Re: [PATCH v3 6/7] Drivers: hv: vmbus: Get the IRQ number from DT

[Roman Kisel]

On 7/27/2024 2:17 AM, Arnd Bergmann wrote:
> On Sat, Jul 27, 2024, at 10:56, Krzysztof Kozlowski wrote:
> > On 27/07/2024 00:59, Roman Kisel wrote:
> > > @@ -2338,6 +2372,21 @@ static int vmbus_device_add(struct platform_device *pdev)
> > >   		cur_res = &res->sibling;
> > >   	}
> > >   
> > > +	/*
> > > +	 * Hyper-V always assumes DMA cache coherency, and the DMA subsystem
> > > +	 * might default to 'not coherent' on some architectures.
> > > +	 * Avoid high-cost cache coherency maintenance done by the CPU.
> > > +	 */
> > > +#if defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE) || \
> > > +	defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU) || \
> > > +	defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL)
> > > +
> > > +	if (!of_property_read_bool(np, "dma-coherent"))
> > > +		pr_warn("Assuming cache coherent DMA transactions, no 'dma-coherent' node supplied\n");
> >
> > Why do you need this property at all, if it is allways dma-coherent? Are
> > you supporting dma-noncoherent somewhere?
>
> It's just a sanity check that the DT is well-formed.
>
> Since the dma-coherent property is interpreted by common code, it's
> not up to hv to change the default for the platform. I'm not sure
> if the presence of CONFIG_ARCH_HAS_SYNC_DMA_* options is the correct
> check to determine that an architecture defaults to noncoherent
> though, as the function may be needed to do something else.
I used the ifdef as the dma_coherent field is declared under these macros:

#if defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE) || \
	defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU) || \
	defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL)
extern bool dma_default_coherent;
static inline bool dev_is_dma_coherent(struct device *dev)
{
	return dev->dma_coherent;
}
#else
#define dma_default_coherent true

static inline bool dev_is_dma_coherent(struct device *dev)
{
	return true;
}

i.e., there is no API to set dma_coherent. As I see it, the options
are either warn the user if they forgot to add `dma-coherent`

if (!dev_is_dma_coherent(dev)) pr_warn("add dma-coherent to be faster\n"),

or warn and force the flag to true. Maybe just warn
the user I think now... The code will be cleaner (no need to emulate
a-would-be set_dma_coherent) , and the user will
know how to make the system perform at its best.

Appreciate sharing the reservations about that piece!

> The global "dma_default_coherent' may be a better thing to check
> for. This is e.g. set on powerpc64, riscv and on specific mips
> platforms, but it's never set on arm64 as far as I can tell.
>
>       Arnd

--
Thank you,
Roman