Re: [PATCH 3/3] spi: bcm2835: Enable shared interrupt support

On Thu, Jun 04, 2020 at 09:05:46AM -0700, Florian Fainelli wrote:

On 6/4/2020 5:32 AM, Mark Brown wrote:

quoted

This feels hacky - it's essentially using the compatible string to set a
boolean flag which isn't really about the IP but rather the platform
integration.  It might cause problems if we do end up having to quirk
this version of the IP for some other reason.

I am not sure why it would be a problem, when you describe a piece of
hardware with Device Tree, even with the IP block being strictly the
same, its very integration into a new SoC (with details like shared
interrupt lines) do warrant a different compatible string. Maybe this is
more of a philosophical question.

The big concern here is trying to support things going forwards - if it
turns out that any quirks are required by this version of the IP then it
gets very confusing and hard to keep DTs stable if you've already
quirked something that clearly isn't the IP version with the compatible
string.  Conversely if we start putting flags into the binding for every
feature that might be changed in a given IP that gets complex as we
can't ever learn new things about an existing IP version without
updating all the DTs which is also bad.

Instead of counting the number of SPI devices we culd request the
interrupt first with flags = IRQF_PROBE_SHARED, if this works, good we
have a single SPI master enabled, if it returns -EBUSY, try again with
flags = IRQF_SHARED and set-up the bcm2835_spi_sh_interrupt interrupt
handler to manage the sharing.

Like you said in a followup patch that doesn't work as the first device
to probe will think the interrupt isn't shared.  You'd need a callback
to change to shared mode from genirq which feels...  inelegant.