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
