[PATCH 15/18] irqchip/apple-aic: Add support for the Apple Interrupt Controller

Hector Martin marcan at marcan.st
Mon Feb 8 10:31:18 EST 2021

On 08/02/2021 20.36, Marc Zyngier wrote:
> On Mon, 08 Feb 2021 10:29:23 +0000,
> Arnd Bergmann <arnd at kernel.org> wrote:
>> On Mon, Feb 8, 2021 at 10:25 AM Marc Zyngier <maz at kernel.org> wrote:
>>> On Thu, 04 Feb 2021 20:39:48 +0000, Hector Martin <marcan at marcan.st> wrote:
>>>> +{
>>>> +     return readl(ic->base + reg);
>>> Please consider using the _relaxed accessors, as I don't think any of
>>> these interacts with memory (apart from IPIs, of course).
>> MSI interrupts require serializing with DMA, so at the minimum I think there
>> needs to be something that ensures that DMA from device into memory
>> has completed before delivering the completion interrupt to a driver. This
>> may already be implied when the AIC is entered, but this is hard to know
>> without actual hardware specs.
> If there is a sync with memory required, it should happen at the point
> where it is Acked, not when masked/unmasked or anything else. And
> given that you want to sync with an external agent (the DMA producer),
> the DMB generated by readl won't save you, as it only orders CPU
> accesses AFAICT.

Found an doc that talks about this, but then... how does the current 
Linux code work anyway for normal use cases?


That says dmb is not enough for DMA-control to DMA-data dependencies due 
to speculation, which is what we have here and the situation I described 
(with an IRQ along the way, but that's irrelevant). But that's what 
readl does: a read followed by a dmb(oshld) followed by a control 
dependency (but that needs an isb to take effect). How does this not 
break drivers that read DMA-accessed memory after a readl of a status 
register? I thought that was the point of the non-relaxed functions.

Hector Martin (marcan at marcan.st)
Public Key: https://mrcn.st/pub

More information about the linux-arm-kernel mailing list