, you can invoke that from
>acpi_dma_configure().
>
>Or make the definition of acpi_dma_configure() itself depend on the
>architecture.
>

So is it better that either removing the masks from acpi_dma_configure (or)
creating the wrapper as Rafael mentioned, than moving
acpi_dma_configure itself , because with something like iommu probe
deferral that is tried, acpi_dma_configure is getting invoked from a device's 
really_probe, a different path again ?

Regards,
 Sricharan