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.
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
