Re: [PATCH 2/2] ARM: arch-shmobile: Use multiple irq vectors for SDHI

[Magnus Damm <magnus.damm@gmail.com>]

On Mon, May 2, 2011 at 8:35 PM, Simon Horman <horms@verge.net.au> wrote:
> On Mon, May 02, 2011 at 12:17:27PM +0200, Guennadi Liakhovetski wrote:
>> On Wed, 27 Apr 2011, Simon Horman wrote:
>>
>> > This patch reverts "ARM: mach-shmobile: sh7372 SDHI vector merge" and
>> > supplies 3 IRQ resources for each SDHI block.
>> >
>> > Some blocks have a 4th IRQ, but this is used for DRM feathres
>> > that I do not have access tot he documentation for and are almost
>> > certainly tainted by licensing issues. So the 4th IRQ is not
>> > hooked-up even if it exists.
>>
>> No technical objections, but - could you maybe explain briefly why we want
>> multiple IRQ sources? Since there are implementations, that use one IRQ
>> for all sources, this mode has to be supported anyway, so, we will have to
>> be able to demux IRQs in software. Adding a multiple IRQ mode adds
>> complexity to the driver, because now both modes will have to be
>> supported, and you'll need more thorough locking to handle possible
>> concurrency, especially on SMP. What advantages does this multiple IRQ
>> mode bring?
>
> Hi Guennadi,
>
> sorry for not explaining the motivation earlier.
>
> As I understand things, on the mackerel/ap4evb/sh7372 the current
> multiplexing situation is actually fine.  There may be some slight gains to
> be had from a locking point of view in having separate IRQ handles.  And
> there may be an argument that breaking things out is more aesthetically
> pleasing. But there are know correctness issues as far as I know.
>
> The real point of this work is that its needed for the ag5evm/sh73a0.
> Currently there is an IRQ multiplexing mechanism for the ag4evm out of
> tree.  And as I understand things it is most likely racy as the all the
> relevant IRQs can't be disabled by the multiplexing mechanism (I believe it
> is because they don't share a clock. Magnus, can you fill in some details
> here?).

The sh73a0 SoC is using GIC as interrupt controller for the ARM core,
compared to the Renesas-specific INTC controller that is used in most
other SH-Mobile SoC offerings from Renesas at this point. On sh73a0
each SDHI block is connected to the GIC with 4 or 3 interrupt lines
but there is no group masking capability in the GIC case. So to use
SDHI on the sh73a0 platform we are finally forced to add support for
multiple interrupt source in the SDHI driver.

I initially tried to add this multiple-interrupt-source-for-SDHI
feature in 2009, but I then received a friendly NAK:
http://kerneltrap.org/mailarchive/linux-kernel/2009/3/11/4418714

Now when Guennadi has separated the tmio base code from SDHI we can
finally add support for multiple interrupt sources for SDHI and still
leave the other tmio-mmc platforms with a single interrupt source.

> So while the current solution has been satisfactory up to date, moving
> forwards there will be hardware that needs this multiple IRQ handlers.
>
> And the reason that I am developing this on mackerel/ap4evb/sh7372
> is that it should work correctly there and it is a platform that
> I have access to for development purposes.

After this is merged we can start moving towards removing the
INTC-specific forwarding hack.

Thanks,

/ magnus